TEST EQUIPMENT
Everyone thinks TEST EQUIPMENT will "solve the problem."
This is a big big MISTAKE.
Test equipment can help solve a problem and it can "lead to frustration,"  "give an incorrect answer,"  "mess you up," and make things worse.
You have to be very careful with test equipment and especially EXPENSIVE equipment because it is very sensitive and can detect pulses and glitches and voltages that are not affecting the operation of the circuit.
You will learn a lot of tricks when reading through this article, but let me say two things.
There are lots of faults and components that you cannot test with "test equipment" (either In or OUT of the circuit) because the fault is either intermittent or the equipment does not load the device to the same extent as the circuit.
And secondly you need both an ANALOGUE multimeter and a DIGITAL meter to cover all the situations.
And if you are working on a car, you only need a $5.00 analogue meter because it will be dropped or fall into a crack, get oily and dirty and you will only lose $5.00
You will learn that a digital meter will pick up spikes and signals on a line and show an incorrect reading.
That's why you need to back-up your readings with an analogue meter.     
When you charge a battery it gets a "floating voltage" and this will be higher than the actual voltage, when the battery is fitted to a project. An analogue meter will draw a slight current and remove the "floating voltage" after a period of time.
Component testers can also give you a false reading, either because the component is out of range of the tester or intermittent and you need to be aware of this.
Oscilloscopes can also display waveforms that are parts of glitches or noise from other chips and these do not affect the operation of the part of the circuit you are investigating.
Sometimes you cannot pickup a pulse because it is not regular and the trigger on the oscilloscope does not show it on the screen. You may think it is missing.
It all depends on the "speed of the oscilloscope" - it's maximum frequency of operation.
Lastly- Power Supplies.  You cannot test globes and motors on a power supply because the starting current can be 5 times more than the operating current. The power supply may not be able to deliver this high current and thus you will think the motor or globe is faulty. Or the power supply is faulty.

You don't have to buy expensive test equipment but you do need it to do the simplest testing and fixing.

Here are the first four things you will need:

LED TESTER - tests LEDs
See the project: HERE

See the project: HERE

The CRO  - OSCILLOSCOPE  -  DIGITAL CRO

If you want to look at the signals in a project, you can get a low cost DIGITAL OSCILLOSCOPE for less than $40.00, such as this:

It has very good resolution and it needs a little bit of understanding to work out the amplitude and frequency of the signal your are displaying.
You can buy the oscilloscope on Aliexpress for about $40.00 and at the same time buy a LED that flashes RGB (either fast or slow) so you can use the CRO to watch the waveforms produced by the LED. Unfortunately the flashing LEDs are only available for $7.00 for 100.
The CRO comes fully built and tested but it needs a stand so you can use it on the work-bench. Wooden blocs 9 x 17 x 50mm are available from $2.00 shops and if you cut the end at 20° and screw them to the board via the two lower holes you will have an ideal stand as sown in the following image:

This is a much better DSO
It has internal battery and a clip-out stand

It has all the features of the other model
and a similar price

SWEEP
Originally all CRO's were analogue devices and used a CATHODE RAY TUBE, in which an electron beam was projected from the back of the tube (the neck) onto the screen. The screen had a phosphor coating and when the electron beam hit the screen it fluoresced and you could see the spot. The spot was then made to travel from the left of the screen to the right and the screen remained illuminated for a short period of time after the beam hit the area. A bit like the tail of a comet. This gave the effect of a brightly illuminated line across the screen.
The screen on this CRO is DIGITAL and all the effects are created by software in the microcontroller.
So that the screen appears like the original CRO's, the software produces a trace across the screen so you know what is happening.
We can pretend the trace is a "flying spot" and this is called the SWEEP.  
The sweep is measured by how long the "spot" takes to travel ONE GRATICULE.  That is from one vertical line to the next.
In our case this is as fast as 10 microseconds. If a wave is being processed by the microcontroller, the "flying spot" moves across the screen from the left-side, rises to the amplitude of the signal, moves across the screen at the same amplitude and then falls to the lowest point of the signal and then across the screen, to the rising edge of the wave and continues in the same way to the right of the screen.


TEST POINT
All CRO's have a TEST POINT (or TEST PIN) that produces a 1kHz square wave with a known amplitude. The tip of the probe touches this point and the screen displays the waveform. This allows the operator to confirm the settings on the screen are correct and the trace is in the middle of the screen.
You don't need to connect the earth lead to view this signal as the earth is connected internally on the board.

 The following diagram shows 3 waveforms:

The frequency of the signals is the same (such as 10kHz) and we are only showing one cycle to explain this feature.
The aim is to get the rise or fall of the wave so that it sits ON a graticule so that you can count the number of cycles across the screen and count the number of graticules.

In the image above we have six full cycles across 12 graticules. You cannot see the rise of the first cycle but we can assume (correctly) that it will be exactly the same as the rise of all the other cycles. And the same applies to the last rise.
We now select 10 graticules and this means we select ten "spacings" and in this distance we can count 5 complete cycles. This makes our calculations easy.
So, we have one cycle in 2 graticule-markings.
Suppose the sweep is set to 10uS. If we have one cycle per graticule, the frequency would be 100kHz. But the frequency takes 2 markings for each cycle and it is has a lower frequency than 100kHz. In fact it is 50kHz.  To prove this, you can change the sweep to 20uS and each cycle will occupy one graticule.

SAFETY
To prevent damage to the input of the CRO, it is best to fit a 1k resistor to the tip and use the lead of the resistor to probe the equipment you are testing. The input of the CRO is fairly high and 1k will not affect the amplitude of the signal.

MAXIMUM FREQUENCY
This CRO is not MHz or GHz CRO.  It is a $40.00 Digital CRO for Audio work and will detect a lot of signals on microcontroller projects that use 1MHz to 4MHz clocking. These projects will have signals up to 250kHz and 1MHz.
The screen consists of pixels and the waveform must be one pixel wide and one pixel spacing, so you need 4 pixels to show cycle. When the frequency is too high, the lines are simply joined together and nothing can be determined.
However you can detect 4 or 5 complete cycles within a graticule and 5 cycles represents a frequency of 500kHz.

Displaying the Maximum frequency
 

SHIFTING THE TRACE
We have covered the feature of connecting the tip of the probe to the module by a direct link (called DC)  or via a capacitor (called AC). This is called COUPLING and is determined by the top left-hand slide-switch.
We can also shift the trace up and down the screen to get more of the waveform onto the screen, especially when we have selected DC coupling and the wave is shifted up or down due to the DC offset.
DC OFFSET is the  the DC voltage on some signals, where say the 12v rail voltage is present and we want to see the ripple. You can either select AC Coupling and see the trace across the centre of the screen or select DC Coupling and move the trace so it can be viewed.
Moving the trace up and down is done by the SELECT button.
The SELECT button highlights the functions at the bottom of the screen and when it highlights the arrow at the left of the screen and changes from yellow to blue, you can move the arrow up and down the screen by pressing the second button to make it go up and the third button to make it go down.
This arrow will take the trace up and down the screen so you can fit waveforms on the screen.

DETECTING SIGNALS
There are many devices you can use to produce signals on the screen. A simple CRO like this is not magic.  It does not produce a waveform immediately as it needs time to process the information and if the signal is from speech, the wave will be very complex. It will consist of large and small waves with varying duration. Clean signals are much easier to understand.
Don't use the CRO near you laptop as the leads pick up scanning signals from the keyboard.
Start with the signal from the TEST POINT and view it on the screen.
You can whistle into a piezo diaphragm or talk loudly into a speaker and see the result.
You can also get a flashing or fading RGB LED and connect it a 12v supply via a 1k resistor. Put the probes across the 1k resistor and you will see the signal from the microcontroller.

TRIGGERING
We need the trace on the screen to be as stable as possible so we can view the signal and work out its characteristics.
To do this there is a detecting circuit that starts to look at the wave when it has increased by a small amount. From this the circuit will start detecting the next cycle at the same point and at least the beginning of each cycle will align with each other.
Waveforms that are constant and accurate are not a problem as they will display cleanly, but the trigger feature is mainly used to stabilize a fluctuating wave.
The CRO has an automatic trigger feature and it "triggers" or "detects" the waveform automatically at a point that is determined by the software.
We can change this point if we think we can do better and the bottom button can be pressed to access the trigger feature. It is set to AUTO and by pressing the second button we can select NORM or SING.
If you want to use the NORM or SING modes, you have to select the next parameter below the screen that shows either a rising edge or falling edge for the trigger and then you have push the SELect button again to get to the right-hand arrow and change it from pink to blue and then press the up/down buttons to select the trigger point.
This feature is mainly used for signals that you are waiting a long time to receive or a waveform that occurs been a long pause.

FREEZE
Digital CRO's are called STORAGE CRO's because they store the information on the screen so it can be processed.
When you are watching the screen it is called the RUN Mode or RUNNING and the waveform may be changing so fast that you cannot see what is happening.
If you press the top button while in the AUTO mode, you will be able to freeze the screen and do some calculations. This is called the HOLD Mode.

With all these features you will able to view audio signals up to 200kHz and digital signals up to maybe 300kHz to 500kHz.

DON'T EXPECT TOO MUCH
Don't think a CRO will solve all your problems. It won't. Most of the time it will just confuse you and show all sorts of glitches and spikes in the signal that are not creating the problem.
You need to combine a CRO with a LOGIC PROBE and also look for power supply problems and smoothing as well as faulty components and dry joints.
This CRO may not be fast enough to pick up the glitch.
However you will learn the basics of operating a CRO and be able to advance to some of the more complex models. 
 

ooo000ooo

A very simple transistor tester:

See the project HERE

TEST EQUIPMENT - the POWER SUPPLY (Make your own)
Everyone and every text book tells you to buy a BENCH POWER SUPPLY.
But I am going to talk from 50 years experience.
If you are designing or repairing a project, you should connect it to a set of weak AA cells to see how it performs. This "Power Supply" will only deliver a small current and if the project has a fault, the voltage will drop considerably and the current will hopefully be small and the current will not damage anything.
This is the way you do your first "Test-Analysis."  By monitoring the voltage of the supply you can work out the current requirement. You can also find out if the project works on a reduced voltage.
Once you know the project does not have any short-circuits, you can use a new set of cells and then advance to a rechargeable set of cells or a BENCH POWER SUPPLY.
This method will also let you find a short circuit or a faulty transistor by feeling each component without blowing it up completely.
The only time I have had to use a Bench Power Supply has been for projects that need a variable power supply to show a cascade of LEDs that indicate the supply voltage.
For everything else I use a weak set of cells, a good set of cells and 12v made from Alkaline cells as it will deliver more than 10 amps.
This covers the whole range of requirements at very little cost.
If you want to power a project for a long period of time, you can buy a Wall Wort or Plug Pack for a few dollars on eBay or use a 5v phone charger or put 2 or 3 discarded chargers in series to get 10v or 15v. Put 1,000u electrolytic across each 5v output to help balance the contribution from each charger.

MULTIMETERS
There are two types:
DIGITAL and ANALOGUE
A Digital Multimeter has a set of digits on the display and an Analogue Multimeter has a scale with a pointer (or needle).
You really need both types to cover the number of tests needed for designing and repair-work.  We will discuss how they work, how to use them and some of the differences between them. 

 DIGITAL AND ANALOGUE MULTIMETERS
 

BUYING A MULTIMETER
There are many different types on the market.
The cost is determined by the number of ranges and also the extra features such as diode tester, buzzer (continuity), transistor tester, high DC current and others.
Since most multimeters are reliable and accurate, buy one with the greatest number of ranges at the lowest cost. 
This article explains the difference between a cheap analogue meter, an expensive analogue meter and a digital meter. You will then be able to work out which two meters you should buy.

Multimeters are sometimes called a "meter",  a "VOM"  (Volts-Ohms-Milliamps or Volt Ohm Meter) or "multi-tester" or even "a tester" - they are all the same.

USING A MULTIMETER
Analogue and digital multimeters have either a rotary selector switch or push buttons to select the appropriate function and range. Some Digital Multimeters (DMMs) are auto ranging; they automatically select the correct range of voltage, resistance, or current when doing a test. However you  need to select the function.

Before making any measurement you need to know what you are checking. If you are measuring voltage, select the AC range (10v, 50v, 250v, or 1000v)  or DC range (0.5v, 2.5v, 10v, 50v, 250v, or 1000v). If you are measuring resistance, select the Ohms range (x1, x10, x100, x1k, x10k). If you are measuring current, select the appropriate current range DCmA 0.5mA, 50mA, 500mA. Every multimeter is different however the photo below shows a low cost meter with the basic ranges. 

The most important point to remember is this:
You must select a voltage or current range that is bigger or HIGHER than the maximum expected value, so the needle does not swing across the scale and hit the "end stop."
If you are using a DMM (Digital Multi Meter), the meter will indicate if the voltage or current is higher than the selected scale, by showing "OL" - this means "Overload." If you are measuring resistance such as 1M on the x10 range the "OL" means "Open Loop" and you will need to change the range.  Some meters show "1'  on the display when the measurement is higher than the display will indicate and some flash a set of digits to show over-voltage or over-current.  A "-1" indicates the leads should be reversed for a "positive reading."
If it is an AUTO RANGING meter, it will automatically produce a reading, otherwise the selector switch must be changed to another range.

The Common (negative) lead ALWAYS fits into
the "COM" socket. The red lead fits into the
red socket for Voltage and Resistance.
Place the red lead (red banana plug)
into "A" (for HIGH CURRENT "Amps")
or mA,uA for LOW CURRENT.
 

The black "test lead" plugs into the socket marked "-"  "Common", or "Com,"  and the red "test lead" plugs into meter socket marked "+" or "V-W-mA."   The third banana socket measures HIGH CURRENT and the positive (red lead) plugs into this. You DO NOT move the negative "-" lead at any time.
The following two photos show the test leads fitted to a digital meter. The probes and plugs have "guards" surrounding the probe tips and also the plugs so you can measure high voltages without getting near the voltage-source.

The question above applies to both (every) type of multimeter and the type of meter you use depends on the accuracy you need. Sometimes you are looking for 1mV change on a 20v rail. Only a DMM will (or a CRO) will produce a result.

Analogue meters have an "Ohms Adjustment" to allow for the change in voltage of the battery inside the meter (as it gets old).

"Ohms Adjust" is also called "ZERO SET"
The sensitivity of this meter is 20,000ohms/volt
on the DC ranges and 5k/v on the AC ranges

Before taking a resistance reading (each time, on any of the Ohms scales) you need to  "ZERO SET" the scale, by touching the two probes together and adjust the pot until the needle reads "0" (swings FULL SCALE). If the pointer does not reach full scale, the batteries need replacing. Digital multimeters do not need "zero adjustment."

THE MULTIMETER I test all my projects with a $5.00 multimeter !! WHY??? Because an analogue multimeter puts a load on a circuit and the reading MUST be genuine. Secondly, an analogue multimeter will show fluctuations in a circuit and show when a certain part of a circuit is not maintaining stability. And thirdly, an analogue multimeter will respond to changes and pulses much faster than a digital meter. Lastly, if I can design and test a circuit with a cheap meter, everyone else should be able to do the same when using a more-expensive meter. Finally, an analogue meter lasts a lifetime. But if you damage it, the cost is only $5.00 And you get 500mA range, a digital meter gives 200mA. Analogue Meters are on eBay I have digital meter when I want to read voltages accurately. If you buy two multimeters, you can test currents up to 1 amp by placing the multimeters in PARALLEL as shown in the following diagram: The red and black probes go to the positive and negative terminals of the project you are testing and you simply ADD the current readings (shown by the pointer on each meter) to get a final value (up to one amp).  Current flows through the multimeter from the positive probe to the negative probe and the arrow on the meter above shows this direction. This is how we arrive at that statement: When taking a measurement of CURRENT, the voltage on the positive probe will be very slightly higher than the voltage on the negative probe, because a very small voltage will be dropped across the CURRENT RESISTOR inside the meter. The meter is actually measuring the voltage across this resistor and you are reading the pointer where the scale says 0-500mA.  We know that current flows from positive to negative and when you trace the circuit above, you can see the meter is part of this circuit. When measuring CURRENT, you use exactly the same reasoning as when you are measuring voltage. Look at the circuit or project and work out which point will have the (slightly) higher voltage. The red probe goes to this point. When measuring CURRENT, even the wires will have a slightly higher voltage at one end. This is the end for the red probe. When measuring CURRENT, the circuit has to be CUT and the probes inserted into the CUT.  You cannot measure the current taken by a component by placing the probes "across it." You have to cut a wire or a track or desolder one of the wires. If you cannot remember how to connect a multimeter when testing CURRENT, tilt it slightly so the positive terminal is higher than the negative terminal and lay the red probe on the bench, HIGHER than the black probe. Now connect the red probe to the positive terminal of the battery and the black probe to the positive "input" of the project. Use another jumper to connect the negative of the battery to the negative (0v) of the project. See how the current has to flow across the meter (from left to right) to make the point read "up-scale". The probes are connected to the battery as shown in the diagram above.

