HOW A DIODE WORKS
(a Self-teaching Guide)

DIODE TEST
 


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More on the diode in: Basic Electronics 1A
 

There is a lot of confusion in text books and on the web about CURRENT FLOW.
WHICH WAY DOES CURRENT FLOW?

Current is a flow of electrons. These electrons are negatively charged particles and they are attracted to the POSITIVE of the supply. This means they flow from NEGATIVE to POSITIVE.
The first inventors and discoverers of electricity did not know this.
They thought electricity flowed from POSITIVE to NEGATIVE.
So, they made a CONVENTION (statement) that electricity (CURRENT) flows from POSITIVE to NEGATIVE.

They were wrong. But hundreds of text books had already been written, so we have TWO situations.

The answer is simple.
When we discuss electrical and electronic circuits, we use the old convention, called CONVENTIONAL CURRENT FLOW (from positive to negative). To get away from any idea of thinking about "electrons" we say "electricity flows from positive to negative." We say this so we can follow all the electrical and electronic circuits using arrows that point in the direction of CONVENTIONAL CURRENT FLOW. 

When discussing ELECTRON-FLOW we use NEGATIVE to POSITIVE.
We keep ELECTRON FLOW arrows within the component we are talking about (such as a radio-valve or transistor-model) and do not put electron-flow arrows on the rest of the circuit.
We have to do this to prevent CONFUSION.

Here is the answer:


The Electron Flow arrow should be within the
component and NOT on the wiring

Don't use any text-books that say current flow is electron-flow as they are omitting CONVENTIONAL CURRENT FLOW and this will confuse you.

We are discussing this point because a diode is an ELECTRONIC device. In other words it involves the flow of electrons because CURRENT will only flow in one direction through a diode.
In all of our discussions we have used CONVENTIONAL CURRENT FLOW as we are talking to beginners in electronics and not PHYSICS students.
Some text books use one convention, then the other convention and nothing can be followed.
Note: the arrow on the emitter of a transistor follows conventional current flow, even though electron flow was known at the time.


A diode is a very simple device and it has a lot of applications. We will cover some of its uses and explain exactly how it works in very simple terms.  
If you don't understand any of the points in this discussion, you can contact Colin Mitchell.

A diode is a device that passes current in only one direction.  It is a bit like a water-valve that prevents water back-flowing into the mains from your property. Or a valve in a pump that prevents the water flowing back down a well.

There are many types of diodes to handle small currents, large currents, high frequencies and high voltages. And there are diodes made from different materials, but they can all be described in a simple way. And that's what we will do.

A diode has two leads. These are called ANODE and CATHODE.

The cathode end is identified in a circuit diagram and on the body of the device.
It may be identified with a line, chamfer or dimple or a symbol. There must be something on the diode that identifies this lead and you have to look for it.


A diode does NOT have a positive or negative end. You see this mistake in so many discussions. A diode will have a positive voltage on the anode and a slightly lower (positive) voltage on the cathode. It will not have a positive on the anode and negative on the cathode.


Incorrect marking with "+" and "-"

In the following diagram only the CATHODE is identified with the letter k (for kathode). The other lead is the ANODE.


Correct marking with "k"

Here is a pictorial way to understand how a diode works:

OR


IT DOES NOT MATTER WHICH WAY YOU DRAW THE CIRCUIT,
THE RESULT IS THE SAME

9v comes out of the battery and when it passes through the diode, 07v is LOST (dropped across the diode), resulting in 8.3v available to operate a motor etc.

The most common type of diode is made from SILICON. It can also be made from GERMANIUM. You need to look in the datasheet to find the composition of the diode you are using.

As mentioned above, a diode does not start to TURN ON until a small voltage is present on its ANODE lead.
For a Germanium diode, this voltage is approx 0.3v.
For a Schottkey diode, this voltage is 0.3v
For a Silicon diode, this voltage is 0.7v. As the current increases, this voltage can rise to about 1.1v (at full current-flow for the diode).