FIXING A MULTIMETER
A multimeter can get "broken" "damaged" and go "faulty."
I don't know why, but eventually they stop working.
It can be something simple like a flat battery, corroded battery contacts, broken switch or something complex, like the circuitry failing.
Multimeters are so cheap, you can buy a new one for less than $10.00
These meters can have a 10 amp range, transistor tester and measure up to 2 meg ohms.
That's why I suggest buying a $10.00 meter.  They are just as good as a $60.00 meter and the cheapest meters last the longest.
Dropping an analogue meter can cause the hair spring to loop over one of the supports and the needle will not zero correctly. You will need to open the cover on the movement and lift the spring off the support with a needle.
A faulty meter can be used in a battery-charger circuit to measure the current or voltage if that scale is still reading-correctly.
Otherwise keep the leads and throw the meter out. It is too dangerous keeping a meter that shows an incorrect reading.
NOTE:  When the battery in a digital meter gets low, the digits on the display start to fade and you need to change the 12v battery.  But before this happens, the low battery voltage will make a voltage reading higher than the actual value and this can fool you.
This happened to me. The 5v regulator voltage increased to 6v, 7v, 8v and I thought the regulator had failed.  Then the display failed and changing the battery solved the problem.

MEASURING FREQUENCY

Before we cover the normal uses for a multimeter, it is interesting to note that some Digital Multimeters (DMM) have features such as Capacitance, Frequency and measuring the gain of a transistor as well as a number of other features using probes such as a temperature probe.  The VICHY VC99 meter above is an example and costs about $40.00.

Basic function	Range
DCV	600mV/6V/60V/600V/1000V
ACV	6V/60/600/1000V
DCA	600uA/6000uA/60mA/600mA/6A/20A
ACA	600uA/6000uA/60mA/600mA/6A/20A
Resistance	600Ω/6kΩ/60kΩ/600kΩ/6MΩ/60MΩ
Capacitance	40nF/ 400nF/4uF/40uF/400uF/2000uF
Frequency	100Hz/1kHz/10kHz/100kHz/1MHz/60MHz
Temperature	-40°C~1000°C
	0°F~1832°F

　

　

　

　

 

MEASURING VOLTAGE
Most of the readings you will take with a multimeter will be VOLTAGE readings.
Before taking a reading, you should select the highest range and if the needle does not move up scale (to the right), you can select another range.
Always switch to the highest range before probing a circuit and keep your fingers away from the component being tested.
If the meter is Digital, select the highest range or use the auto-ranging feature, by selecting "V." The meter will automatically produce a result, even if the voltage is AC or DC.
If the meter is not auto-ranging, you will have to select if the voltage is from a DC source or if the voltage is from an AC source. DC means Direct Current and the voltage is coming from a battery or supply where the voltage is steady and not changing and AC means Alternating Current where the voltage is coming from a voltage that is rising and falling. 
You can measure the voltage at different points in a circuit by connecting the black probe to chassis. This is the 0v reference and is commonly called "Chassis" or "Earth" or "Ground" or "0v."
The red lead is called the "measuring lead" or "measuring probe" and it can measure voltages at any point in a circuit. Sometimes there are "test points" on a circuit and these are wires or loops designed to hold the tip of the red probe (or a red probe fitted with a mini clip or mini alligator clip).
You can also measure voltages ACROSS A COMPONENT. In other words, the reading is taken in PARALLEL with the component. It may be the voltage across a transistor, resistor, capacitor, diode or coil. In most cases this voltage will be less than the supply voltage.
If you are measuring the voltage in a circuit that has a HIGH IMPEDANCE, the reading will be inaccurate, up to 90% !!!, if you use a cheap analogue meter.

Here's a simple case.
The circuit below consists of two 1M resistors in series. The voltage at the mid point will be 5v when nothing is connected to the mid point. But if we use a cheap analogue multimeter set to 10v, the resistance of the meter will be about 100k, if the meter has a sensitivity of 10k/v and the reading will be incorrect.
Here how it works:
Every meter has a sensitivity. The sensitivity of the meter is the sensitivity of the movement and is the amount of current required to deflect the needle FULL SCALE.
This current is very small, normally 1/10th of a milliamp and corresponds to a sensitivity of 10k/volt (or 1/30th mA, for a sensitivity of 30k/v).
If an analogue meter is set to 10v, the internal resistance of the meter will be 100k for a 10k/v movement.
If this multimeter is used to test the following circuit, the reading will be inaccurate.
The reading should be 5v as show in diagram A.
But the analogue multimeter has an internal resistance of 100k and it creates a circuit shown in C.
The top 1M and 100k from the meter create a combined PARALLEL resistance of 90k. This forms a series circuit with the lower 1M and the meter will read less than 1v  
If we measure the voltage across the lower 1M, the 100k meter will form a value of resistance with the lower 1M and it will read less than 1v
If the multimeter is 30k/v, the readings will be 2v. See how easy it is to get a totally inaccurate reading.




This introduces two new terms:
HIGH IMPEDANCE CIRCUIT and "RESISTORS in SERIES and PARALLEL."

If the reading is taken with a Digital Meter, it will be more accurate as a DMM does not take any current from the circuit (to activate the meter). In other words it has a very HIGH input impedance. Most Digital Multimeters have a fixed input resistance (impedance) of 10M - no matter what scale is selected. That's the reason for choosing a DMM for high impedance circuits.  It also gives a reading that is accurate to about 1%.


MEASURING VOLTAGES IN A CIRCUIT
You can take many voltage-measurements in a circuit. You can measure "across" a component, or between any point in a circuit and either the positive rail or earth rail (0v rail). In the following circuit, the 5 most important voltage-measurements are shown. Voltage "A" is across the electret microphone. It should be between 20mV and 500mV. Voltage "B" should be about 0.6v. Voltage "C" should be about half-rail voltage. This allows the transistor to amplify both the positive and negative parts of the waveform. Voltage "D" should be about 1-3v. Voltage "E" should be the battery voltage of 12v.

MEASURING VOLTAGES IN A CIRCUIT

MEASURING CURRENT
You will rarely need to take current measurements, however most multimeters have DC current ranges such as 0.5mA, 50mA, 500mA and 10Amp (via the extra banana socket) and some meters have AC current ranges.  Measuring the current of a circuit will tell you a lot of things.  If you know the normal current, a high or low current can let you know if the circuit is overloaded or not fully operational.

Current is always measured when the circuit is working (i.e: with power applied).
It is measured IN SERIES with the circuit or component under test.
The easiest way to measure current is to remove the fuse and take a reading across the fuse-holder. Or remove one lead of the battery or turn the project off, and measure across the switch.
If this is not possible, you will need to remove one end of a component and measure with the two probes in the "opening."
Resistors are the easiest things to desolder, but you may have to cut a track in some circuits. You have to get an "opening" so that a current reading can be taken.
The following diagrams show how to connect the probes to take a CURRENT reading. 
Do not measure the current ACROSS a component as this will create a "short-circuit."
The component is designed to drop a certain voltage and when you place the probes across this component, you are effectively adding a "link" or "jumper" and the voltage at the left-side of the component will appear on the right-side. This voltage may be too high for the circuit being supplied and the result will be damage.

Measuring current through a resistor



Measuring the current of a globe

Do NOT measure the CURRENT of a battery
(by placing the meter directly across the terminals)
A battery will deliver a very HIGH current
and damage the meter

Do not measure the "current a battery will deliver" by placing the probes across the terminals. It will deliver a very high current and damage the meter instantly. There are special battery testing instruments for this purpose.
When measuring across an "opening" or "cut," place the red probe on the wire that supplies the voltage (and current) and the black probe on the other wire. This will produce a "POSITIVE" reading.
A positive reading is an UPSCALE READING and the pointer will move across the scale - to the right. A "NEGATIVE READING" will make the pointer hit the "STOP" at the left of the scale and you will not get a reading. If you are using a Digital Meter, a negative sign "-" will appear on the screen to indicate the probes are around the wrong way. No damage will be caused. It just indicates the probes are connected incorrectly. 
If you want an accurate CURRENT MEASUREMENT, use a digital meter.

MEASURING 1 AMP
Most digital multimeters only go to 200mA and most cheap analogue meters only go to 500mA.
But a clever way to measure up to 1 amp is to put 2 cheap analogue meters in parallel and read the two screens. Just add the combined values and you will be able to read up to 1 amp.
Just another time when a cheap $5.00 analogue meter comes in handy.

Here is anther way to increase the current range:

I have converted one of the meters above to 0-5AMP
using 8 x 1 ohm 0.25watt resistors in parallel on the 100mA range and the reading is 0 - 5 amp on the 50v scale.

Turning an old meter into a valuable 0-5Amp meter

This is how you do it.
Get an old meter for conversion. 
Get a good meter with a 0-200mA or 0-500mA scale.
Get a 12v supply and 3 or 5 watt wire wound resistors 8R2 and 3R3 etc. As many as you can.

Make a resistive circuit that draws say 100mA and check the current with the good meter (separately) and then with the meter you are going to modify to make sure both meters are detecting the correct amount of current.
Now you have a starting-point.

Make the SHUNT - the resistors soldered in parallel on the matrix board above - using 8 x one ohm resistors.

Connect the two meters in series with (wire wound resistors) to the 12v supply until the good meter (called the calibrating meter) reads FULL DEFLECTION.
If you add another resistor, the needle reading will go DOWN SCALE. If you remove a resistor the needle will move UP-SCALE.
Now adjust the number of resistors until the meter you are calibrating reads the same current. 
This is as far as you can go. 
You don't need any complex mathematics.  Just a simple addition or removal of resistors. You can use 2R2 or 3R3 to make smaller increments or decrements in the position of the pointer.
This is not the best way to calibrate a meter but it is is the best we can do. Any inaccuracy in our calibration will be multiplied 5 times in the final reading - but this is a simple way to turn an old meter into something valuable.
Now remove the calibrating meter and reduce the number of wire wound resistors and the needle will move UP-SCALE.  Reduce the number of wire wound resistors and the remaining resistors will get HOT and the needle will move up scale to the 5 amp reading.
You now have a valuable 0-5 amp CURRENT METER.
Called  an AMMETER   (0-5AMP)
Called a 5-Amp Meter or
5-Amp AMMETER (this is the best name).

Here is another way to convert a digital meter:
Most digital meters only read to to 200mA. You can increase this to 1 amp with the following simple set of resistors called SHUNT RESISTORS.
Place the multimeter in series with a load that is taking 200mA.
Now get some one ohm, 2.2ohm and 3.3 ohm resistors. Place them, one at a time, across the two probes and you will find the reading will reduce every time you add a resistor. 
You can put them directly across the probes or two in series to get a larger résistance.
Keep experimenting until the reading on the meter is "40."  You know this reading is really 200mA, so, when the probes are put on a circuit that reads "200"  the real current will be 200 x 5 = 1,000mA or 1AMP! 
 

MEASURING AC CURRENT
Measuring AC current is very difficult to do because the waveform is rising and falling and when the waveform is "on and off" such as the DCC waveform in a DCC Model Railway set-up, the actual current taken by a module will be impossible to measure with a cheap multimeter. 
However a simple way to find out the current flowing is to place a 500 ohm pot in the positive line and connect a LED from the middle pin to one of the outer pins and then connect another LED across the LED but in the opposite direction. This will only be suitable for a current up to about 50mA.
As you turn the pot from zero ohms, the LEDs will start to come on.
We are NOT measuring the brightness but the point at which the LED detects a voltage of about 3.2v across the pot. (actually across about half the pot)
Now put the "tester" on a variable power supply and connect a 220R as the load. As you increase the voltage, one of the LEDs will come on with the same very weak brightness. Now place a DC milliamp meter in line with the tester and measure the current. The value will be very close to the AC current flowing in the original circuit.
The same principle can be used to measure higher currents by using a low-resistance resistor and 2 LEDs.
Suppose you have a 10 ohm resistor and LEDs that illuminate at 3.2v 
When the current peaks at 320mA, the LEDs will be illuminated with very low brightness, but because the peak will only be for a very small portion of the cycle, the actual current-flow will not be equal or the same as 320mA DC current. We are just measuring a PEAK.
You have to be careful when making this type of "tester" to prevent damaging the LEDs. Start with a set of say 5 resistors in parallel with each value 47 ohms or slightly higher or lower. As you remove each resistor, the LEDs will start to come ON.
This will let you know that some point in the cycle the current is 320mA.
If it is a square-wave, such as the DCC waveform for Model Railways, the DC current-flow will be very nearly the same as the AC current measured by this tester. 

CURRENT SHARING
This is a term we use when two or more devices are placed in parallel and we hope each device will dissipate half the heat.
Suppose you have a component that gets too hot. You can add a heatsink or place another component "across it."
The first thing you have to remember is this: a lot of the heat from the component goes down the leads and into the tracks on the printed circuit board.
If you add another component you are sending the heat from two devices to the same tracks. You may need to keep the two devices apart.
However if the device is a transistor or diode the original device will drop a higher voltage when the full current flows and that's why it is getting so hot.
If you add another device on top of it, each device will pass less current and the voltage-drop across the combination will be less and the overall heat loss will be less. 
So you will be solving 3 problems at the same time.
1. The voltage-drop across the combination will be less,
2. The heat will be distributed via more devices, and
3. The set-up will fail less often because it is not being driven so hard.

A 1-amp diode can be connected across a 2-amp diode to reduce the heating of each component and the effectiveness needs to be tested with your fingers.
There is no law or rule for this but a silicon diode drops about 0.7v when half the specified current is flowing and rise to 1.1v when full current flows. Buy adding another diode across the first, the overall characteristic voltage drop across the combination can reduced considerably.

The same concept of current sharing applies to resistors and they can be placed in parallel or series to distribute the heat, but make sure the tracks on the board can dissipate the heat.
The rating of a resistor ONLY applies when it is connected close to the PC board and when the tracks are thick enough to dissipates the heat. Resistors "up in the air" can dissipate very little.

The same concept of current sharing can also be used for 3-terminal regulators and to find if they are equal-sharing, place your fingers on both at the same time and see if you let-go at the same time.

The same concept of current sharing can be used for zener diodes, as the current-capability of a high-voltage zener is less than a low voltage zener.  So, two zeners, 6v2 and 6v2 can be used for a 12v supply and the current capability of the set-up will be 64mA whereas a 12v zener of the same 400mW rating will be 33mA. Sometimes you need the zener to dissipate all the wattage when a LOAD is not connected to the circuit.  This is the concept of a zener diode as a SHUNT REGULATOR. See: Zener Diode - about halfway down the article we describe a SHUNT REGULATOR.

MEASURING RESISTANCE
Turn a circuit off before measuring resistance.
If any voltage is present, the value of resistance will be incorrect.
In most cases you cannot measure a component while it is in-circuit. This is because the meter is actually measuring a voltage across a component and calling it a "resistance." The voltage comes from the battery inside the meter. If any other voltage is present, the meter will produce a false reading.
If you are measuring the resistance of a component while still "in circuit," (with the power off) the reading will be lower than the true reading.  
                                    Measuring resistance

Measuring resistance of a heater
(via the leads)

Measuring the resistance of a piece of resistance-wire

Measuring the resistance of a resistor

Do not measure the "Resistance of a Battery"

1. Do not measure the "resistance of a battery." The resistance of a battery (called the Internal impedance) is not measured as shown in the diagrams above. It is measured by creating a current-flow and measuring the voltage across the battery.  Placing a multimeter set to resistance (across a battery) will destroy the meter. 
2. Do not try to measure the resistance of any voltage or any "supply."

Resistance is measured in OHMs.
The resistance of a 1cm x 1cm bar, one metre long is 1 ohm.
If the bar is thinner, the resistance is higher. If the bar is longer, the resistance is higher.
If the material of the bar is changed, the resistance is higher.
When carbon is mixed with other elements, its resistance increases and this knowledge is used to make RESISTORS. (However, when carbon is mixed with non-conducting powders, the resistance decreases. Such as mixing carbon with depolariser chemicals in a "dry cell.")
Resistors have RESISTANCE and the main purpose of a resistor is to reduce the CURRENT FLOW.
It's a bit like standing on a hose. The flow reduces.
When current flow is reduced, the output voltage is also reduced and that why the water does not spray up so high. Resistors are simple devices but they produce many different effects in a circuit.
A resistor of nearly pure carbon may be 1 ohm, but when non-conducting "impurities" are added, the same-size resistor may be 100 ohms, 1,000 ohms or 1 million ohms.
Circuits use values of less than 1 ohm to more than 22 million ohms.