Question
For a diode, does current flow from anode to cathode, or cathode to anode?

Answer
Current flows from anode to cathode. The arrow on the symbol shows the direction of CONVENTIONAL CURRENT flow.


WE WILL TAKE THE EXAMPLE OF A SILICON DIODE

A silicon diode is just like the wall of a dam. As soon as the water reaches the top of the wall, it overflows. The silicon diode has a height of 0.7v and as the voltage from the power supply increases, nothing happens until the voltage reaches 0.7v. At this voltage CURRENT STARTS TO FLOW and if the voltage is increased, the CURRENT INCREASES and the diode is destroyed. It's just like all the water flowing over the top of the dam.


DIODE VOLTAGE NOT CONSTANT
We have said the voltage across a silicon diode is 0.7v. This voltage increases slightly as the current increases.
For a 1 amp diode, this voltage will increase to about 1.1v when 1 amp is flowing.
For a 3 amp diode, this voltage will increase to about 1.1v when 3amp is flowing.
For a 10 amp diode, this voltage will increase to about 1.1v when 10 amp is flowing.


The voltage across a diode increases with current-flow

In the diagrams above, the current-flow is determined by the voltage of the Power Supply AND the current required by the load. The diode does NOT determine the current-flow. By increasing the voltage of the Power Supply, the current will increase.
The LOAD can be a motor, globe or high-wattage resistor.
In diagram A, the voltage drop (voltage lost) across the diode is 0.7v. This voltage increases to 1.1v in diagram D due to the increase in current.
Because the current and voltage increases in each diagram, the wattage (heat generated) by the diode increases at a very fast rate:
In diagram A  the wattage generated by the diode is: 0.3 x 0.7 = 0.21watts
In diagram B  the wattage generated by the diode is: 0.5 x 0.8 = 0.4watts
In diagram C  the wattage generated by the diode is: 0.7 x 0.9 = 0.63watts
In diagram D  the wattage generated by the diode is: 1.1 x 1 = 1.1watts

REVERSE VOLTAGE
If you connect a diode around the wrong way, no current will flow. But if you increase the voltage to 100v, 200v or 300v, the diode will suddenly break-down and a large current will flow.
The voltage at which this occurs is called the REVERSE BREAKDOWN VOLTAGE.
It could be as low as a few volts or as high as 1,000v.
This voltage is always provided in the data sheet as it is most-important.
If a diode is used in a mains BRIDGE RECTIFIER (to be discussed later), it will see a voltage as high as 325v for the 240v mains and the diode will need to be a 400v device.


A 1N4004 is a 400v diode - This is its REVERSE VOLTAGE rating

In the circuit above, the diode will not be destroyed when the voltage reaches 400v (for a 1N4004) because the current-limiting SAFETY RESISTOR has been included.


Question
For a silicon diode, what is the approximate voltage-drop across its leads when it is delivering about 10% of its rated current?

Answer
0.7v    This is is the voltage we use for current up to about 40% of maximum current.



Question
Which lead of a silicon diode is identified on a circuit diagram?

Answer
The cathode lead is identified with the letter "k"



Question
In the following diagram, is the diode conducting?



Answer
Yes



Question
In the following diagram, is the diode conducting?



Answer
No. The diode is reverse-biased. The arrow on the diode indicates the direction of current-flow.


Question
In the following diagram, is the diode conducting?



Answer
No. The diode is reverse-biased. The arrow on the diode indicates the direction of current-flow.


Question
In the following diagram, is the diode conducting?



Answer
Yes



DIODE APPLICATIONS

Here are some applications for a diode (or set of diodes):

THE DIODE AS A PROTECTION DIODE

In the following diagram, the diode does not conduct if the battery is connected around the wrong way. It is called a PROTECTION DIODE. This means the circuit will not see a reverse voltage and will not be damaged.
However the circuit will see a voltage 0.7v less than the voltage of the battery due to the 0.7v drop (lost) across the diode.


       Protection diode

In the following circuit, the diode conducts if the battery is connected around the wrong way and creates a SHORT-CIRCUIT. This will burn-out the fuse. The diode is called a PROTECTION DIODE.


       Protection diode

The advantage of this circuit is the diode does not drop 0.7v between the supply and the voltage on the project. The full voltage of the supply is connected directly to the project. This is a big advantage for projects working on 12v (battery) and require a high current, however the fuse will be damaged if the battery is connected around the wrong way.

Question
In the following diagram, describe the fault:



Answer
The diode has been drawn around the wrong way.

 

THE DIODE CONVERTS AC TO DC

A diode can be used to convert AC to DC:
The diode converts AC (called ALTERNATING CURRENT) to DC (called DIRECT CURRENT).
The original household voltage was 120v DC (direct current). DC is the same type of voltage that comes from a battery. It does not rise and fall but is steady at the specified voltage.
If you apply 120v DC to a 120v globe, it illuminates brightly.
You can also operate motors, toasters, heaters and other simple pieces of equipment from DC.     (But you cannot operate a piece of equipment containing a transformer and that's why it has such limited use.)
And you need to locate the generator very close to each household because you cannot use transformers to deliver the supply and there is a voltage drop in the street wiring due to the current flowing in the wiring.
This needed lots of small generating plants and it was a very expensive way to deliver electricity.
The solution was to convert to AC (alternating current) and locate a single generating plant at either a source of coal or water for hydro electricity.
Buy since AC is rising and falling, the heat produced by 120v AC in a toaster (for example) will not be as much as 120v DC because part of the waveform is less than 120v for a portion of the cycle.
The answer was to make the 120v AC rise higher than 120v for part of the cycle so that it produced the same heat as 120v DC.
This means the 120v AC rises to 170v at its peak.
For 240v AC mains, the voltage rises to 336v.
That's why the 120v or 240v mains is so dangerous. Your body actually detects the 336v and this is what kills you.
Now, let's understand how this 120v/240v is delivered.
The mains actually consists of a single wire. This is called the ACTIVE.
The other wire is the GROUND or EARTH or NEUTRAL.
Sometimes the Neutral is also delivered as a wire, but let's take the case of a single ACTIVE wire.
The voltage in the active wire is rising 336v HIGHER than earth then it falls to 336v LOWER than earth. It is rising and falling like this 50 or 60 times per second.
Once you understand this concept, you will able to see how a diode converts AC to DC.

Placing a diode on the active line of the mains will remove the part of the waveform that falls 336v LOWER than earth.
The result is a set of 50 or 60 pulses per second that rise 336v higher than earth. In actual fact a pulse rises 336v then falls to zero. There is no waveform during the time when the original waveform falls below earth.
The following diagram shows the result of adding a diode to the active line. The result is not DC but PULSATING DC and needs to be smoothed via an electrolytic to get DC.

The following diagram shows the result of adding an electrolytic:





Using a single diode is called HALF-WAVE RECTIFICATION.
Adding an electrolytic is called SMOOTHING. It can also be called REDUCING THE RIPPLE.


FULL WAVE RECTIFICATION
The negative portions of an AC waveform can be combined with the positive portions to produce a pulsing DC waveform.
See: http://www.electronics-tutorials.ws/diode/diode_6.html 
or:
http://www.eecs.tufts.edu/~dsculley/tutorial/diodes/diodes3.html
for an explanation on how the diodes in the bridge "guide" the incoming waveform to produce pulsing DC:
 

When an electrolytic is added to the circuit, it charges during the peaks and delivers energy when the waveform drops. The result is called DC with Ripple.




USING A DIODE AS A VOLTAGE REFERENCE

In the following diagram we see a silicon diode connected to a power supply via a safety resistor:

The power supply is adjusted from 1v to 5v and the voltage across the diode remains constant at 0.7v.