Resistors are identified on a circuit with numbers and letters to show the exact value of resistance - such as 1k    2k2    4M7
The letter W  (omega  - a Greek symbol) is used to identify the word "Ohm."
but this symbol is not available on some word-processors, so the letter "R" is used.  The letter "E" is also sometimes used and both mean "Ohms."
A one-ohm resistor is written "1R" or "1E."  It can also be written "1R0" or "1E0."  
A resistor of one-tenth of an ohm is written  "0R1" or "0E1."   The letter takes the place of the decimal point.
10 ohms  =  10R  
100 ohms = 100R
1,000 ohms = 1k (k= kilo = one thousand)
10,000 ohms = 10k
100,000 ohms = 100k
1,000,000 ohms = 1M   (M = MEG = one million)

The size of a resistor has nothing to do with its resistance. The size determines the wattage of the resistor - how much heat it can dissipate without getting too hot.
Every resistor is identified by colour bands on the body, but when the resistor is a surface-mount device, numbers are used and sometimes letters.
You MUST learn the colour code for resistors and the following table shows all the colours for the most common resistors from 1/10th of an ohm to 22 Meg ohms for resistors with 5% and 10% tolerance. 



If 3rd band is gold, Divide by 10
If 3rd band is silver, Divide by 100
(to get 0.22ohms etc)

 

Reading 4-band resistors
The most "common" type of resistor has 4 bands and is called the 10% resistor. It now has a tolerance of 5% but is still called the "10% type" as the colours increase by 20% so that a resistor can be 10% higher or 10% lower than a particular value and all the resistors produced in a batch can be used.
The first 3 bands produce the resistance and the fourth band is the "tolerance" band. Gold = 5%
(Silver =10% but no modern resistors are 10%!! - they are 5%  2% or 1%)

 

Here is another well-designed resistor colour code chart:

Download the program and save it on your desk-top for future reference:
ColourCode.exe  (520KB)
ColourCode.zip   (230KB)
ColourCode.rar   (180KB)

RESISTORS LESS THAN 10 OHMS
When the third band is gold, it indicates the value of the "colors" must be divided by 10.
Gold = "divide by 10" to get values 1R0 to 8R2
When the third band is silver, it indicates the value of the "colors" must be divided by 100. (Remember: more letters in the word "silver" thus the divisor is "a larger division.")
Silver = "divide by 100" to get values R1 to R82
e.g: 0R1 = 0.1 ohm     0R22 =  point 22 ohms  
See 4th Column above for examples.

The letters "R, k and M" take the place of a decimal point.
e.g: 1R0 = 1 ohm     2R2 = 2 point 2 ohms   22R = 22 ohms  
2k2 = 2,200 ohms     100k = 100,000 ohms
2M2 = 2,200,000 ohms


HOW TO REMEMBER THE COLOUR CODE:
Each colour has a "number" (or divisor) corresponding to it.
Most of the colours are in the same order as in the spectrum. You can see the spectrum in a rainbow. It is:  ROY G BIV  and the colours for resistors are in the same sequence.

black 
brown     -  colour of increasing temperature
red
orange
yellow
green
blue
(indigo - that part of the spectrum between blue and violet)
violet
gray  
white
 

colour	value	No of zero's
silver	-2	divide by 100
gold	-1	divide by 10
black	0	No zeros
brown	1	0
red	2	00
orange	3	,000 or k
yellow	4	0,000
green	5	00,000
blue	6	M
violet	7
gray	8
white	9

Here are some common ways to remember the colour code:
Bad Beer Rots Our Young Guts, But Vodka Goes Well
Bright Boys Rave Over Young Girls But Violet Gets Wed
Bad Boys Rave Over Young Girls But Violet Gets Wed with Gold and Silver.

Reading 5-band resistors:
5-band resistors are easy to read if you remember two simple points. The first three bands provide the digits in the answer and the 4th band supplies the number of zero's.  

Reading "STANDARD VALUES" (on 5-band resistors)
5-band resistors are also made in "Standard Values" but will have different colours to 4-band "common" resistors - and will be confusing if you are just starting out. For instance, a 47k 5% resistor with 4-bands will be: yellow-purple-orange-gold. For a 47k 1% resistor the colours will be yellow-purple-black-red-brown. The brown colour-band represents 1%. 
The first two colour-bands for a STANDARD VALUE or "common value" in 1% or 5% will be the SAME. These two bands provide the digits in the answer. 
It's the 3rd band for a 5% resistor that is expanded into two bands in a 1% resistor. But it's easy to follow. 
For a standard value, the 3rd band in a 1% resistor is BLACK. This represents a ZERO in the answer. (For 5-band resistors BLACK represents a ZERO when in the third band. This is different to 4-band resistors where black represents the word OHMS! If the third band is BROWN, the answer will be 1).
So the 4th band has to represent one-less ZERO and is one colour UP THE COLOUR CHART!  In other words the 3rd and 4th bands (combined) on a 1% resistor produces the same number of zero's as the 3rd band on a 5% resistor! 

Resistors come in a range of values and the two most common are the E12 and E24 series. The E12 series comes in twelve values for each decade. The E24 series comes in twenty-four values per decade.

E12 series - 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82

E24 series - 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91

Here is the complete list of 1%  1/4watt resistors from:
CIRCUIT SPECIALISTS. The following list covers 10 ohms (10R) to 1M.
To buy 1% resistors from Circuit Specialists, click: HERE.

10R 12R1 15R 18R2 22R1 27R4 30R1 33R2 36R5 39R2 47R5 49R9 51R1 56R2 68R1 75R 82R5 90R9 100R

121R 150R 182R 200R 221R 240R 249R 274R 301R 332R 348R 392R 402R 475R 499R 565R 604R 681R 750R

806R 825R 909R 1k0 1k21 1k5 1k82 2k 2k21 2k2 2k43 2k49 2k67 2k74 3k01 3k32 3k48 3k57 3k74

3k83 3k92 4k02 4k22 4k64 4k75 4k7 4k87 4k99 5k11 5k23 5k36 5k49 5k62 5k76 5k9 6k04 6k19 6k81

7k15 7k5 7k87 71k5 8k06 8k25 8k45 8k66 8k87 9k09 9k31 9k53 9k76 10k 11k 12k 12k1 12k4 13k

14k7 15k 15k8 16k9 17k4 17k8 18k2 20k 22k1 22k6 23k7 24k9 27k4 29k4 30k1 33k2 34k8 36k5 38k3

39k2 40k2 44k2 46k4 47k 47k5 49k9 51k1 53k6 56k2 61k9 68k1 69k8 75k0 82k5 90k 90k9 95k3 100k

121k 147k 150k 182k 200k 212k 221k 226k 249k 274k 301k 332k 357k 392k 475k 487k 499k 562k 604k 1M

Here is the list of 1% resistors from suppliers (such as Farnell):

1R0 1R2 1R5 2R2 2R7 3R3 3R9 4R7 5R6 6R2 6R8 7R5 8R2 9R1 10R 11R 12R

13R 15R 16R 18R 20R 22R 24R 27R 30R 33R 36R 39R 43R 47R 51R 56R 62R

68R 75R 82R 91R 100R 110R 120R 130R 150R 160R 180R 200R 220R 240R 270R 300R 330R

360R 390R 430R 470R 510R 560R 620R 680R 750R 820R 910R 1k 1k1 1k2 1k3 1k5 1k6

1k8 2k0 2k2 2k4 2k7 3k 3k3 3k6 3k9 4k3 4k7 5k1 5k6 6k2 6k8 7k5 8k2

9k1 10k 11k 12k 13k 15k 16k 18k 20k 22k 24k 27k 30k 33k 36k 39k 43k

47k 51k 56k 62k 68k 75k 82k 91k 100k 110k 120k 130k 150k 160k 180k 200k 220k

240k 270k 300k 330k 360k 390k 430k 470k 510k 560k 620k 680k 750k 820k 910k 1M

Surface Mount Resistors

3-digit Surface Mount resistors on a PC board

4-digit Surface Mount resistors on a PC board

The photo above shows surface mount resistors on a circuit board. The components that are not marked are capacitors (capacitors are NEVER marked).
All the SM resistors in the above photos conform to a 3-digit or 4-digit code. But there are a number of codes, and the 4-digit code caters for high tolerance resistors, so it's getting very complicated.
Here is a basic 3-digit SM resistor:

A 330k SM resistor

The first two digits represent the two digits in the answer. The third digit represents the number of zero's you must place after the two digits. The answer will be OHMS. For example: 334 is written 33 0 000. This is written 330,000 ohms. The comma can be replaced by the letter "k". The final answer is: 330k.
222 = 22 00 = 2,200 = 2k2 
473 = 47 000  = 47,000 = 47k
474 = 47 0000 = 470,000 = 470k
105 = 10 00000 = 1,000,000 = 1M = one million ohms
There is one trick you have to remember. Resistances less than 100 ohms are written: 100, 220, 470. These are 10 and NO zero's = 10 ohms = 10R 
or 22 and no zero's = 22R  or 47 and no zero's = 47R.  Sometimes the resistor is marked: 10, 22 and 47 to prevent a mistake.

Remember:
R = ohms
k = kilo ohms = 1,000 ohms
M = Meg = 1,000,000 ohms
The 3 letters (R, k and M) are put in place of the decimal point. This way you cannot make a mistake when reading a value of resistance.

Surface Mount CURRENT SENSING Resistors
Many new types of CURRENT SENSING surface-mount resistors are appearing on the market and these are creating lots of new problems.
Fortunately all resistors are marked with the value of resistance and these resistors are identified in MILLIOHMS.  A miili ohm is one thousandth or an ohm and is written 0.001 when writing a normal mathematical number.
When written on a surface mount resistor, the letter R indicates the decimal point and it also signifies the word "OHM" or "OHMS" and one milli-ohm is written R001 
Five miiliohms is R005   and one hundred milliohms is R100
Some surface mount resistors have the letter "M" after the value to indicate the resistor has a rating of 1 watt. e.g: R100M   These surface-mount resistors are specially-made to withstand a high temperature and a surface-mount resistor of the same size is normally 250mW or less.
These current-sensing resistors can get extremely hot and the PC board can become burnt or damaged.
When designing a PC board, make the lands very large to dissipate the heat.
Normally a current sensing resistor is below one ohm (1R0) and it is easy to identify them as R100 etc.
You cannot measure the value of a current sensing resistor as the leads of a multimeter have a higher resistance than the resistor and few multimeters can read values below one ohm.
If the value is not visible, you will have to refer to the circuit.
Before replacing it, work out why it failed.
Generally it gets too hot. Use a larger size and add tiny heatsinks on each end.
Here are some surface=mount current-sense resistors:

THE COMPLETE RANGE OF SM RESISTOR
MARKINGS
Click to see the complete range of SM resistor markings for 3-digit code: 

0R1 = 0.1ohm R22 = 0.22ohm R33 = 0.33ohm R47 = 0.47ohm R68 = 0.68ohm R82 = 0.82ohm 1R0 = 1R 1R2 = 1R2 2R2 = 2R2 3R3 = 3R3 4R7 = 4R7 5R6 = 5R6 6R8 = 6R8 8R2 = 8R2 100 = 10R 120 = 12R 150 = 15R 180 = 18R 220 = 22R 270 = 27R 330 = 33R 390 = 39R 470 = 47R 560 = 56R 680 = 68R 820 = 82R 101 = 100R 121 = 120R 151 = 150R 181 = 180R 221 = 220R 271 = 270R 331 = 330R 391 = 390R 471 = 470R 561 = 560R 681 = 680R 821 = 820R 102 = 1k0 122 = 1k2 152 = 1k5 182 = 1k8 222 = 2k2 272 = 2k7 332 = 3k3 392 = 3k9 472 = 4k7 562 = 5k6 682 = 6k8 822 = 8k2 103 = 10k 123 = 12k 153 = 15k 183 = 18k 223 = 22k 273 = 27k 333 = 33k 393 = 39k 473 = 47k 563 = 56k 683 = 68k 823 = 82k 104 = 100k 124 = 120k 154 = 150k 184 = 180k 224 = 220k 274 = 270k 334 = 330k 394 = 390k 474 = 470k 564 = 560k 684 = 680k 824 = 820k 105 = 1M0 125 = 1M2 155 = 1M5 185 = 1M8 225 = 2M2 275 = 2M7 335 = 3M3 395 = 3M9 475 = 4M7 565 = 5M6 685 = 6M8 825 = 8M2 106 = 10M0

Click to see the complete range of SM resistor markings for 4-digit code: 

0000 =00R 00R1 = 0.1ohm 0R22 = 0.22ohm 0R47 = 0.47ohm 0R68 = 0.68ohm 0R82 = 0.68ohm 1R00 = 1ohm 1R20 = 1R2 2R20 = 2R2 3R30 = 3R3 6R80 = 6R8 8R20 = 8R2 10R0 = 10R 11R0 = 11R 12R0 = 12R 13R0 = 13R 15R0 = 15R 16R0 = 16R 18R0 = 18R 20R0 = 20R 22R0 = 22R 24R0 = 24R 27R0 = 27R 30R0 = 30R 33R0 = 33R 36R0 = 36R 39R0 = 39R 43R0 = 43R 47R0 = 47R 51R0 = 51R 56R0 = 56R 62R0 = 62R 68R0 = 68R 75R0 = 75R 82R0 = 82R 91R0 = 91R 1000 = 100R 1100 = 110R 1200 = 120R 1300 = 130R 1500 = 150R 1600 = 160R 1800 = 180R 2000 = 200R 2200 = 220R 2400 = 240R 2700 = 270R 3000 = 300R 3300 = 330R 3600 = 360R 3900 = 390R 4300 = 430R 4700 = 470R 5100 = 510R 5600 = 560R 6200 = 620R 6800 = 680R 7500 = 750R 8200 = 820R 9100 = 910R 1001 = 1k0 1101 = 1k1 1201 = 1k2 1301 = 1k3 1501 = 1k5 1601 = 1k6 1801 = 1k8 2001 = 2k0 2201 = 2k2 2401 = 2k4 2701 = 2k7 3001 = 3k0 3301 = 3k3 3601 = 3k6 3901 = 3k9 4301 = 4k3 4701 = 4k7 5101 = 5k1 5601 = 5k6 6201 = 6k2 6801 = 6k8 7501 = 7k5 8201 = 8k2 9101 = 9k1 1002 = 10k 1102 = 11k 1202 = 12k 1302 = 13k 1502 = 15k 1602 = 16k 1802 = 18k 2002 = 20k 2202 = 22k 2402 = 24k 2702 = 27k 3002 = 30k 3302 = 33k 3602 = 36k 3902 = 39k 4302 = 43k 5102 = 51k 5602 = 56k 6202 = 62k 6802 = 68k 7502 = 75k 8202 = 82k 9102 = 91k 1003 = 100k 1103 = 110k 1203 = 120k 1303 = 130k 1503 = 150k 1603 = 160k 1803 = 180k 2003 = 200k 2203 = 220k 2403 = 240k 2703 = 270k 3003 = 300k 3303 = 330k 3603 = 360k 3903 = 390k 4303 = 430k 4703 = 470k 5103 = 510k 5603 = 560k 6303 = 620k 6803 = 680k 7503 = 750k 8203 = 820k 9103 = 910k 1004 = 1M 1104 = 1M1 1204 = 1M2 1304 = 1M3 1504 = 1M5 1604 = 1M6 1804 = 1M8 2004 = 2M0 2204 = 2M2 2404 = 2M4 2704 = 2M7 3004 = 3M0 3304 = 3M3 3604 = 3M6 3904 = 3M9 4304 = 4M3 4704 = 4M7 5104 = 5M1 5604 = 5M6 6204 = 6M2 6804 = 6M8 7504 = 7M5 8204 = 8M2 9104 = 9M1 1005 = 10M

0000 is a value on a surface-mount resistor. It is a zero-ohm LINK!
Resistances less than 10 ohms have  'R' to indicate the position of the decimal point.
Here are some examples:

Three Digit Examples	Four Digit Examples
330 is 33 ohms - not 330 ohms	1000 is 100 ohms - not 1000 ohms
221 is 220 ohms	4992 is 49 900 ohms, or 49k9
683 is 68 000 ohms, or 68k	1623 is 162 000 ohms, or 162k
105 is 1 000 000 ohms, or 1M	0R56 or R56 is 0.56 ohms
8R2 is 8.2 ohms

A new coding system has appeared on 1% types. This is known as the EIA-96 marking method. It consists of a three-character code. The first two digits signify the 3 significant digits of the resistor value, using the lookup table below. The third character - a letter - signifies the multiplier.

code	value		code	value		code	value		code	value		code	value		code	value
01	100		17	147		33	215		49	316		65	464		81	681
02	102		18	150		34	221		50	324		66	475		82	698
03	105		19	154		35	226		51	332		67	487		83	715
04	107		20	158		36	232		52	340		68	499		84	732
05	110		21	162		37	237		53	348		69	511		85	750
06	113		22	165		38	243		54	357		70	523		86	768
07	115		23	169		39	249		55	365		71	536		87	787
08	118		24	174		40	255		56	374		72	549		88	806
09	121		25	178		41	261		57	383		73	562		89	825
10	124		26	182		42	267		58	392		74	576		90	845
11	127		27	187		43	274		59	402		75	590		91	866
12	130		28	191		44	280		60	412		76	604		92	887
13	133		29	196		45	287		61	422		77	619		93	909
14	137		30	200		46	294		62	432		78	634		94	931
15	140		31	205		47	301		63	442		79	649		95	953
16	143		32	210		48	309		64	453		80	665		96	976

The multiplier letters are as follows:

letter	mult		letter	mult
F	100000		B	10
E	10000		A	1
D	1000		X or S	0.1
C	100		Y or R	0.01

22A is a 165 ohm resistor, 68C is a 49900 ohm (49k9) and 43E a 2740000 (2M74). This marking scheme applies to 1% resistors only.

A similar arrangement can be used for 2% and 5% tolerance types. The multiplier letters are identical to 1% ones, but occur before the number code and the following code is used:

2%						5%
code	value		code	value		code	value		code	value
01	100		13	330		25	100		37	330
02	110		14	360		26	110		38	360
03	120		15	390		27	120		39	390
04	130		16	430		28	130		40	430
05	150		17	470		29	150		41	470
06	160		18	510		30	160		42	510
07	180		19	560		31	180		43	560
08	200		20	620		32	200		44	620
09	220		21	680		33	220		45	680
10	240		22	750		34	240		46	750
11	270		23	820		35	270		47	820
12	300		24	910		36	300		48	910

With this arrangement, C31 is 5%, 18000 ohm (18k), and D18 is 510000 ohms (510k) 2% tolerance.
Always check with an ohm-meter (a multimeter) to make sure.

Chip resistors come in the following styles and ratings:
 Style: 0402, 0603, 0805, 1206, 1210, 2010, 2512, 3616, 4022
 Power Rating: 0402(1/16W), 0603(1/10W), 0805(1/8W), 1206(1/4W), 1210(1/3W), 2010(3/4W), 2512(1W), 3616(2W), 4022(3W)
 Tolerance: 0.1%, 0.5%, 1%, 5%
 Temperature Coefficient: 25ppm 50ppm 100ppm

EIA marking code for surface mount (SMD) resistors
01S = 1R 02S = 1R02 03S = 1R05 04S = 1R07 05S = 1R1 06S = 1R13 07S = 1R15 08S = 1R18 09S = 1R21 10S = 1R24 11S = 1R27 12S = 1R3 13S = 1R33 14S = 1R37 15S = 1R4 16S = 1R43 17S = 1R47 18S = 1R5 19S = 1R54 20S = 1R58 21S = 1R62 22S = 1R65 23S = 1R69 24S = 1R74 25S = 1R78 26S = 1R82 27S = 1R87 28S = 1R91 29S = 1R96 30S = 2R0 31S = 2R05 32S = 2R10 33S = 2R15 34S = 2R21 35S = 2R26 36S = 2R32 37S = 2R37 38S = 2R43 39S = 2R49 40S = 2R55 41S = 2R61 42S = 2R67 43S = 2R74 44S = 2R80 45S = 2R87 46S = 2R94 47S = 3R01 48S = 3R09 49S = 3R16 50S = 3R24 51S = 3R32 52S = 3R4 53S = 3R48 54S = 3R57 55S = 3R65 56S = 3R74 57S = 3R83 58S = 3R92 59S = 4R02 60S = 4R12 61S = 4R22 62S = 4R32 63S = 4R42 64S = 4R53 65S = 4R64 66S = 4R75 67S = 4R87 68S = 4R99 69S = 5R11 70S = 5R23 71S = 5R36 72S = 5R49 73S = 5R62 74S = 5R76 75S = 5R9 76S = 6R04 77S = 6R19 78S = 6R34 79S = 6R49 80S = 6R65 81S = 6R81 82S = 6R98 83S = 7R15 84S = 7R32 85S = 7R5 86S = 7R68 87S = 7R87 88S = 8R06 89S = 8R25 90S = 8R45 91S = 8R66 92S = 8R87 93S = 9R09 94S = 9R31 95S = 9R53 96S = 9R76	01R = 10R 02R = 10R2 03R = 10R5 04R = 10R7 05R = 11R 06R = 11R3 07R = 11R5 08R = 11R8 09R = 12R1 10R = 12R4 11R = 12R7 12R = 13R 13R = 13R3 14R = 13R7 15R = 14R 16R = 14R3 17R = 14R7 18R = 15R 19R = 15R4 20R = 15R8 21R = 16R2 22R = 16R5 23R = 16R9 24R = 17R4 25R = 17R8 26R = 18R2 27R = 18R7 28R = 19R1 29R = 19R6 30R = 20R0 31R = 20R5 32R = 21R0 33R = 21R5 34R = 22R1 35R = 22R6 36R = 23R2 37R = 23R7 38R = 24R3 39R = 24R9 40R = 25R5 41R = 26R1 42R = 26R7 43R = 27R4 44R = 28R0 45R = 28R7 46R = 29R4 47R = 30R1 48R = 30R9 49R = 31R6 50R = 32R4 51R = 33R2 52R = 34R0 53R = 34R8 54R = 35R7 55R = 36R5 56R = 37R4 57R = 38R3 58R = 39R2 59R = 40R2 60R = 41R2 61R = 42R2 62R = 43R2 63R = 44R2 64R = 45R3 65R = 46R4 66R = 47R5 67R = 48R7 68R = 49R9 69R = 51R1 70R = 52R3 71R = 53R6 72R = 54R9 73R = 56R2 74R = 57R6 75R = 59R0 76R = 60R4 77R = 61R9 78R = 63R4 79R = 64R9 80R = 66R5 81R = 68R1 82R = 69R8 83R = 71R5 84R = 73R2 85R = 75R0 86R = 76R8 87R = 78R7 88R = 80R6 89R = 82R5 90R = 84R5 91R = 86R6 92R = 88R7 93R = 90R9 94R = 93R1 95R = 95R3 96R = 97R6	01A = 100R 02A = 102R 03A = 105R 04A = 107R 05A = 110R 06A = 113R 07A = 115R 08A = 118R 09A = 121R 10A = 124R 11A = 127R 12A = 130R 13A = 133R 14A = 137R 15A = 140R 16A = 143R 17A = 147R 18A = 150R 19A = 154R 20A = 158R 21A = 162R 22A = 165R 23A = 169R 24A = 174R 25A = 178R 26A = 182R 27A = 187R 28A = 191R 29A = 196R 30A = 200R 31A = 205R 32A = 210R 33A = 215R 34A = 221R 35A = 226R 36A = 232R 37A = 237R 38A = 243R 39A = 249R 40A = 255R 41A = 261R 42A = 267R 43A = 274R 44A = 280R 45A = 287R 46A = 294R 47A = 301R 48A = 309R 49A = 316R 50A = 324R 51A = 332R 52A = 340R 53A = 348R 54A = 357R 55A = 365R 56A = 374R 57A = 383R 58A = 392R 59A = 402R 60A = 412R 61A = 422R 62A = 432R 63A = 442R 64A = 453R 65A = 464R 66A = 475R 67A = 487R 68A = 499R 69A = 511R 70A = 523R 71A = 536R 72A = 549R 73A = 562R 74A = 576R 75A = 590R 76A = 604R 77A = 619R 78A = 634R 79A = 649R 80A = 665R 81A = 681R 82A = 698R 83A = 715R 84A = 732R 85A = 750R 86A = 768R 87A = 787R 88A = 806R 89A = 825R 90A = 845R 91A = 866R 92A = 887R 93A = 909R 94A = 931R 95A = 953R 96A = 976R	01B = 1k 02B = 1k02 03B = 1k05 04B = 1k07 05B = 1k1 06B = 1k13 07B = 1k15 08B = 1k18 09B = 1k21 10B = 1k24 11B = 1k27 12B = 1k3 13B = 1k33 14B = 1k37 15B = 1k4 16B = 1k43 17B = 1k47 18B = 1k5 19B = 1k54 20B = 1k58 21B = 1k62 22B = 1k65 23B = 1k69 24B = 1k74 25B = 1k78 26B = 1k82 27B = 1k87 28B = 1k91 29B = 1k96 30B = 2k0 31B = 2k05 32B = 2k10 33B = 2k15 34B = 2k21 35B = 2k26 36B = 2k32 37B = 2k37 38B = 2k43 39B = 2k49 40B = 2k55 41B = 2k61 42B = 2k67 43B = 2k74 44B = 2k80 45B = 2k87 46B = 2k94 47B = 3k01 48B = 3k09 49B = 3k16 50B = 3k24 51B = 3k32 52B = 3k4 53B = 3k48 54B = 3k57 55B = 3k65 56B = 3k74 57B = 3k83 58B = 3k92 59B = 4k02 60B = 4k12 61B = 4k22 62B = 4k32 63B = 4k42 64B = 4k53 65B = 4k64 66B = 4k75 67B = 4k87 68B = 4k99 69B = 5k11 70B = 5k23 71B = 5k36 72B = 5k49 73B = 5k62 74B = 5k76 75B = 5k9 76B = 6k04 77B = 6k19 78B = 6k34 79B = 6k49 80B = 6k65 81B = 6k81 82B = 6k98 83B = 7k15 84B = 7k32 85B = 7k5 86B = 7k68 87B = 7k87 88B = 8k06 89B = 8k25 90B = 8k45 91B = 8k66 92B = 8k87 93B = 9k09 94B = 9k31 95B = 9k53 96B = 9k76	01C = 10k 02C = 10k2 03C = 10k5 04C = 10k7 05C = 11k 06C = 11k3 07C = 11k5 08C = 11k8 09C = 12k1 10C = 12k4 11C = 12k7 12C = 13k 13C = 13k3 14C = 13k7 15C = 14k 16C = 14k3 17C = 14k7 18C = 15k 19C = 15k4 20C = 15k8 21C = 16k2 22C = 16k5 23C = 16k9 24C = 17k4 25C = 17k8 26C = 18k2 27C = 18k7 28C = 19k1 29C = 19k6 30C = 20k0 31C = 20k5 32C = 21k0 33C = 21k5 34C = 22k1 35C = 22k6 36C = 23k2 37C = 23k7 38C = 24k3 39C = 24k9 40C = 25k5 41C = 26k1 42C = 26k7 43C = 27k4 44C = 28k0 45C = 28k7 46C = 29k4 47C = 30k1 48C = 30k9 49C = 31k6 50C = 32k4 51C = 33k2 52C = 34k0 53C = 34k8 54C = 35k7 55C = 36k5 56C = 37k4 57C = 38k3 58C = 39k2 59C = 40k2 60C = 41k2 61C = 42k2 62C = 43k2 63C = 44k2 64C = 45k3 65C = 46k4 66C = 47k5 67C = 48k7 68C = 49k9 69C = 51k1 70C = 52k3 71C = 53k6 72C = 54k9 73C = 56k2 74C = 57k6 75C = 59k0 76C = 60k4 77C = 61k9 78C = 63k4 79C = 64k9 80C = 66k5 81C = 68k1 82C = 69k8 83C = 71k5 84C = 73k2 85C = 75k0 86C = 76k8 87C = 78k7 88C = 80k6 89C = 82k5 90C = 84k5 91C = 86k6 92C = 88k7 93C = 90k9 94C = 93k1 95C = 95k3 96C = 97k6	01D = 100k 02D = 102k 03D = 105k 04D = 107k 05D = 110k 06D = 113k 07D = 115k 08D = 118k 09D = 121k 10D = 124k 11D = 127k 12D = 130k 13D = 133k 14D = 137k 15D = 140k 16D = 143k 17D = 147k 18D = 15k 19D = 154k 20D = 158k 21D = 162k 22D = 165k 23D = 169k 24D = 174k 25D = 178k 26D = 182k 27D = 187k 28D = 191k 29D = 196k 30D = 200k 31D = 205k 32D = 210k 33D = 215k 34D = 221k 35D = 226k 36D = 232k 37D = 237k 38D = 243k 39D = 249k 40D = 255k 41D = 261k 42D = 267k 43D = 274k 44D = 280k 45D = 287k 46D = 294k 47D = 301k 48D = 309k 49D = 316k 50D = 324k 51D = 332k 52D = 340k 53D = 348k 54D = 357k 55D = 365k 56D = 374k 57D = 383k 58D = 392k 59D = 402k 60D = 412k 61D = 422k 62D = 432k 63D = 442k 64D = 453k 65D = 464k 66D = 475k 67D = 487k 68D = 499k 69D = 511k 70D = 523k 71D = 536k 72D = 549k 73D = 562k 74D = 576k 75D = 590k 76D = 604k 77D = 619k 78D = 634k 79D = 649k 80D = 665k 81D = 681k 82D = 698k 83D = 715k 84D = 732k 85D = 750k 86D = 768k 87D = 787k 88D = 806k 89D = 825k 90D = 845k 91D = 866k 92D = 887k 93D = 909k 94D = 931k 95D = 953k 96D = 976k	01E = 1M 02E = 1M02 03E = 1M05 04E = 1M07 05E = 1M1 06E = 1M13 07E = 1M15 08E = 1M18 09E = 1M21 10E = 1M24 11E = 1M27 12E = 1M3 13E = 1M33 14E = 1M37 15E = 1M4 16E = 1M43 17E = 1M47 18E = 1M5 19E = 1M54 20E = 1M58 21E = 1M62 22E = 1M65 23E = 1M69 24E = 1M74 25E = 1M78 26E = 1M82 27E = 1M87 28E = 1M91 29E = 1M96 30E = 2M0 31E = 2M05 32E = 2M10 33E = 2M15 34E = 2M21 35E = 2M26 36E = 2M32 37E = 2M37 38E = 2M43 39E = 2M49 40E = 2M55 41E = 2M61 42E = 2M67 43E = 2M74 44E = 2M80 45E = 2M87 46E = 2M94 47E = 3M01 48E = 3M09 49E = 3M16 50E = 3M24 51E = 3M32 52E = 3M4 53E = 3M48 54E = 3M57 55E = 3M65 56E = 3M74 57E = 3M83 58E = 3M92 59E = 4M02 60E = 4M12 61E = 4M22 62E = 4M32 63E = 4M42 64E = 4M53 65E = 4M64 66E = 4M75 67E = 4M87 68E = 4M99 69E = 5M11 70E = 5M23 71E = 5M36 72E = 5M49 73E = 5M62 74E = 5M76 75E = 5M9 76E = 6M04 77E = 6M19 78E = 6M34 79E = 6M49 80E = 6M65 81E = 6M81 82E = 6M98 83E = 7M15 84E = 7M32 85E = 7M5 86E = 7M68 87E = 7M87 88E = 8M06 89E = 8M25 90E = 8M45 91E = 8M66 92E = 8M87 93E = 9M09 94E = 9M31 95E = 9M53 96E = 9M76	01F = 10M 18F = 15M 30F = 20M

 If you want an accurate RESISTANCE measurement, remove the resistor from the circuit and use a Digital meter.
 

SURFACE MOUNT COMPONENTS - PACKS
Talking Electronics has packs of components for the repairman. The following packs are available:

SURFACE MOUNT RESISTOR PACK consists of 1 off each standard value 
10 ohms to 1M & 2M2 (60 resistors) 
$14.20    including pack and post

SURFACE MOUNT CAPACITOR PACK consists of:
2 - 10p   5 - 47p  5 - 100p  5 - 470p  5 - 1n  5 - 10n   5 - 22n   5 - 100n 
5 - 1u 16v electrolytic    5 - 10u 16v electrolytic  
(40 components)    
$23.80  including pack and post

SURFACE MOUNT DIODE PACK consists of:
5 - 1N 4148 (marked as "A6") 
$10.00    including pack and post

SURFACE MOUNT TRANSISTOR PACK consists of:
5 - BC 848 (marked as "1K")  NPN
5 - BC858  PNP
$10.00     including pack and post

email Colin Mitchell for details on how to pay by credit card or PayPal.

CREATING ANY VALUE OF RESISTANCE
Any value of resistance can be created by connecting two resistors in PARALLEL or SERIES.
You can also create a higher wattage resistor by connecting them in SERIES OR PARALLEL.
We are only going to cover two EQUAL VALUE resistors in SERIES or in PARALLEL.
If you want to create a "Special Value," simply connect two resistors and read the value with a Digital Meter. Keep changing the values until you get the required value. We are not going into series or Parallel formulae.  You can easily find a value with a multimeter.