We can place two or more diodes in series to increase the output voltage:

Instead of using lots of diodes, we can use a single diode that "breaks-down" at a specified voltage. This type of diode is called a ZENER DIODE.


THE ZENER DIODE
 Zener diodes come in voltages from 3v3 to more than 47v. You can consider them to be ordinary diodes that have "failed."  For instance a 1N4001 diode has a reverse breakdown voltage of 50v. It is a 50v ZENER DIODE.


Zener Symbol and voltage

A zener diode should always be identified in a circuit with its voltage next to the symbol. You can include the wattage but this can be worked out by looking at the voltage and current flowing though the zener. Don't use part-numbers to identify the zener voltage as the reader will have to look-up a product-list to find the zener voltage. Circuit diagrams should be instantly readable and instantly understandable.

A zener diode drops 0.7v when connected around one way and drops its ZENER VOLTAGE when connected around the other way. Here is a diagram to show this:

The following diagram shows a zener diode producing a constant 5v6 output when the power supply ranges from 6v to more than 8v.

If the power supply is adjusted from 0v to 8v, no current flows in the circuit when the voltage is below 5v6. As soon as the power supply reaches 5v6, current flows through the safety resistor and the diode.
If  the power supply voltage is increased, more current flows through the safety resistor and through the zener diode. The voltage across the leads of the zener diode remains a constant 5v6. 
If the voltage of the power supply is increased further, the current through the zener diode will increase. It will get hotter and eventually burn out. 

You can produce any zener reference voltage by combining zener diodes and ordinary diodes:

Notes:
You can see the two 5v6 zener diodes in the diagram above are connected around the opposite way to the ordinary diode. 
 

Question
In the following diagram, what is the combined zener voltage?



Answer
7v

 

Question
In the following diagram, what is the combined zener voltage?



Answer
One diode is around the wrong way. The circuit will not produce a zener reference voltage. The output voltage will be 9v and the string of diodes will have not effect on determining the output voltage.

 

Question
In the following diagram, If the safety resistor is removed and the 9v supply is connected directly to the two diodes and zener, explain what will happen:



Answer
A very high current will flow through the diodes and zener because the combined zener of the three items is 7v. They will be damaged.


Question
In the following diagram, explain what will happen:



Answer
The zener value of the three components is 11.9v    This voltage is higher than the supply (9v) and NO CURRENT will flow in the circuit.

 

Question
Here's the most absurd explanation of "Voltage Flow" in a zener diode. Can you see the mistake?



Answer
Voltage does not "flow."    Voltage is a potential. It "exists" at each point in a circuit. When the voltage on the cathode is higher than the anode, the zener diode will break-down and current will flow from the cathode to anode. When this happens the zener diode is given a number to represent its voltage (break-down voltage).

 

Question
Zener diodes can be connected in series to produce any voltage. Simply add the zener voltages to provide the resulting output voltage. What is the output voltage of this combination: 



Answer
15v6

 

Question
Insert zener diode A or B to produce an output voltage of 19.4v 



Answer
Zener diode B

 

THE SHUNT REGULATOR
We will now combine all the facts we have learnt from above to produce a circuit called a SHUNT REGULATOR.
A Shunt Regulator takes a high voltage (containing ripple) and produces a lower FIXED voltage with very little ripple. 
We have learnt that:
1. A BRIDGE converts AC to DC (pulsating DC.   DC with ripple).
2. An electrolytic smoothes the pulsating DC and reduces the ripple.

A shunt regulator FURTHER REDUCES THE RIPPLE and produces A FIXED VOLTAGE.
A SHUNT REGULATOR consists of two components: A resistor (called the current limiting resistor) and a zener diode

Here's how the SHUNT REGULATOR works.

Firstly, the zener diode and resistor work just like a dam with an overflow pipe at the top. If the water level in the dam does not reach the pipe, NO water overflows.  When the water reaches the pipe, it overflows through the zener pipe. If the water level rises further, more water flows through the zener.

The height of the water is never above the small overflow pipe.
The small overflow pipe is connected to the globe and the brightness of the globe is constant because the voltage on the cathode of the zener is fixed (by the action of the zener diode).
It does not matter how much ripple is present in the incoming waveform, the value of the resistor is chosen to eliminate ALL THE RIPPLE.

The mathematics to work out the value of the resistor is very complex as it involves the resistance of the resistor, its wattage and the wattage of zener.
This is covered in the article: The Power Supply
The height of the small overflow pipe can be taken to a LOAD such as GLOBE.
The load can only be a SMALL LOAD. In other words, it can only require a small current, such as a small torch globe.
If the load takes ALL THE CURRENT (coming from the resistor), the zener will get NO CURRENT and it will DROP OUT OF REGULATION.
In other words, the voltage on the cathode of the zener will drop.
This is exactly like putting a pump on the small overflow pipe and sucking all the water from the zener:


 

We can now add the bridge and smoothing electrolytic:



Waveform
A is an AC voltage and you can see it rises to a peak above the zero-voltage line and equally below the 0v line.
Waveform
B represents the waveform as it emerges from the bridge. (The electrolytic is not connected to detect this waveform.)
Waveform
C shows the effect of the electrolytic. It stores energy from the peaks and delivers the energy when the waveform drops. The result is DC with a small amount of ripple.
Waveform
D is very smooth DC at a voltage determined by the voltage of the zener.

Question
In the diagram above, describe the shape of the output voltage D if the electrolytic is removed


Answer
The pulsing DC will produce a waveform on the cathode of the zener as shown in the following diagram:

This will not matter if the load is a globe but if the load is an amplifier, the power supply will produce HUM.

 

Here is an unusual use for a zener diode in the bridge of a power supply:



This is a very special type of power supply called a TRANSFORMERLESS POWER SUPPLY and it shows the effect of putting a diode in the bridge that has a very low breakdown voltage. Normally the 4 diodes will have a peak reverse voltage (breakdown voltage) of 200v or  400v and this effect will not be noticed. But when  two 18v zeners are included to demonstrate the effect of "breakdown voltage" the output of the supply is 18v minus 0.7v across the lower diode.

CURRENT SHARING
The current though the zener and the current though the small globe is called CURRENT SHARING. They share the current coming from the resistor. The resistor is called a FEED RESISTOR or FEEDER RESISTOR or CURRENT LIMITING RESISTOR or CURRENT DETERMINING RESISTOR. It is NOT a LOAD RESISTOR.
It is designed to deliver CURRENT to the zener and globe. The globe is the LOAD.
A shunt regulator consisting of a zener (and resistor) is designed to deliver a small current. If a large current is required, a transistor is added to the circuit, called a PASS TRANSISTOR or a number of different circuits can be used.

We will now cover CURRENT SHARING:


 
Diagram A above shows the correct CURRENT SHARING between the zener and globe. The zener should only be taking a few milliamps as this current is wasted and the zener is only required to provide a fixed voltage.
However if the load is removed, all the current taken by the load will not flow through the zener and that's why the zener must be capable of dissipating this wattage. 
Diagram B shows what will happen if the supply voltage increases. The CURRENT through the feed resistor will increase. This is similar to more water flowing though the pipe containing the feed resistor and the extra water will flow though the zener.
Here is the reason:
The voltage on the cathode of the zener is fixed (the voltage delivered to the globe is fixed). This means the current taken by the globe will remain constant. Thus any extra current can only flow though the zener.