TWO EQUAL-VALUE RESISTORS IN SERIES
Two equal-value resistors IN SERIES creates a value of DOUBLE. You simply ADD the values.
This can be done with any to two values as shown. Three equal-value resistors in series is three times the value.

TWO EQUAL-VALUE RESISTORS IN PARALLEL
Two equal-value resistors IN PARALLEL creates a value of HALF. Three equal-value resistors in parallel is equal to one-third the value.

If you want a particular value and it is not available, here is a chart.
Use 2 resistors in series or parallel as shown:

Required Value	R1	Series/ Parallel	R2	Actual value:
10	4R7	S	4R7	9R4
12	10	S	2R2	12R2
15	22	P	47	14R9
18	22	P	100	18R
22	10	S	12	22
27	22	S	4R7	26R7
33	22	S	10	32R
39	220	P	47	38R7
47	22	S	27	49
56	47	S	10	57
68	33	S	33	66
82	27	S	56	83

There are other ways to combine 2 resistors in parallel or series to get a particular value. The examples above are just one way.  4R7 = 4.7 ohms      

TESTING A RESISTOR  
To check the value of a resistor, it should be removed from the circuit. The surrounding components can affect the reading and make it lower.
Resistors VERY RARELY change value, but if it is overheated or damaged, the resistance can increase. You can take the reading of a resistor "in-circuit" in one direction then the other, as the surrounding components may have diodes and this will alter the reading.
You can also test a resistor by feeling its temperature-rise. It is getting too hot if you cannot hold your finger on it (some "metal film" resistors are designed to tolerate quite high temperatures).  

HOW TO WORK OUT THE VALUE OF A RESISTOR  
This is a question from a reader.
"How do I work out the value of a resistor for biasing or illuminating a LED?"
The value of ALL resistors is worked out by the use of Ohm's Law.
Ohm's Law is:

I = V    Current is equal to voltage divided by resistance.
      R

Here's one way to work out the resistance:
The first thing you do is decide on the current you want to flow. It may be 1mA for the base current for a transistor or 10mA for a LED. It could be as low as 0.1mA or even 1uA. But you must decide on what we call "a current-flow."
Next, you need to know the voltage that will be at the top of the resistor and the voltage at the bottom. Let's say it is 12v at the top and 2v at the bottom.
This means 10v will be across the resistor.
Now we use Ohm's Law:       I = V/R       In other words, Current = voltage divided by resistance.
In all 3-term formulas, you must know the value of two items to be able to work out the value of the remaining item. These items are called "terms" or "variables"
Here are some examples:  
If the resistor is 10,000 ohms (10k) the current will be 1mA
If the resistor is 1,000 ohms (1k) the current will be 10mA
If the resistor is 100 ohms (100R) the current will be 100mA

The base-line for you to remember is this:  one volt across a 10k resistor will allow 1mA to flow.

For all other values you do not need a calculator.  
Just follow these examples:
If 10k will pass 1mA, 4k7 will pass 0.5mA and 3k3 will pass 0.3mA.
If 1k will pass 10mA, 470R will pass 20mA and 330R will pass 30mA.
If 100R will pass 100mA, 47R will pass 200mA and 33R will pass 300mA.

If 1M will pass 1uA, 470k will pass 2uA.

If the voltage is increased to 20v, all the current values will double.
If the voltage is reduced to 5v, the current-values will be halved. No calculator is needed.

If you don't know the voltage across the resistor, you will need to start with a 1M resistor then 100k then 10k then 1k, making sure nothing is getting too hot or being destroyed.   This called "trial-and-error" and is the basis of experimenting. 

If the resistor is in a high-current circuit, you will also need to work out its wattage.
Normally a 250mW standard through-hole resistor will be suitable for most applications (or a 100mW surface-mount resistor).
But when the current is more than say about 100mA, the resistor will get warm or hot.
You will need to work out the "losses" in the resistor. This is the wattage lost when the current is flowing.
This is called the POWER FORMULA.  It is: Power = volts x amps   (answer is watts)
We are going to keep the discussion simple and only cover the losses of 1watt or more.
When 100mA flows through a resistor and 10v is across the resistor, the wattage lost by the resistor (the dissipation of the resistor) will be:
Power =  10volts  times 0.1amp = 1 watt
The formula is:   Power = Volts x Amps and the answer is watts.
If the voltage is 10v and the current = 1 amp,  the watts dissipated by the resistor will be 10 watts.
This is called HEAT or WASTED ENERGY or HEAT LOSS and it is very difficult to dissipate from your project.   You may need a large heat-fin or a fan and is beyond the scope of this discussion.

 TESTING AN "AC" RESISTOR 
There is no such thing as an "AC" resistor. Resistors are just "resistors" and they can be in AC circuits or DC circuits.  Resistors can be given names such as "Safety Resistor"  "Ballast Resistor"  "LOAD Resistor" "Feed Resistor"   "Dropper Resistor" or "Supply Resistor." These are just normal resistors with a normal resistance - except a "Safety Resistor."
A safety resistor is made of a flame-proof material such as metal-oxide-film and not carbon-composition. It is designed to "burn out" when too much current flows BUT NOT CATCH FIRE.
It is a low-value resistor and has a voltage-drop across it but this is not intentional. The voltage-drop is to create a "heating-effect" to burn out the resistor. In all the other types of resistor, the voltage-drop is intentional.
A Ballast resistor is a normal resistor and can be called a Power resistor, Dropper resistor, Supply resistor or Feed resistor. It is designed to reduce the voltage from one source and deliver a lower voltage.  It is a form of: "in-line" resistor. 
A Load Resistor is generally connected across the output of a circuit and turns the energy it receives, into heat.

RESISTOR NETWORKS 
To reduce the number of components in a circuit, some engineers use a set of identical resistors in  a  package called a Single-In-Line (SIL) resistor network. It is made with many resistors of the same value, all in one package. One end of each resistor is connected all the other resistors and this is the common pin, identified as pin 1 and has a dot on the package.
These packages are very reliable but to make sure all the resistors are as stated, you need to locate pin 1. All values will be identical when referenced to this pin.

RESISTOR NETWORKS

Some resistor networks have a "4S" printed on the component. The 4S indicates the package contains 4 independent resistors that are not wired together inside. The housing has eight leads as shown in the second image.
Independent resistors have an even number of pins and measuring between each pair will produce identical values. Resistance between any pair will indicate leakage and may be a fault. If you know how they are connected, and the value, and you think they are faulty, you can replace an array with 8 small resistors soldered together in a similar way to the diagrams below.  The network below has an "in house" number and does not identify any values.

WIRE WOUND RESISTOR
A wire wound resistor is also called a POWER RESISTOR. This type of resistor can have a resistance as low as 0.1 ohms (one-tenth of an ohm) or as high as about 10k.
The image shows a 0.68 ohm resistor as the letter "R" represents the DECIMAL POINT and R68 is the same a .68 and this is 0.68 ohms. The wattage is 9 watts.
This resistor will allow xxx amps to flow.   To work out the current, use the formula:
Power = Current x Current x resistance
   9     = Current x Current  x .68
Divide both sides by 0.68
13.2   =  Current x Current
Find the square root of 13.2
Current = 3.6 amps

When 3.6 amps flow through the resistor, the voltage appearing across it will be:
V = current x resistance
   =  3.6      x  0.68
   =  2.5v and the wattage (heat) loss will be 9 watts.

The purpose of a resistor like this is to stop or reduce "ripple." Ripple is the noise or hum in an amplifier when the sound is turned up.
There are many reasons why you need to reduce the level of hum and this resistor will remove ripple as large as 2.5v when 3.6 amps is flowing, provided you have filter electrolytics on both side of the resistor to assist in removing the ripple.

If the letter "R" is in a different position, the value of resistance would be:

68R = 68Ω
6R8 = 6.8Ω
R68 = 0.68Ω

If you replace the R68 resistor a 6R8 resistor by mistake, the voltage across it will rise to 25v and if 3.6 amps flows, the wattage will be: 90 watts!!!
The resistor will glow red and burn out.  


TESTING A POSISTOR 

A Posistor is a resistor that connects in series with the degaussing coil around the picture tube or Monitor. When cold, it has a very low resistance and a large current flows when the monitor or TV is switched on. This current heats up the Posistor and the resistance increases. This causes the current to decrease and any magnetism in the shadow mask is removed. The posistor can one or two elements and it is kept warm so the resistance remains high. Many Posistors have a second element inside the case that connects directly to the supply to keep the Positive Temperature Coefficient resistor high so that the current through the degaussing coil falls to almost zero. This constant heat eventually destroys the package.  
The heavy current that flows when a set is turned ON also causes the posistor to crack and break and this results in poor purity on the screen - as the shadow mask gradually becomes magnetic..
Posistors have different resistance values from different manufacturers and must be replaced with an identical type.
They can be checked for very low resistance when cold but any loose pieces inside the case will indicate a damaged component.

 A "BURNT" RESISTOR - normally and technically called a "burnt-out" resistor.
The resistance of a "burnt" resistor can sometimes be determined by scraping away the outer coating - if the resistor has a spiral of resistance-material. You may be able to find a spot where the spiral has been damaged.

Note the spirals of conductive carbon.
The number of spirals has nothing to with the resistance.
It is the amount of carbon particles in the "track" that
determines the resistance. It is also the thickness and width
of the track that determines the resistance.
And then it is the overall size of the resistor that determines the wattage.
And then the size of the leads, the closeness to the PCB and
the size of the lands that eventually determines how hot the resistor
will get.

Clean the "spot" (burnt section of the spiral) very carefully and make sure you can get a good contact with the spiral and the tip of your probe. Measure from one lead of the resistor to the end of the damaged spiral. Then measure from the other lead to the other end of the spiral.
Add the two values and you have an approximate value for the resistor. You can add a small amount for the damaged section.
This process works very well for damaged wire-wound resistors. They can be pulled apart and each section of the resistance-wire (nichrome wire) measured and added to get the full resistance.

There is another way to determine the value of a damaged resistor.
Get a set of resistors of the same wattage as the damaged component and start with a high value. It's handy to know if the resistor is in the range: 10ohm to 100ohms or 1k to 10k etc, but this is not essential.
Start with a very high value and turn the circuit ON. You can perform voltage tests and if you know the expected output voltage, decrease the resistance until this voltage is obtained.
If you do not know the expected voltage, keep reducing the value of resistance until the circuit works as designed.
This is the best advice in a situation where you do not know the value of a resistor.

There is a third way to determine the value and this requires measuring the voltage drop across the resistor and the current-flow. By multiplying the two you will get a wattage and this must be less than the wattage of the resistor being replaced.

 A "SHUNT" RESISTOR
A SHUNT RESISTOR is a power-resistor and you will find them in multimeters to provide the CURRENT READING.
This type of resistor is also called A CURRENT SHUNT (Current Shunt Resistor) or CURRENT SENSE RESISTOR. You will also find them in many other circuits where the current is required to be measured.
A CURRENT SENSE RESISTOR is always very low resistance so it does not alter the performance of the circuit. 
As current flows though the resistor, a voltage is produced across the resistor and this voltage is measured by a detecting circuit and the designer of the circuit already knows how much current is flowing for each mV developed across the resistor.
But, if too much current flows, these resistors can burn-out and it is impossible to work out the value of the resistor. Mainly because the resistance can be as low as 0.1 ohms.
But there is an easy way to replace the resistor.
You will need a set of resistors and the cheapest way to start is with one ohm resistors (0.25watt).
Place two of them in parallel and connect the project in series with a multimeter set to 10 amp range and then place a 10 ohm wire-wound resistor in series with the two instruments and a 12v supply.
You know the current should be about 1.2 amps and if the reading on your damaged instrument is reading 3 amps, you will need to place another one-ohm resistor in parallel with the other two. Keep doing this until the reading on the damaged meter corresponds to the reading on the functioning meter.
You can add 2 or 3 one-ohm resistors in series to get a fine adjustment.
When you have finished, you can work out  the value of the combination by realising two resistors in parallel is equal to 0.5 ohms and 3 is 0.33 ohms and 4 is 0.25 ohms.
You may be able to buy shunt resistors of the required value or maybe use surface mount resistors but you also have to take into account the size of the original resistor. You must match the size.

MILLI-OHM RESISTORS
Many SHUNT RESISTORS have a very low resistance, mainly because the resistor is measuring a high current and you don't want a lot of heat to be created by the resistor and you don't want the voltage you are measuring to reduce in value.
That's why many SHUNT RESISTORS have a resistance in the MILLI-OHM range.
A milliohm is ONE THOUSANDTH OF AN OHM and if one amp is flowing through the resistor, one-millivolt will be developed across it.
If you go through all the samples below, the same reasoning produces 5mV for a 5 milli-ohm resistor and one quarter of a milli-volt for a 0m25 resistor. Thus 4 amp through this resistor produces 1mV.
Because a high current will flow through the resistor and through the tracks, there will be some degree of heating and these resistors can get warm/hot and even overheat and melt the solder. They can also go open and that's where your skill comes in  . . to replace them with the correct value and wattage.    

TESTING AND MEASURING MILLI-OHM RESISTORS
Very low-ohm resistors can be measured in two ways.
Digital multimeters will measure resistors from 0.1 ohms but the leads add 0.3 ohms to the reading.
You can double-check the reading by adding a 1 ohm resistor and measuring the milli-ohm resistor with the 1 ohm and with it removed.
If the milliohm resistor is less than 0.1 ohm, you can place the resistor in series with a multimeter set to current (1 amp) and add an 8 ohm wire wound resistor in series.
Connect the circuit to a variable power supply and increase the current to exactly 1 amp.
Now measure the voltage across the milli-ohm resistor with a multimeter set to say 2,000mA.
Each mV will represent 1milliohm. 
For example, if the reading is 200mV, the resistor is 200milli-ohm (0.2 ohm).
If the reading is 1,000mV (1volt) the resistor will be 1,000milli-ohm (1 ohm)  
A 0m25 resistor (shown above) will show 0.25mV on the multimeter.
A 5M0 resistor above will show 5mV on the multimeter.
A R015 resistor will show 15mV on the multimeter.

SAFETY RESISTOR
Finally we come to the use of a resistor as a SAFETY RESISTOR.
Whenever you are testing a circuit, you need to use a supply that will not deliver a high current. This will prevent things "going up in smoke" and burning the PCB tracks. The easiest is to use "dry cells" (AA) and even though the motor or output device may not work correctly, you can be sure a short-circuit is not present.
If you are going to use a 12v battery, you need to include a one-ohm (0.25watt) resistor in series with the positive lead.
This resistor ill allow 250mA to pass with damage. At 500mA the resistor will get very hot and any current above this will burn out the resistor.
You will be able to work out how quickly the resistor "goes up in smoke" and compare it to the current requirement of the circuit.
If you leave the resistor permanently in the positive line, it becomes a SAFETY RESISTOR, but don't forget it will drop a small voltage and if the circuit takes peaks of current, this resistor can fail.
For the cost of one-cent you can protect a project and make sure it is working before connecting it permanently to the supply.

FUSIBLE RESISTOR
A fusible resistor is a low value resistor and should be made of non-combustible materials.
The value is chosen so it does not get hot during normal operation but if twice the current flows, it "burns out."
Sometimes it is housed in a non-combustible sleeve.
If you think it has burnt out for no reason, replace it with a one-ohm resistor (0.25 watt) and feel its temperature. If it is in a DC line, the voltage across it will be 250mV max.

SUBSTITUTING A RESISTOR
You can get a resistor substitution box for $18.00 plus postage.
I bought one 40 years ago and have only used it ONCE.
Just get a a resistor on jumper leads and if it burns out, you have only lost 10 cents.
When designing a circuit, you may need to go 10% higher or lower to see the effect.
That's why it is best to try a resistor on leads.

Cost: $18.00 plus postage.  Just use an individual
resistor on jumper leads. Save $20.00

VOLTAGE RATING OF CAPACITOR
Capacitors have a voltage rating, stated as WV for working voltage, or WVDC. This specifies the maximum voltage that can be applied across the capacitor without puncturing the dielectric. Voltage ratings for "poly," mica and ceramic capacitors are typically 50v to 500 VDC. Ceramic capacitors with ratings of 1kv to 5kv are also available. Electrolytic capacitors are commonly available in 6v, 10v 16v, 25v, 50v, 100v, 150v, and 450v ratings.