We will now change the globe for a motor:



If the supply voltage remains constant and the motor takes more current, it robs the current from the zener. This shown in diagram B above. If the motor takes even more current, it will take ALL THE CURRENT from the zener. This shown in diagram C above.
Up to now the voltage across the motor is constant. But if the motor wants more current, the SHUNT REGULATOR will drop out of regulation and the voltage across the motor will DROP.
The motor may or may not get more energy from the supply, but we will not go into this condition because the SHUNT REGULATOR has ceased to perform.  

The SHUNT REGULATOR is called a SHUNT REGULATOR because the zener is connected directly across the voltage being delivered to the LOAD and it "shunts" (takes away) ALL the current being delivered by the resistor (if the load is removed). When a LOAD is connected, it takes current from the zener and it can do this until almost all the current is taken.
You must leave a small current through the zener to keep the circuit in REGULATION.
The SHUNT REGULATOR is a very wasteful design as current is flowing ALL THE TIME. If the load is not using the current, it is being wasted through the zener.

CURRENT SHARING can also be also be applied to placing two equal diodes in parallel. This can be ordinary diodes or zener diodes.
Take the case of "power diodes."
Suppose you have a 1-amp power supply and intend to use four 1N4004 diodes in a bridge. 
We have already mentioned that this type of diode has a voltage drop of 0.7v when the current is about 500mA but increases to 1.1v when the current is 1 amp.
By using an extra set of four diodes in parallel with the first set, the current through each diode will be shared.The current may not be shared equally, but it will be much less than 1amp (through each diode) and the overall wattage-loss will be less and this will be shared between two diodes. Overall, a very good outcome.

GATING - how a diode can be used as a GATE
The technical way to describe a diode is:
A diode only allows current to pass in one direction. 
This also means a diode only allows voltage to appear on the cathode when the voltage on the anode is above 0.7v.

We can use this feature to INHIBIT (stop) an oscillator and also produce circuits where two or more inputs determine the output of a circuit.
This is called GATING.
We will cover two features, Gating An Oscillator and creating Diode Gates

GATING AN OSCILLATOR
We are not interested in how the oscillator works. We want to STOP or INHIBIT the oscillator via an input signal.
The simplest type of circuit to "control" or "gate" is a DIGITAL OSCILLATOR. This is an oscillator made up of a BUILDING BLOCK contained in an INTEGRATED CIRCUIT.
The Building Block is also called a GATE and we have two terms called "GATE" in this discussion.
The Building Block may be an AND gate, OR gate, Inverter or Schmitt Trigger.
The other "gate" in this discussion is the action of turning ON or turning-OFF the oscillator.
The "gate" is the diode on the CONTROL LINE and it is taken HIGH or LOW to control the oscillator.  

In the following circuit, the INTEGRATED CIRCUIT (IC) contains 6 Schmitt Triggers and each Schmitt Trigger is called a BUILDING BLOCK or Schmitt Gate. It oscillates due to the resistor R and capacitor C.

The voltage on the capacitor "C" rises to 2/3 rail voltage (via resistor "R") and this is detected by the IC to make the output LOW. The capacitor now discharges to 1/3 rail voltage via resistor R and the IC detects a LOW to make the output HIGH. An animation of the circuit is available from Talking Electronics on the CD of the whole site. Subscribe to the CD for $10.00.

A gating diode can be placed on the "control line" to control (inhibit) the oscillator:

When the gating diode is taken HIGH, the oscillator is "jammed" (inhibited - frozen):


The oscillator is INHIBITED

In the diagram above, the voltage through the gating diode will keep the capacitor charged and prevent the IC changing state. The oscillator is INHIBITED.


The oscillator is NOT inhibited

In the diagram above, the gating diode does NOT inhibit the oscillator. The oscillator produces a square-wave output.


If the gating diode is reversed, a HIGH on the cathode will NOT inhibit the oscillator:


The oscillator is NOT inhibited

A gating diode connected to 0v, as shown below, will inhibit the oscillator:


The oscillator is INHIBITED

The gating diode will only allow a voltage of 0.7v to appear across its leads. This voltage is too low for the input of the IC and thus the circuit will not change state. The oscillator is INHIBITED.

The gating diode is normally connected to the output of a controlling circuit, as shown below:

The first oscillator controls the second oscillator via the gating diode.
The operation of the Schmitt Trigger oscillators is fully covered in the Interactive section of Talking Electronics website and on the CD of the whole site. Subscribe to the CD for $10.00.
We are only covering the operation of a diode in this discussion. 


THE GATING DIODE
It is important to understand the effect of a gating diode on the operation of an oscillator  (as shown in the diagrams above).

Question
Is the oscillator inhibited (stopped - halted - frozen) when the anode of the gating diode is HIGH?



Answer
Yes. The voltage via the diode will keep the capacitor charged and the oscillator will be frozen with the output low.

 

Question
Is the oscillator inhibited when the cathode of the gating diode is HIGH? 



Answer
No. The voltage on the cathode will not pass through the diode and the operation of the oscillator will not be affected.

 

Question
Is the oscillator inhibited when the cathode of the gating diode is LOW?



Answer
Yes. The 0v on the cathode will put a maximum of 0.6v on the anode and this will keep the capacitor discharged and the oscillator will be frozen with the output high.

 

Question
Is the oscillator inhibited when the anode of the gating diode is LOW?



Answer
No. 0v on the anode will not pass through the diode. The diode will allow a voltage of at least 100v to appear on the cathode and thus the operation of the oscillator will not be affected.

 

DIODE GATES
Diodes can be connected so that two or more CONTROL LINES determine the state of an output.
This is called GATING and more-specifically DIODE GATING.
The two diode gates are: AND GATE    OR GATE.

An AND GATE is HIGH when both inputs are HIGH.
An OR GATE is HIGH when either input is HIGH.

Notes:
1. For a Diode Gate, we are not concerned with the 0.6v to 0.7v drop across the diode.
2. A Diode Gate is classified as  DIGITAL GATE. In other words, the output is either HIGH or LOW.
3. When it is HIGH, the output is as close to the rail voltage of the system as possible. This voltage is normally 5v, but it can be 3v3 for a 3v3 system or 12v of other systems.
4. When it is LOW, the output is as close to 0v as possible.

THE DIODE
OR GATE
The voltage and current is passed to the output by the incoming signal and a pull-up resistor is not needed. A pull-down resistor will prevent the output "floating." 
The output will go HIGH when line A OR line B is HIGH (and also when BOTH inputs are HIGH).
A voltage-drop of 0.6v is lost across the diode but this will not affect any circuit using this gate.




 

THE DIODE AND GATE
For the diode AND gate, BOTH input A AND input B must be HIGH for the output to be HIGH. 
The diode AND gate works in a slightly different way to the diode OR gate. 
The pull-up resistor delivers the output voltage and current. 
The input lines ALLOW the output voltage to rise when BOTH inputs are HIGH. The output current of the AND gate is determined by the value of resistor R.  When the output is low, this current is termed BLEED CURRENT and flows through the diode(s) to the 0v rail. This current is "waste" current and must be kept to a minimum for good design.

Note:
NAND and NOR gates cannot be produced with diodes and resistors as these gates involve INVERSION and a diode cannot provide inversion. A transistor or IC is needed to provide inversion. 
The OR and AND gates above are called DIGITAL GATES because the output is HIGH or LOW. This condition is created because the input lines are DIGITAL LINES - they are either HIGH or LOW.
 

Question
Is the following circuit an AND gate or OR gate:



Answer
The two diodes are connected around the wrong way. The circuit is not and AND gate or OR gate. 


Question
Is the following circuit an AND gate or OR gate:



Answer
The gate is an OR gate. 


THE TRANSISTOR
A transistor can be considered to have a diode between between the base and emitter leads because these two leads behave just like a diode. We have shown an NPN transistor in the diagram below, but the same applies to a PNP transistor.