THE SIZE OF A CAPACITOR  - RIPPLE FACTOR
The size of a capacitor depends on a number of factors, namely the value of the capacitor (in microfarads etc) and the voltage rating. But there is also another factor that is most important. It is the RIPPLE FACTOR. Ripple Factor is the amount of current-fluctuation the capacitor (electrolytic) can withstand without getting too hot.
When current flows in and out of an electrolytic, it gets hot and this will eventually dry-out the capacitor as some of the liquid inside the capacitor escapes through the seal. It's a very slow process but over a period of years, the capacitor looses its capacitance.
If you have two identical 1,000u  35v electrolytics and one is smaller, it will get hotter when operating in a circuit and that's why it is necessary to choose the largest electrolytic. 

CAUTION
If a capacitor has a voltage rating of 63v, do not put it in a 100v circuit as the insulation (called the dielectric) will be punctured and the capacitor will "short-circuit." It's ok to replace a 0.22uF 50WV capacitor with 0.22uF 250WVDC.
An electrolytic will withstand 110%  voltage - that is 10% higher than the marked value, but a voltage above this will leak through the oxide insulation and eventually create a pin-hole short-circuit.

SAFETY
A capacitor can store a charge for a period of time after the equipment is turned off. High voltage electrolytic caps can pose a safety hazard. These capacitors are in power supplies and some have a resistor across them, called a bleed resistor, to discharge the cap after power is switched off.
If a bleed resistor is not present the cap can retain a charge after the equipment is unplugged.

How to discharge a capacitor
Do not use a screwdriver to short between the terminals as this will damage the capacitor internally and the screwdriver.
Use a 1k 1 watt or 3watt or 5watt resistor on jumper leads (or held with pliers) and keep them connected for up to 15 seconds to fully discharge the electro. You can even go as low as 100 ohms or 10 ohms for a 25v or 35v electrolytic and watch the spark. Then test it with a voltmeter to make sure all the energy has been removed.

Before testing any capacitors, especially electrolytics, you should look to see if any are damaged, overheated or leaking. Swelling at the top of an electrolytic indicates heating (and pressure inside the case) and will result in drying out of the electrolyte. Any hot or warm electrolytic indicates leakage and ceramic capacitors with portions missing indicates something has gone wrong (such as it being "blown apart").

Here is a 120u 330v electrolytic from a flash circuit in an old-fashioned film camera.
If the flash does not "fire," the electrolytic will be charged to about 350 volts !!
Use a 1k resistor (held with pliers) to slowly discharge it. It may take 15 seconds to fully discharge. 

TESTING A CAPACITOR
There are two things you can test with a multimeter:
1. A short-circuit within the capacitor
2. Capacitor values above 1u.

You can test capacitors in-circuit for short-circuits. Use the x1 ohms range.
To test a capacitor for leakage, you need to remove it or at least one lead must be removed. Use the x10k range on an analogue or digital multimeter.
For values above 1u you can determine if the capacitor is charging by using an analogue meter. The needle will initially move across the scale to indicate the cap is charging, then go to "no deflection." Any permanent deflection of the needle will indicate leakage.
You can reverse the probes to see if the needle moves in the opposite direction. This indicates it has been charged. Values below 1u will not respond to charging and the needle will not deflect.
This does not work with a digital meter as the resistance range does not output any current and the electrolytic does not charge.

Rather than spending money on a capacitance meter, it is cheaper to replace any suspect capacitor or electrolytic.
Capacitors can produce very unusual faults and no piece of test equipment is going to detect the problem.
In most cases, it is a simple matter to solder another capacitor across the suspect component and view or listen to the result.
This saves all the worry of removing the component and testing it with equipment that cannot possibly give you an accurate reading when the full voltage and current is not present.
It is complete madness to even think of testing critical components such as capacitors, with TEST EQUIPMENT. You are fooling yourself. If the Test Equipment says the component is ok, you will look somewhere else and waste a lot of time.

FINDING THE VALUE OF A CAPACITOR
If you want to find the value of a surface-mount capacitor or one where the markings have been removed, you will need a CAPACITANCE METER. Here is a simple circuit that can be added to your meter to read capacitor values from 10p to 10u.
The full article can be found HERE.

ADD-ON CAPACITANCE METER
 

CAPACITOR SUBSTITUTION BOX

Cost: $18.00 plus postage.  Just use individual capacitors
and solder them directly to the board.

SUPER CAPACITORS
Super-capacitors are coming on the market with surprisingly high capacitance but still in the low-voltage range. They are suitable for some applications but not for: JUMP-STARTER PACKS.

A super capacitor does not take the place of a rechargeable battery. It may be able to deliver a very high current for a short period of time, (for say a spot welder) but some manufacturers sell a JUMP STARTER PACK for a car, using super-capacitors.
Don't be tricked by the output voltage and huge current of these packs. There is more to learn out jump-starting a car.
Jump-starting a car uses a very clever trick in the first place.
If your car does not start on a cold morning, it will be due to the battery as starting to get old and its voltage will drop when a high current is required. This is due to the chemical on the plates increasing in resistance. When the voltage drops by 1, 2 3 or 4 volts, the current drops as well due to the simple fact of Ohm's Law coming into the equation. The combination of these two lower values causes the starter motor to revolve as a lower RPM and the car does not start.
 A jump-starter pack connects across your battery and it does deliver some of the current but more important is the fact that it keeps the VOLTAGE at a higher value. This is the secret.  It keeps the VOLTAGE high. You can also say it "raises" the voltage.
A jump-starter pack containing a rechargeable battery will keep the voltage high for more than 30 seconds.
This is because it contains a NEW battery (very similar in construction to your car battery) and the cells will maintain an output of 2.1v for at least 30 seconds.
But a super capacitor will only deliver its full rated voltage for less than one second and the voltage drops very quickly.
This means a super-capacitor jump starter pack is totally unsuitable for jump-starting a car.     The following two graphs show the voltage of a rechargeable battery remains fairly high for a long period of time. The  super-capacitor voltage drops very quickly (after a fraction of a second). The super-cap helps the car battery but only for a fraction of a second. It may take 5 to 10 seconds to start the car and the super-cap will not help.
There are other technical points to this discussion including the amount of energy in a super-cap and a jump-start battery pack. The energy in a super-cap starter pack is less than a pack of torch cells. If 8 Alkaline cells could deliver the current, eight "D" cells would start your car 10 times. (432,000watt-sec  from the cells   and the starter motor takes 42,000watt-sec). A 1,000F supercap @12v only delivers 70watt-secs but its useful delivery is only for the portion that is between 12v and down to 9v and this is just 15watt-seconds. So you can see the uselessness of a super-capacitor for this application.

The negative wire is RED. The positive wire is "Copper"

HEATSINKING
Heat generated by current flowing between the collector and emitter leads of a transistor causes its temperature to rise. This heat must be conducted away from the transistor otherwise the rise may be high enough to damage the P-N junctions inside the device. Power transistors produce a lot of heat, and are therefore usually mounted on a piece of aluminium with fins, called a HEATSINK.
This draws heat away, allowing it to handle more current. Low-power signal transistors do not normally require heat sinking. Some transistors have a metal body or fin to connect to a larger heatsink. If the transistor is connected to a heatsink with a mica sheet (mica washer), it can be damaged or cracked and create a short-circuit. (See Testing Mica Washers).  Or a small piece of metal may be puncturing the mica. Sometimes white compound called Heatsink Compound is used to conduct heat through the mica. This is very important as mica is a very poor conductor of heat and the compound is needed to provide maximum thermal conduction.

TRANSISTOR FAILURE
Transistor can fail in a number of ways. They have forward and reverse voltage ratings and once these are exceeded, the transistor will ZENER or conduct and may fail. In some cases a high voltage will "puncture" the transistor and it will fail instantly. In fact it will fail much faster via a voltage-spike than a current overload.

It may fail with a "short" between any leads, with a collector-emitter short being the most common. However failures will also create shorts between all three leads.
A shorted transistor will allow a large current to flow, and cause other components to heat up.
Transistors can also develop an open circuit between base and collector, base and emitter or collector and emitter.
The first step in identifying a faulty transistor is to check for signs of overheating. It may appear to be burnt, melted or exploded. When the equipment is switched off, you can touch the transistor to see if it feels unusually hot. The amount of heat you feel should be proportional to the size of the transistor's heat sink. If the transistor has no heat sink, yet is very hot, you can suspect a problem.
DO NOT TOUCH A TRANSISTOR IF IT IS PART OF A CIRCUIT THAT CARRIES 240VAC. Always switch off the equipment before touching any components.

TRANSISTOR REPLACEMENT
If you can't get an exact replacement, refer to a transistor substitution guide to identify a near equivalent.

The important parameters are:
- Voltage
- Current
- Wattage
- Maximum frequency of operation

The replacement part should have parameters equal to or higher than the original.

Points to remember:
- Polarity of the transistor i.e. PNP or NPN.
- At least the same voltage, current and wattage rating.
- Low frequency or high frequency type.
- Check the pinout of the replacement part
- Use a desoldering pump to remove the transistor to prevent damage to the
      printed circuit board.
- Fit the heat sink.
- Check the mica washer and use heat-sink compound
- Tighten the nut/bolt - not too tight or too loose.
- Horizontal output transistors with an integrated diode should be replaced with the
     same type.

DIGITAL TRANSISTORS
There is no such thing as a DIGITAL TRANSISTOR or an AUDIO TRANSISTOR.
All transistors are just "TRANSISTORS" and the surrounding components as well as the type of signal, make the transistor operate in DIGITAL MODE or ANALOGUE MODE.
But we have some transistors that have inbuilt resistors to make them suitable for connecting to a digital circuit without the need for a base resistor.
Here is the datasheet for an NPN transistor BCR135w and PNP datasheet for BCR185w.
These transistors are called "Digital Transistors" because the "base lead" can be connected directly to the output of a digital stage. This "lead" or "pin" is not really the base of the transistor but a 4k7 (or 10k) resistor connected to the base allows the transistor to be connected to the rest of a digital circuit. 
You cannot actually get to the base. The resistor(s) are built into the chip and the transistor is converted into a "Digital Transistor" because it will accept 5v on the "b" lead.
The 47k is not really needed but it makes sure the transistor is fully turned OFF if the signal on the "b" lead is removed (in other words - if the input signal is converted to a high-impedance signal  - see tri-state output from microcontrollers for a full explanation).  
This transistor is designed to be placed in a circuit where the input changes from low to high and high to low and does not stop mid-way. This is called a DIGITAL SIGNAL and that is one reason why the transistor is called a digital transistor. (However you could stop half-way but the transistor may heat up and get too hot).  
Any transistor placed in a digital circuit can be called a "digital transistor" but it is better to say it is operating in DIGITAL MODE.  

Lithium-ion cells need to be charged individually so the charger can be turned off when the cell reaches a certain voltage. This voltage is pre-programmed into the charger.
If you have a faulty battery made up of a number of Li-ion cells, you can detect the faulty cell in a number of ways.
The simplest is to buy a single-cell charger for Aliexpress for $1.50 and connect jumper leads to the output. Clip it onto a cell and the red LED will illuminate on the charger module. When the blue LED illuminates, the cell is charged.
These chargers generally output about 500mA so a lithium cell of 4AHr will take at least 10 to 12 hours to charge. 18650 cells have a capacity of 1AHr to more than 4AHr, depending on the amount of and length of material wrapped in a spiral inside the cell. A very light cell will have almost nothing inside.
The output voltage can be stated as anything between 3.2v and 3.7v and will have a "floating charge" as high as 4.2v when it is removed from a charger. These voltages are not very important because they will soon drop to between 3.4v and 4.2v for perfectly good cells from different manufacturers.
Even 10AHr battery banks will show just 3.9v after charging.
To test the cell you can put a 10R 5watt wire wound resistor across the cell and include a blue or white LED with 100R resistor across the 10R resistor.
Leave the 10R to discharge the cell and take a note of how long it takes. Do the same with the other cells and you will be able to work out which is the faulty cell. This may take a few days but you will be able to return a dead battery into a good replacement.

BATTERY CHARGER FOR CARS (12v)
Testing a 12v battery for a car battery is a very difficult thing to do if it an automatic charger.
Some of these chargers don't start to charge if the battery is below 5v and some take time to test the battery before starting to charge. If you have one that is failing to charge you can convert it to a simple charger and get rid of the hassles.
If you want to build a charger, here are some of the features you will need to know.
The problem with all chargers is the current they will deliver when the battery voltage is low. Chargers do not have any current-limiting features and the current will be limited to the VA rating of the transformer.
But this means the current will be very high when you touch the two leads together and also when you connect a flat battery. That's why automatic chargers do not start up or they deliver brief pulses to the battery to charge it and it will produce a "floating charge" to oppose the voltage delivered by the charger and then the charger will start to charge it normally. But this all take a lot of electronics.

The normal output voltage of a 12v charger is 17.7v to 18.3v and to measure this voltage you need to connect a 1,000u to 4,700u electrolytic across the leads and measure the DC voltage.
Without the electrolytic the voltage is pulsing DC and the meter will read 11v (which is quite inaccurate).
If you have a charger and it will not start to deliver a voltage, connect 12v set of AA cells across the electrolytic to start the charger and then immediately remove them. Now you can read the DC voltage.
If your automatic charger has failed, you can convert it to a simple charger by re-connecting either the positive or negative wire from the transformer to the positive or negative of the bridge. On the automatic chargers I have converted, the negative lead was connected to the negative terminal of the bridge and the thermal trip and on-board ammeter remained in circuit so the cut-out tripped when excess current is delivered and the MOVING IRON ammeter showed the current flowing.

With all chargers, you should only deliver a maximum of the capacity of the battery and then use the battery to see if it "held its charge." 
For example a 40AHr battery should be charged for 12 hours at 4 amps.
Most batteries don't disclose the capacity but give a meaningless CCA (cold crank amp) rating that you are suppose to co-relate to the current required by the starter motor.
The best way to work out the capacity is by the weight of the battery:

1.5kgm	4AHr
5.5kgm	16AHr
6.25kgm	20Ahr
7.2kgm	27AHr
10.3kgm	40AHr
14.5kgm	40AHr
10kgm	50AHr
15.5kgm	75AHr

The smallest capacity 12v battery for a car is 40AHr and a battery can be charged at 10% of its rating for a period of time AFTER it is fully charged and it will produce only a small amount of "gassing." Some batteries have pressure relieve holes or ports or tracks to release this gas but if you are not sure and the battery says: FULLY SEALED - can be used in any position" You need to be very careful as these batteries need a REGULATED charger that stops charging when the battery reaches a terminal voltage of about 14.5v. These type of batteries do not produce any gassing AT ALL below about 15v as they have a plate chemistry that prevents gassing below this voltage and come as MAINTENANCE FREE. But this only applies when the charger is limited to 15v charging voltage.  
You have to know what to do.  If you don't following this: If you have left the lights ON and the battery is dead, you can give it a boost of 4 hours and start the can and use the car for a 30 minute trip. Or you can charge the battery for a maximum of 12 hours.
If the battery is small and you don't know its condition, charge it for one or two hours and test it.
Be very careful because you don't know the condition of the battery and you are not using a regulated battery charger and you don't know the specified charging current or the shut-off voltage. 
The danger comes when you overcharge a battery and don't know the safety relief for the bubbles of gas produced by the plates.  

The simplest charger for a 12v battery is a transformer and bridge. Use a 3 to 10 amp bridge with heatsink and any type of transformer. Connect it up and if the bridge gets warm you know current is flowing. For each amp the bridge will dissipate 1.5watts. If you cannot hold your finger on the bridge, it is getting too hot. The same with the transformer.
It does not matter how long it takes to charge a battery. The only problem is charging it too fast or over-charging. Charge for a few hours and test it. If it does not last very long, charge it for longer. This way you can get an idea how long to charge it. Old batteries will be very inefficient in holding a charge.

FIXING A 12V BATTERY
If you are charging a 12v battery and the charging current remains high but the battery does not start the car  - the problem will be a faulty cell. When the charged battery is put under load such as turning on the HIGH BEAM lights, the battery voltage will immediately drop to about 10v.
If you want to turn this battery into a DONKEY BATTERY and use it for say your solar array, you can drill through the top plastic cover of the battery until you get to the individual connectors between the cells and fit a self-tapping screw to each connector. You can now measure the individual voltage of each cell and find the problem. Gradually discharge the faulty cell and short across the two screws with wire and you have a good 10v battery. You will need to get another faulty battery with a single cell to produce a 12v battery or you can use it in a high voltage solar array.