An NPN Transistor

This fact is important to know when analysing circuits such as this one:


Lie Detector

The circuit will not start to turn ON until the voltage on the base of the NPN transistor is 0.6v.
This voltage is provided by your finger on the Touch Pads and the top 100k resistor.

 
At the beginning of this article we mentioned the fact that the voltage across a diode can be increased from 0v to 0.55v and nothing flows through the diode.
This is exactly the same effect with the base of a transistor.
The transistor does not conduct until the voltage on the base reaches 0.6v and if the voltage is increased, above 0.55v, current will start to flow "through the diode" - "through the base lead to the emitter."
Although a diode and base-emitter junction of a transistor are NOT the same thing - we can use this simple explanation to understand how the two components work.

In the case of a diode, as the voltage is increased over 0.6v, more current flows through the diode. In the case of the base of a transistor, as the voltage is increased over 0.6v, the transistor turns ON more and more current flows through the collector-emitter terminals.
(The voltage 0.6v can range from 0.6v to 0.9v, depending on the type of diode and the current flowing.)
The whole point to understand is this: NO current flows until the voltage across the diode is 0.55v and as the voltage is increased,  more current will flow (in the case of a transistor).
In the case of a diode,  you can add this interpretation: As the circuit requires more current, the additional current will flow though the diode and create a higher voltage-drop across it.   

BREAKDOWN
When a diode, zener diode and/or transistor sees a HIGH VOLTAGE across ANY of the leads, it will BREAKDOWN.
No semiconductor device can withstand a very high voltage and we will look at what happens.
This is a very important topic because we use this breakdown feature in some circuits and other times we need to know how to prevent breakdown.
If a transistors sees a high voltage such as a spike (and the voltage has a reasonable amount of current) the transistor will be instantly damaged.
If the current is low, the transistor can be repeatedly operated with the feature.
Here's what will happen to a diode, zener diode and transistor:

The voltage across a signal diode or power diode or zener diode will be equal to rail voltage when the voltage is below the breakdown or Avalanche voltage of the diode. In other words, no current will flow through the safety resistor and no voltage will be dropped across this resistor.
However, when the voltage reaches the Peak Inverse Voltage for a Diode, or the avalanche voltage for a zener, the device will breakdown and cause current to flow through the device so that the voltage on the cathode will not rise any further. As you increase the voltage, the current through the device will increase but the voltage across it will not change. The device will get hotter and hotter until it fails with overheating.

The same will occur with a transistor. When the voltage on the collector reaches the maximum for the transistor, it will breakdown and the voltage across the collector-emitter will remain at this voltage. 

Fig
D shows how a diode (or zener diode) can be used to pass voltages that are higher than the breakdown voltage of the device. Nothing flows though the device until the breakdown voltage is reached and then the exact-same-waveform flows though the device.  

Note: When the three devices are connected in the reverse direction the diode and zener will drop (breakdown) at 0.6v and the transistor will breakdown at a very low voltage. 

HEATSINKING
You might not think a power diode needs to be HEATSINKED because there are no heat-fins on a diode. But the leads and pads on the PC board form a very important part of getting rid of the heat generated.
The author has seen examples of one pad being smaller than the other and the lead heated up the solder to produce a dry joint.
In another case, the small pads resulted in the diode overheating and "shorting;" and other diode "going open."
So, the quality of the heatsink is VERY IMPORTANT.
If you cannot hold your finger on a diode for 10 seconds, it is TOO HOT.
Adding extra-wide pads and tracks on the underside of the board is a very good idea.
Power diodes are very robust, however it is good engineering to add extra track-work to prevent over-heating.

Now go to:

       DIODE TEST
 

You will find a lot of helpful material on these pages:

Spot Mistakes:   P1  P2   P3   P4  . .   P11   P12   P13   P14  
                          P15   P16   P17   P18   P19   P20  
 
 

Now go to Basic Electronics 1A:



 

27/2/2014