SOLAR PANELS
There is quite a lot to learn and understand about solar installations because a Solar Panel is a very "sloppy" power supply and changes its impedance throughout the day according to the illumination it gets.
That's why you can connect them in series and parallel and they will combine their output energy, even though each panel is contributing a varying amount.
The first thing you should know is the advantages of series connection and the advantages of parallel connection.
No matter how you connect them, the charge controller will convert a high voltage at low current into a low voltage at high current to charge the battery pack.
Working with high voltage DC is very dangerous.
Solar arrays can be anything from 40v to 80v or 300v to 600v or more.
Anything over 100v DC needs special care and attention.
An 80v AC voltage will give you a "tingle."  100v -150v AC will give you quite a shock.
But you will not be able to feel a 120v DC voltage.  That's why DC is so dangerous.
Solar arrays also produce a very high current and the panels need to have the voltage  covered to 12v DC or up to 120v DC for storage into batteries or converted to 240v AC for delivery to a household or exported to the mains.
We will not be covering voltages or currents as these vary enormously.
We will be covering the connection of panels in series and in parallel and explaining the faults that may occur.

PANELS IN SERIES
Solar panels can be connected in series and the voltage of each panel is added to produce the output voltage.
Suppose you have a 5kW array and suppose the inverter has a maximum input voltage of 500v. This means the input current will be 5,000/500 = 10 amps.
Most solar panels have an output wattage of about 250 to 300 watts and an output voltage of about 30v. This means the output current will be about 8 to nearly 10 amps. With a 20 panel array the system will produce about 5kW and the inverter will need to have an input voltage capability of up to 700v.
The only problem with series connection is shading. If one panel becomes shaded due to a tree or cloud, the current from the whole system is reduced as well as the voltage.
A small amount of shading can reduce the current considerably.  But that's the effect you get when the intensity of the sun changes due to clouds in the sky.
Remember, when installing or servicing the array, the 700v DC will be present or may be present at any part of the wiring because you do not know if any part of the wiring is damaged and touching any of the metal mounting brackets of frames. And this voltage will be present at any part of the day when the sun is shining bright or dull.
The whole system is insulated and isolated and does not have a "ground" connection and no point has zero voltage.
To test a solar panel you need to disconnect it completely from all the other panels and use a LOAD.
The LOAD is a tester made from 3 12v car headlamps of 100watts each.  These are connected in series.
Connect the solar panel and observe the brightness.
If the sun has constant brightness you can compare all the panels.
This is the simplest, cheapest and best way to find the faulty panel.

PANELS IN PARALLEL
Solar panels can be connected in parallel and the current of each panel is added to produce the output current.
Panels need to be connected in parallel if you have an inverter that has an input limit of 100v to 200v.
But then you have to make sure the current does not exceed the rating of the inverter.


  

TESTING PIEZO DIAPHRAGMS and PIEZO BUZZERS
There are two types of piezo devices that produce a sound.
They are called PIEZO DIAPHRAGMS and PIEZO BUZZERS.
A piezo diaphragm consists of two metal plates with a ceramic material between. The ceramic expands and contracts when an alternating voltage is placed on the two plates and this causes the main plate to "dish" and "bow."
This creates a high-pitched sound. There are no other components inside the case and it requires an AC voltage of the appropriate frequency to produce a sound.
A piezo buzzer has a transistor and coil enclosed and when supplied with a DC voltage, the buzzer produces a sound.
Both devices can look exactly the same and the only way to tell them apart is by connecting a 9v battery. One device may have "+' and "-" on the case to indicate it is a piezo buzzer, but supplying 9v will make the buzzer produce a sound while the piezo diaphragm will only produce a "click."   

A piezo diaphragm will produce a click
when connected to 9v DC.
A piezo buzzer will produce a tone when
connected to a DC voltage.

How a PIEZO BUZZER WORKS
A Piezo Buzzer contains a transistor, coil, and piezo diaphragm and produces sound when a voltage is applied. The buzzer in the circuit above is a PIEZO BUZZER. 

The circuit starts by the base receiving a small current from the 220k resistor. This produces a small magnetic flux in the inductor and after a very short period of time the current does not increase. This causes the magnetic flux to collapse and produce a voltage in the opposite direction that is higher than the applied voltage.
3 wires are soldered to pieces of metal on the top and bottom sides of a ceramic substrate that expands sideways when it sees a voltage. The voltage on the top surface is passed to the small electrode and this positive voltage is passed to the base to turn the transistor ON again.  This time it is turned ON more and eventually the transistor is fully turned ON and the current through the inductor is not an INCREASING CURRENT by a STATIONARY CURRENT and once again the magnetic flux collapses and produces a very high voltage in the opposite direction. This voltage is passed to the piezo diaphragm and causes the electrode to "Dish" and produce the characteristic sound. At the same time a small amount is "picked-off" and sent to the transistor to create the next cycle.

TESTING A SPEAKER
A speaker (also called a loud speaker) has  coil of wire wrapped around a magnet but it does not touch the magnet as it is wound on a thin cardboard former so the coil will be pulled closer to the flux produced by the magnet when a current flows in the coil.
When the current flows in the other direction, the coil moves away from the magnetic flux.
This coil is called a  voice coil and it is connected to a sheet of thin card called a CONE and as the cone vibrates, the speaker reproduces music or noise.
Use a multimeter on a low ohms scale to read the value of resistance of the coil.
It can be as low as 2 ohms or as high as 100 ohms.

REPLACING A SPEAKER
Replacing a speaker is easy. Just get one the same size and impedance and the job is done.
But there is a lot more to understand this simple replacement.
The principle of operation of a speaker is called ELECTROMAGNETISM and the strength of the "Pull" depends on the strength of the PERMANENT MAGNET and the magnetism produced by the coil.
The strength of the coil can be produced in two different ways.
The coil can consist of a few turns and a high current flows. Or it can consist of many turns with a very low current.
When you multiply the number of turns and the current you get a result called AMP-TURNS.
It is the AMP-TURNS that produces the FLUX (called MAGNETIC FLUX) and this flux interacts with the MAGNETIC LINES OF FORCE produced by the PERMANENT MAGNET to produce REPULSION or ATTRACTION.
Here's the amazing part: You can replace an 8-ohm speaker with 16 ohm, 32 ohm or 50 ohm and get the same (or even higher) output. That's because the number of turns on an 8-ohm coil multiplied by the current flowing through the coil may be equal to the number of turns on a 50 ohm coil multiplied by the current flowing through the coil. If the answers are identical, both speakers will produce the same output.
So, don't bypass the possibility of replacing a speaker with one having a higher impedance voice coil (VC). If the new speaker has a super-magnet, the output will be very impressive. A higher impedance will also put less stress on the output of the circuit and it will sometimes allow a higher voltage to be delivered to the speaker and thus allow proportionately higher current to flow.   
A speaker with a weak magnet will produce a low output. Throw it out.

CONTINUITY TESTER
Now is an ideal time to introduce a simple piece of test equipment that will test all sorts of devices and circuits.
It is a CONTINUITY TESTER

This piece of test equipment is available from Talking Electronics for $2.50 plus postage.
It is very handy handy and very clever because it has 2 levels of continuity.
The "short-circuit" probe detects low resistance and beeper-buzzer produces a noise when a low resistance is present (up to about 50 ohms).
The High Sensitivity probe is amplified by the transistor and will detect up to about 30k.
It's ideal for measuring and comparing a fault project with a project that works as you can hear the different tones from the buzzer and detect quite small differences in resistance.
You don't realise the importance of a simple piece of test gear like this, until you get one. We use it all the time. 

Now, back to the speaker discussion:
Most speakers have an 8R voice coil and the actual resistance may be slightly lower than this.
Some speakers have a resistance of 16R, 32R or 50R and even 75 ohms.
You would think putting a 16R speaker in place of 8R would reduce the sound output, but this is not always the case.
You can even use 50R or 75R and get the same performance.
This may sound amazing, but here is the reason.
The cone is deflected a certain amount due to the current flowing and the number of turns.
These two values are multiplied together to produce a value called AMP-TURNS.
If we have an 8R speaker with 80 turns and 100mA, the result is 0.1 x 80 = 8.
If we use a 16R speaker, the average current flow will be 50mA and the number of turns will be about 160.   The multiplication of 0.05 x 160 = 8.
The author then tried a 50R speaker and the sound output was equal to 8R and the same with 75R speaker.
This might not apply in all situations, but the 75R speaker was slightly larger and the ticking sound form the Metal Detector kit was louder than using an 8R mini speaker.

To see if the cone of a speaker is undamaged, push it slightly and it will move towards the magnet. If it does not move, it is bent or damaged. If the cone is scratchy when pushed, it is rubbing against the magnet.
A cone should be able to be pushed and pulled from its rest-state. If not, it will produce a distorted sound.

TESTING A CIRCUIT
Whenever you test a circuit, the TEST EQUIPMENT puts "a load" or "a change" on it.
It does not matter if the test equipment is a multimeter, Logic Probe, CRO, Tone Injector or simply a LED and resistor.
There are two things you need to know.
1. The IMPEDANCE of the circuit at the location you are testing, and
2. The amount of load you are adding to the circuit via the test equipment.

There is also one other hidden factor. The test equipment may be injecting "hum" due to its leads or the effect of your body at absorbing hum from the surroundings or the test equipment may be connected to the mains.
These will affect the reading on the test equipment and also any output of the circuit.
Sometimes the test equipment will prevent the circuit from working and sometimes it will just change the operating conditions slightly. You have to be aware of this.
The last section of this eBook covers High and Low Impedance and understanding impedance is something you need to know.
The point to note here is the fact that the equipment (and the reading) can be upset by hum and resistance/capacitance effects of test equipment. This is particularly critical in high impedance and high frequency circuits.


TESTING INTEGRATED CIRCUITS   (IC's)
Integrated Circuits can be tested with a LOGIC PROBE. A Logic Probe will tell you if a line is HIGH, LOW or PULSING.
Most logic circuits operate on 5v and a Logic Probe is connected to the 5v supply so the readings are accurate for the voltages being tested.
A Logic Probe can also be connected to a 12v CMOS circuit.
You can make your own Logic Probe and learn how to use it from the following link:

http://www.talkingelectronics.com/projects/LogicProbeMkIIB/LogicProbeMk-IIB.html

LOGIC PROBE with PULSE
This is a very simple transistor circuit to provide HIGH-LOW-PULSE indication for digital circuits. It can be built for less than $5.00 on a piece of matrix board or on a small strip of copper clad board if you are using surface mount components. The probe will detect a HIGH at 3v and thus the project can be used for 3v, 5v and CMOS circuits.

LOGIC PROBE using CD4001 and CD4011
Here is a simple Logic Probe using a single chip. The circuits have been designed for the CD4001 CMOS quad NOR gate and CD4011 CMOS NAND gate.  The output has an active buzzer that produces a beep when the pulse LED illuminates (the buzzer is not a piezo-diaphragm but an active buzzer containing components).

SUPER PROBE MkII has 20 different features including a Logic Probe, capacitance tester, Inductance tester, and more.

SUPER PROBE MkII Circuit

SUPER PROBE MkII

To test an IC, you need a circuit diagram with waveforms. These diagrams will show the signals and are very handy if a CRO (cathode ray Oscilloscope ) is used to diagnose the problem. The CRO will reproduce the waveform and prove the circuit is functioning correctly.
A Logic Probe will just show activity and if an output is not producing a "pulse" or "activity," you should check the power to the IC and test the input line.
It is beyond the scope of this eBook to explain how to diagnose waveforms, however it is important to know if signals are entering and exiting an IC and a Logic Probe is designed for this.  

SIGNAL INJECTOR
This circuit is rich in harmonics and is ideal for testing amplifier circuits. To find a fault in an amplifier, connect the earth clip to the 0v rail and move through each stage, starting at the speaker. An increase in volume should be heard at each preceding stage. This Injector will also go through the IF stages of radios and FM sound sections in TV's.

TESTING AUDIO AMPLIFIERS and AUDIO IC's
The Super Probe MII described above has a "noise" function and a tone function that allows you to inject a signal into an audio stage, amplifier (made from discrete components) or an audio chip, and detect the output on a speaker.
Audio stages are very difficult to work-with if you don't have a TONE GENERATOR or SIGNAL INJECTOR.
The signals are very small and not detected by a multimeter.
You can start anywhere in an amplifier and when a tone is heard, you can keep probing until the signal is not present or louder. From this you can work out which way the signal is travelling.
A Signal Injector is very handy for finding shorts and broken wires in switches, plugs, sockets and especially leads to headphones.
You can determine the gain of a stage (amplification) by probing before and after a chip or transistor and listen for the relative increase in volume from the speaker.
You can also use your finger to produce "hum" or "buzz" if a Signal Injector is not available.
Nearly all audio problems are plugs, sockets and cracks in the PC board, but finding them takes a lot of time and skill. 

TESTING IC's - also called "CHIPS"
An Integrated Circuit is also called a "chip." It might have 8 pins or as many as 40.
Some chips are ANALOGUE. This means the input signal is rising and falling slowly and the output produces a larger version of the input.
Other chips are classified as DIGITAL and the input starts at 0v and rises to rail voltage very quickly. The output does exactly the same - it rises and falls very quickly.
You might think the chip performs no function, because the input and output voltage has the same value, but you will find the chip may have more than one output and the others only go high after a number of clock-pulses on the input, or the chip may be outputting when a combination of inputs is recognised or the output may go HIGH after a number of clock pulses.

ANALOGUE CHIPS (also see above)
Analogue chips are AUDIO chips or AMPLIFIER chips.
To test these chips you will need three pieces of test equipment:
1. A multimeter - this can be digital or analogue.
2. A Signal Injector
3. A Mini Bench Amplifier.
 

The Mini Bench Amplifier is available as a kit.

MINI BENCH AMPLIFIER

MINI BENCH AMPLIFIER CIRCUIT

Start by locating the power pin with a multimeter.
If the chip is receiving a voltage, you can use the Mini Bench Amplifier to detect an output.
Connect the Ground Lead of the Mini Bench Amplifier to 0v and touch the Probe tip on each of the pins.
You will hear faint audio on the Input pin and very loud audio on the Output pin.
If no input is detected, you can use a Signal Injector to produce a tone.
Connect the clip of the Signal Injector to 0v  and the probe to the input pin of the amplifier chip. At the same time, connect the Mini Bench Amplifier to the output pin and you will hear a very loud tone. 
These pieces of test equipment can also be used to diagnose an amplifier circuit constructed with individual components.
Amplifier circuits using discrete components are very hard to trouble-shoot and these pieces of test equipment make it very easy.

DIGITAL CHIPS 
It is always best to have data on the chip you are testing, but if this is not available, you will need three pieces of equipment:
1. A multimeter - this can be digital or analogue.
2. A Logic Probe,
3. A logic Pulser.
Firstly test the chip to see if power is being delivered. This might be anything from 3v3 to 15v.
Place the negative lead of the multimeter on the earth rail of the project - this might be the chassis, or the track around the edge of the board or some point that is obviously 0v.
Try all the pins of the chip and if you get a reading, the chip will have "supply."
Identify pin 1 of the chip by looking for the "cut-out" at the end of the chip and you may find a small dimple below the cut-out (or notch). This is pin 1 and the "power pin" can be directly above or any of the other pins.
Next you need to now if a signal is entering the chip.
For this you will need a LOGIC PROBE.
A Logic Probe is connected to the same voltage as the chip, so it will detect a HIGH and illuminate a red LED.
Connect the Logic Probe and touch the tip of the probe on each pin.
You will not know if a signal is an input or output, however if you get two or more active pins, you can assume one is input and the other is output. If none of the pins are active, you can assume the signal is not reaching this IC.
If only one pin is active, you can assume the chip is called a CLOCK (or Clock Generator). This type of chip produces pulses. If more than two pins are active, you can assume the chip is performing its function and unless you can monitor all the pins at the same time, you don't know what is happening.
This is about all you can do without any data on the chip.

If you have data on the chip, you can identify the input(s) and output(s).
A Logic Probe on each of these pins will identify activity.
A Logic Probe has 3 LEDs. Red LED indicates a HIGH, Green indicates a LOW and Orange indicates a PULSE (activity).
Some Logic Probes include a piezo and you can hear what is happening, so you don't take your eyes off the probe-tip.
It is important not to let the probe tip slip between the pins and create a short-circuit.

LOGIC PULSER
If you have a board or a single chip and want to create activity (clock pulses), you can use a Logic Pulser. This piece of test equipment will produce a stream of pulses that can be injected into the clock-line (clock input) of a chip.
You can then use a Logic Probe at the same time on the outputs to observe the operation of the chip.

You can also use the Mini Bench Amplifier to detect "noise" or activity on the inputs and outputs of digital chips.
This only applies if the frequency is in the audio range such as scanning a keyboard or switches or a display. 

This is how to approach servicing/testing in a general way. There are thousands of digital chips and if you want to test a specific chip for its exact performance, you will need to set-up a "test-bed."

REMOTE CONTROLS
There are two types of remote control - Infrared and RF. Infrared is used for short-range, line-of-sight for TV's DVD's etc.
A few faults can be fixed, but anything complex needs a new remote control.
Check the batteries and battery-contacts. See if the IR LED is illuminating by focusing it into a digital camera and looking on the screen for illumination.
The only other things are a sticky button, a worn-out button or a crack in the PC board. Water damage is generally too much work to repair.
RF remote controls for cars, garage doors etc need a second working unit to check the power output.
Here is a simple circuit that can be connected to an analog multimeter to detect the signal strength at a very close range:

To hear the tone from a transmitter, the Mini Bug Detector circuit can be used:

Any further investigation requires a circuit diagram so you can work out what is actually being sent from the transmitter.
Most of the time it is a faulty switch, battery or contacts. Make sure the setting is correct on the "dip switches" and use a working unit to compare all your testing.

TESTING VOLTAGES ON (in) A CIRCUIT
There are basically two different types of circuit.
1. ANALOGUE CIRCUIT
An analogue circuit can also be called an AUDIO CIRCUIT and the voltages at different points in a circuit can be measured with a multimeter but the changes (the waveforms) will be quite small or changing at a rapid rate and cannot be detected by a multimeter.
You need a CRO to "see" the signals or a Signal Injector to inject a waveform into the circuit and hear the result on the circuit's speaker.

2. DIGITAL CIRCUIT
A digital circuit can also be called a "Computer Circuit" or "Logic Circuit" and some of the voltages can be measured with a multimeter (such as supply voltages) but the "signal lines" will be be changing from HIGH to LOW to HIGH very quickly and these signals are detected with a Logic Probe. 

Here are some circuits with details of how to test the voltages.
Most circuits do not show voltages at various different points and we will explain what to expect on each "stage."

TESTING A MODULE
This is the way to test all modules. You may know what they do but you may know nothing. The point is to be careful. Don't allow any of your probing to create a short-circuit as the energy in large capacitors (electrolytics) will destroy almost anything.
You can see the module has 3 inputs. They all convert the input voltage to 25v (or deliver 25v) to the top diode and this diode turns on a Darlington transistor that gradually rises "higher and higher"  (as the 4 electrolytics charge). The transistor is turned ON via the 3 x 1k resistors and the base voltage is monitored by three zeners and a LED. When this voltage gets to 9.1v + 9.1v + 5.1v + 3.3v (for the LED), it cannot rise any further and the emitter reaches 0.6v less than this.
The first thing to do is connect a low voltage to the DC input and see of a voltage appears on the electrolytics.
If not, you measure the voltage on the ends of the 1k resistors and the 3 leads of the transistor. No voltage will mean the wiring or the top diode is not conducting or around the wrong way.
When you have voltage, increase the input voltage to more than 27v and the LED will illuminate. If not the zeners and or LED are faulty. If the LED comes on below 25v, a zener will be faulty.
The 3 voltages on the transistor should be almost the same after a short period of charging. If not, the transistor will be damaged.
Do no short-circuit the output terminals.  The high current from the electrolytics will BLOW UP the output power diode.
Touch a 8R 5watt power resistor on the top of the output screw terminals and look for the spark. Or you can use a fairly large 12v motor. If it does not work, the diode is damaged.
The transistor does not pass a high current at any time and is not heat-sinked. When the output is delivery energy to a load, the output voltage drops to a few volts and the red wire on the circuit diagram reduces the voltage on the base to a low value and prevents the circuit charging the capacitors.

This module is used to show how to perform "testing".

A "STAGE"
A stage is a set of components with an input and output. A "stage" can also be called a "Building Block."
Sometimes it has a capacitor on the input and one on the output.
This means the stage is completely isolated as far as DC is concerned.
The stage has a supply (a DC supply) and it is producing its own voltages on various points on the "stage." It can only process (amplify) "AC." (signals).
Sometimes the stage can be given a name, such as small-signal amplifier, push-pull amplifier or output. 
If the stage has a link or resistor connected to a previous stage, the previous stage will have a "DC effect" on the stage. In other words it will be biasing or controlling the voltages on the stage. The stage may be called a "timer" or "delay" or "DC amplifier."

It is important to break every circuit into sections. This makes testing easy. If you have a capacitor at the input and output, you know all the problems lie within the two capacitors.
In a digital circuit (no capacitors) you need to work on each IC (integrated Circuit) and test the input for activity and all the outputs.

Once you have determined if the circuit is Analogue or Digital, or a combination of both, you have to look at the rail voltage and work out the size or amplitude of the voltage or waveform.
This is done before making a test, so your predictions are confirmed.
You will need a multimeter (either Digital or Analogue) a Logic Probe and a Signal Injector (Tone Generator). An analogue meter has the advantage that it will detect slight fluctuations of voltage at a test-point and its readings are faster than a digital meter. A digital meter will produce an accurate voltage-reading - so you should have both available.

HIGH IMPEDANCE AND LOW IMPEDANCE
Every point in a circuit has a characteristic called "IMPEDANCE." This has never been discussed before in any text book. That's why it will be new to you.
In other words, every point will be "sensitive to outside noise."
An audio amplifier is a good example. If you put your finger on the active input, it will produce hum or buzz in the speaker. This is because it is a HIGH IMPEDANCE line or high impedance section of the circuit.
The same applies to every part in a circuit and when you place Test Equipment on a line for testing purposes, the equipment will "upset" the line. It may be very slight but it can also alter the voltage on the point CONSIDERABLY.
We have already mentioned (above) how a cheap multimeter can produce a false reading when measuring across a 1M resistor. That's why you need high impedance test Equipment so you do not "load" the point you are testing and create an inaccurate reading.
The word Impedance really means resistance, but when you have surrounding components such as diodes, capacitors, transistors, coils, Integrated Circuits, supply-voltages and resistors, the combined effect is very difficult to work out as a "resistance" and that's why we call it "Impedance."  
The term "High and Low Impedance" is a relative term and does not have any absolute values but we can mention a few points to help you decide.
In general, the base of a transistor, FET input of an IC are classified as HIGH IMPEDANCE.
The output of these devices are LOW IMPEDANCE.
Power rails are LOW IMPEDANCE.
An oscillator circuit and timing circuit are HIGH IMPEDANCE.
A LOAD is low impedance.
And it gets tricky: An input can be designed to accept a low-impedance device (called a transducer or pick-up) and when the device is connected, the circuit becomes LOW impedance, but the input circuitry is actually high impedance.
The impedance of a diode or LED is HIGH before the device sees a voltage higher than the junction voltage and then it becomes LOW Impedance.
Impedance is one of the most complex topics however it all comes down to testing a circuit without loading it.
That's why test equipment should have an input impedance higher than 1M.

The first circuit we will investigate is the Mini Bug Detector, shown above and below. Points on the circuit have been labelled A, B, C etc: 

Point A - The first transistor is "self-biased" and will have 0.6v on the base. The antenna is connected to a 20 turn coil and you might think the coil will "short" the signals to earth.
But the coil and 470p capacitor form a circuit that oscillates at a high frequency when the antenna wire picks up stray signals. The coil and capacitor actually amplify the signals (see Talking Electronics website: Spy Circuits to see how a TANK CIRCUIT works) and these signals enter the base of the first transistor.
This is classified as a HIGH Impedance section because the signals are small and delicate and any loading via test equipment will kill them. The first transistor amplifies the signals about 70 times and they appear at Point B.

The signal passes though a 22n to Point C and the transistor amplifies the signal about 70 times to point D. Point C is classified as high impedance as any voltage measurement at this point will upset the biasing of the stage as a few millivolts change in base-voltage will alter the voltage on the collector considerably. Point D is classified as low impedance as any voltage-testing will not alter the voltage appreciably.
The output of the second stage passes through a capacitor to the join of two diodes. These two diodes are not turned on because the voltage at Point E can never rise above 0.7v as this is the voltage produced by the base-emitter of the third transistor.
The purpose of the two diodes is to remove background noise. Background noise is low amplitude waveforms and even though the transistor is turned on via the 220k, low amplitude signals will not be received. The third transistor works like this: It cannot be turned ON any more because any waveform from the 22n will be "clipped" by the bottom diode and it will never rise above 0.6v.
So, the only signal to affect the transistor is a negative signal - to turn it OFF.
Firstly we have to understand the voltage on the 22n. When the second transistor is sitting at mid-rail voltage, the 22n gets charged via the 2k2 and lower diode. When the transistor gets tuned ON, the collector voltage falls and the left side of the 22n drops. The right side of the 22n also drops and when it drops 0.6v, the top diode starts to conduct and when the voltage on the 22n drops more than 0.6v the third transistor starts to turn OFF. This effect is amplified by the transistor at least 100 times and appears at Point F. All the voltages around the two diodes are classified as HIGH Impedance as any piece of test equipment will upset the voltage and change the output.
There are some losses in amplitude of the signal as it passes through the 22n coupling capacitors but the end result is a very high strength signal at point G. The  4th transistor drives a 10mH choke and the mini piezo is effectively a 20n capacitor that detects the "ringing" of the inductor to produce a very loud output. 
The 22n capacitor on the collector eliminates some of the background noise.  The choke and piezo form an oscillatory circuit that can produce voltages above 15v, even though the supply is 3v.
The 47n capacitor at Point J is to keep the supply rails "tight" (to create a LOW Impedance) to allow weak cells to operate the circuit.
The "Power-ON" LED tells you to turn the device off when not being used and Point L is the power supply - a low impedance line due to the 47u electrolytic. 

Testing the Mini Bug Detector
To test the Mini Bug Detector, you will need a Signal Injector.
Place the Injector on Point G and you will hear a tone. Then go to E, C and A. The tone will increase in volume. If it does not increase, you have pin-pointed the faulty stage.


The next circuit is a combination of digital and analogue signals. It is a Logic Probe:

The voltage on a circuit (to be tested) is detected by the probe at Point A of the circuit above and the "tip" is classified as "reasonably high impedance" as it has a 220k resistor between the tip and 0v rail. The 1M reduces the impedance by about 20% but the inputs of the two inverters have no effect on the "tip" impedance as they are extremely high input-impedance devices.
The 1M trim pot is designed to put put a voltage on point B that is slightly higher than mid-rail so the green LED is turned off.
Point A will see a voltage below mid-rail and point C will be HIGH. Point C and F are low-impedance outputs.
When the tip of the probe is connected to a LOW voltage, Point B sees a LOW and Point F goes LOW to illuminate the green LED. At the same time it removes the "jamming voltage" produced by the diode between pin 4 of the 4049 and pin 3 of the 74C14 and the oscillator between points H and J produces a low-tone via the 100k resistor and 22n to indicate a LOW.
When the probe tip sees a HIGH, a lot more things happen.
Point C goes LOW and turns on the red LED.  At the same time the 100p is in an uncharged state and the right lead goes LOW. This takes the left lead LOW as the left lead connects to a HIGH Impedance line and pin 9 goes LOW. This makes point E HIGH
and since the 1u is in an uncharged state, pin 11 goes HIGH. This makes point G LOW and the diode between pins 9 and 12 keeps pin 9 LOW and takes over from the pulse from the 100p. The yellow LED is illuminated. The 1u starts to charge via the 470k and when it is approx half-charged, pin 11 sees a HIGH and point G goes low. This creates the length of pulse for the yellow LED.
At the same time, Point L goes LOW because the "jamming diode" from pin 2 of the 4049 goes low and allows the inverter between point L and N to produce a tone for the piezo.
In addition, Point I goes HIGH and quickly charges a 1u electrolytic. This removes the effect of the jamming diode on pin 5 of the 74C14 and a low frequency oscillator made up of 68k and 1u between pins 5&6 turns on and off an oscillator between points O and R to get a beep. The mini piezo is driven n bridge mode via the two gates between points QT and PS.
Point U is a 1u electrolytic to reduce the impedance of the power rail and Point V is a protection diode to prevent damage if the probe is connected to the supply around the wrong way.  

Testing the Logic Probe
You can test the Logic Probe with the simple Logic Probe with Pulse project described above. It will let you know if each point in the circuit is HIGH or LOW. You will also find out the difficulty in testing the points that are HIGH Impedance, as the Probe will upset the voltage levels and the reading may be inaccurate.


More circuits will be added here in the future.

THE VOLTAGE DIVIDER - this topic could fill a book.
You need to read lots of other sections in this eBook, including the section on measuring across a resistor with a multimeter, and high impedance circuits, to fully understand the complexities of a VOLTAGE DIVIDER CIRCUIT. 
It is one of the most important BUILDING BLOCKS to understand. Even though it  may consists of two components, you have to understand what is happening between these two components. You have to realise there is a voltage at their join that will be rising and falling due to one of the components changing RESISTANCE.
Sometimes you can work out the voltage at the join by using Ohm's LAW but quite often it will be impossible as it is changing (rising and falling) during the operation of the circuit.
At the beginning of this discussion we will only dealing with DC circuits and the voltage across a particular component will be due to its RESISTANCE. We are not going into any formulas, as it is very easy to measure the voltages with a multimeter set to VOLTS and you will have an accurate result.
The simplest two components in series are resistors. They always have the same resistance during the operation of a circuit and the voltage across each will not change.
In a further discussion we will cover "resistors" that change value according to the temperature. These are called THERMISTORS. And we have "resistors" that change value according to the light they receive.  These are called LIGHT DEPENDENT RESISTORS (LDR's) or PHOTO RESISTORS.
A transistor that is partly or fully turned ON can be considered to be similar to a resistor.
In these 3 cases we need to measure the voltage at the join with a voltmeter as it will be a lot of work to measure the resistance and work out a value.
You can also keep a voltmeter on the joint and watch the voltage change.
Finally we have some components that produce a fixed voltage across them (or nearly fixed) and the remaining voltage is dropped across a resistor. These components MUST have a resistor connected in series to limit the current and allow the component to pass the specified in the datasheet.
These devices include LEDs, diodes and zener diodes. A LED will have a fairly fixed voltage across it from 1.7v to 3.6v depending on the colour. A diode will have a voltage of 0.7v across it when it is connected to a voltage via a resistor. And a zener diode will have a fixed voltage across it when it is connected with the cathode to the positive rail via a resistor. The voltage across it will be as marked on the zener.
The concept of a VOLTAGE DIVIDER is very simple, but it takes a lot of understanding because both VOLTAGE and CURRENT are involved in the UNDERSTANDING-PROCESS.
Each component has a resistance and this can be measured with a multimeter. When two components are connected in series, a current will flow and a voltage will develop across each item.
More voltage will develop across the item with the higher resistance and the addition of each voltage will always equal the supply voltage.
That's the simple answer.
There is a little more involved . . . It is the word CURRENT.  Here is an explanation:
Suppose we have a 1k and 2k resistor on a 12v supply. The voltage at the join will be 4v.
In other words, there will be 4v across the 1k and 8v across the 2k.
If we have a 10k and 20k resistors in series, the voltage will also be 4v at the join.
If we have a 100k and 200k resistors, the voltage will also be 4v at the join.
The voltage will be the same in all cases, but the current will be different. The current in the second case will be one-tenth and only one hundredth in the third case.
If you want to go further, place a one ohm and two ohm in series and get 4v. But the resistors will get very hot and burn out very quickly.
Why do we have to chose between using 1k, 10k and 100k set-ups? Because different amounts of current will flow in each set-up. This current is called BLEED CURRENT and is basically "WASTED CURRENT." But it may be only way to design a circuit.
How do you choose? Basically you measure the current flowing through the voltage divider and one-fifth of this current will be the current you can "tap-off" to the circuit you are supplying the 4v to. You can turn the circuit around the other way and deliver 8v.  In the above example with 1k and 2k, the "bleed current" will be 4mA and you can only supply 1mA to your project.
WHY?
Because the current you take from the voltage divider will make the 4v reduce to a lower voltage (3.5v) and if you take more than 1mA, the voltage will fall to less than 3v.
A voltage divider is like supplying a circuit with a very old and weak battery.

THE VOLTAGE DOUBLER
Many circuits use a capacitor to increase the voltage.
Firstly you have to look at a circuit and then realise "it is designed to increase the voltage" and then look at the features of this type of circuit.
If the incoming voltage is 12vAC, the peak will be 17v and the output can be as high as 32v.  And it will be DC.
The electrolytic used in this type of circuit may get hot or warm, depending on the current and it will gradually dry out. This will reduce the output voltage -the voltage will sag when a load is applied.
This is the first component to replace.
The simplest voltage doubler is called a CAPACITOR INPUT VOLTAGE DOUBLER:

If the output voltage is very high, you can use 2 electrolytics:

The same circuit can be used to produce a positive and negative output:

If the input voltage to the doubler section is DC, as shown in the following 555 VOLTAGE DOUBLER circuit, you will have to "jack-up" the voltage on the electrolytic by taking the negative lead of the electrolytic to the negative rail to charge it and then take the negative lead to the positive rail to deliver the voltage to the output section: