HOW A CAPACITOR WORKS


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More on the capacitor in: Basic Electronics 1A
 

There are hundreds of different types of capacitors
THEY ALL WORK THE SAME
They are ALL similar to a RECHARGEABLE BATTERY.
A capacitor takes time to charge.
A rechargeable battery takes time to charge.
A miniature rechargeable battery stores 10,000 times more energy than the same-size capacitor, but they are both very similar as they both store "electricity - energy."
Of course there are lots of differences between a battery and a capacitor, but this discussion is designed to simplify HOW A CAPACITOR WORKS as some capacitors are "jumping up and down" in a circuit while being charged and discharged while others are just "sitting" and being charged and discharged. It all depends where they are located in a circuit.
Some capacitors pass a signal from one stage to the next (these are "jumping up and down" while others reduce the amplitude of a signal (called smoothing or filtering).
A capacitor takes a period of time to charge (and discharge) and this time is detected by a circuit to produce an OSCILLATOR. It is the basis of a circuit called a TIME DELAY and needs a resistor in series with the capacitor to produce the time delay.
A capacitor can create a waveform OUT OF NO-WHERE when combined with an inductor (coil) and this amazing capability produces another type of oscillator - called a sine-wave oscillator.
In fact a capacitor can perform more than 10 different functions and produce many different effects and that's why you need to read this article so you can determine what each capacitor is "doing" in a circuit.
You may have a circuit with 10 capacitors and each is performing a different function.   You need to know what each is doing and if it is "standing still" or "jumping up and down."
That's one of the first things you need to know when looking at a circuit to "see" how it is operating.
When you can "see" a capacitor "rising and falling" you can see how energy it is passing from one lead of the capacitor to the other by looking at the value of the surrounding components.
That's why anyone who draws a circuit without component values ON THE CIRCUIT is not a real electronics engineer. He obviously cannot see a circuit working and that's why the component values are of no importance.
The skill of being able to "see" a capacitor "jumping up and down" in a circuit has never been described before in any text book or covered in any lecture and that's why so few people really understand how a circuit works.
This concept is what makes Tesla, Armstrong and other pioneers so brilliant. They could see the circuit working and they how they "thought-of" and improved the design.
They didn't have any text books to refer-to, they had to invent the circuit.
That's why we have included animations. They show how the capacitor works.

CAPACITOR SAMPLES:


Different types of capacitor and symbol


Electrolytics





PHOTO of capacitor and battery

A capacitor is like a shock absorber in a car. Without a shock absorber, the wheel will move up and down very quickly and the car will bounce up and down. When a shock absorber is fitted, the car glides smoothly.
Thus a capacitor (called an electrolytic) will smooth a rail (a rail voltage) that is jumping "up and down."
This is because a capacitor takes time to charge and discharge.

When a capacitor is combined with a resistor or inductor, other effects can be produced.

Here are some of the effects:

1. It can charge and discharge slowly via a resistor. This is called a timing circuit or delay circuit.
The voltage is detected and amplified to produce a waveform called an oscillation or waveform or oscillator.
2. It can prevent a waveform rising or falling quickly, just like a "shock absorber" in a car.
This is called a reservoir capacitor or "spike prevention" capacitor.
3. It can pass a waveform from one part of a circuit to another. 
This is called a coupling capacitor.
4. It can produce a waveform (called a sinewave) when connected to a coil (called and inductor).
5. It can store energy and release it when required.
This is called a reservoir capacitor (electrolytic).

In other words, it can do lots of things and that's what we will study.

CAPACITOR SYMBOL

The symbol for a capacitor is two parallel lines with a space between the lines. This is exactly what a capacitor is: two plates that do not touch. There are slightly different variations on this to show a "normal" capacitor (generally a capacitor from 1p (1 picrofard) to 1 microfarad - 1u and made by interleaving aluminium foil between paper, plastic or mica). These are also called non-polarised capacitors because they can be fitted to a circuit around either way.
A polarised capacitor must be fitted so the positive lead sees a positive voltage to keep the oxide layer inside the capacitor in good condition.


NON-POLARISED CAPACITORS


POLARISED and ELECTROLYTIC CAPACITORS


VARIABLE CAPACITORS

An electrolytic is a capacitor has the aluminium foil etched to increase the surface area by up to 100 times and a liquid (electrolyte) is added to contact this surface to produce the high capacitance.

A capacitor can work on its own as a STORAGE or RESERVOIR capacitor (as explained in 3 below) or operate with a series resistor.
When a resistor is in series with a capacitor, the capacitor will take a period of time to charge and discharge.
From this you can work out what is happening to the signal at the join of the two components.
Here is how the slow-charging and slow-discharging works:

CHARGING A CAPACITOR
A battery and a capacitor are very similar. They both take time to charge.
In most circuits a capacitor is connected in series with a resistor and this makes the capacitor charge slowly.
As the capacitor charges, the voltage across it INCREASES but the increase is not linear. The voltage increases quickly at the beginning but gets slower and slower.
This looks like an amazing effect but it is really due to the voltage on the capacitor giving the incoming voltage less "pressure" to charge it.
It is just like a man eating 15 hamburgers. He is quick at the start but after the 13th hamburger, his full stomach slows down the eating process.


The capacitor charges to nearly
the voltage of the battery


CHARGING A CAPACITOR

The time scale is the X-axis and if you get a perpendicular ruler and move it across the graph at a constant speed, you will find the voltage rises very quickly at the start and slows down as the voltage on the capacitor increases to almost supply-voltage.
As the voltage on the capacitor rises to almost the voltage of the battery, the increase gets slower and slower.
There is nothing magic about this.
The voltage on the capacitor is initially zero and the rail voltage will initially deliver a high current and thus start to rapidly charge the capacitor, as the voltage on the capacitor increases, the difference between the rail voltage and the capacitor-voltage gets smaller and smaller and thus the current entering the capacitor gets less and less and thus the increase slows down. 
You don't have to worry about this.
Just remember a capacitor takes time to charge (and discharge).


1b. DISCHARGING A CAPACITOR
The discharge time for a capacitor is exactly the same as the charge-time. If it take 5 seconds for capacitor to charge from 1v to 7v, it will take 5 seconds to discharge back to 1v.
The discharge curve is shown in the following diagram:

2. THE DELAY CIRCUIT
Here is a circuit using a capacitor (electrolytic) to produce a DELAY. The LED illuminates 2 seconds after the power is applied. This is a "speeded-up diagram:


Delay circuit

3. THE RESERVOIR CAPACITOR
A capacitor can be used to reduce the oscillations on a supply rail.
The capacitor takes time to charge and discharge and it also stores energy when the voltage on the supply rail is HIGH  and delivers energy when the supply rail is LOW. 
This produces two results. The frequency of oscillation of the supply-rail will reduce and the amplitude of the oscillations will reduce.
This is called STABILIZING the supply rail.
It is also called TIGHTENING UP the supply rail.
It reduces the HUM in audio amplifiers.
It reduces the spikes and glitches in digital circuits.
It is also called the FILTER capacitor.
It is also called a DECOUPLING capacitor as it prevents a spike or glitch from one section of a circuit reaching another section.
A reservoir capacitor (electrolytic) has a HIGH VALUE so it has the greatest effect on reducing fluctuations.


A RESERVOIR electrolytic

4. A CAPACITOR SEPARATES TWO STAGES
 
 (in other words: A Capacitor blocks DC - it does not allow energy to pass from one plate (lead) to the other)
The symbol for a capacitor is two lines with a space between. This means one plate is NOT connected to the other.
If you connect 12v to one of the plates, NO VOLTAGE will appear on the other plate, as shown in the diagram. This diagram is showing that the capacitor will not deliver any energy from the lead identified with an arrow. If you connect anything to the lead, the capacitor will charge and give a "spark" or pulse of energy BUT when it is fully charged, it will not deliver any more energy from the lead. This is because the two plates are not touching each other.


Read the text above to understand this diagram

Here is a circuit with a capacitor separating two stages:

The voltage on the collector of the first transistor is about 6v and the voltage on the base of the second transistor is about 0.6v.
These voltages are produced by the transistors and surrounding resistors. You cannot connect these two points directly as the voltages are different. If you connect them directly, the circuit will NOT WORK.
The capacitor connects these points and allows the signal from the first stage to pass to the second stage, while keeping the 6v separated from the 0.6v.
In other words: The Capacitor is BLOCKING DC.

How does the capacitor pass a signal?
It works just like a magnet on each side of a door. If you raise the magnet on one side, the other magnet rises too.
If you raise the left lead, you increase the gap between the two plates and energy flows into the right lead to fill the gap. The components connected to the right-lead see this as a voltage almost identical to what is happening to the left lead.
This is obviously only a simply way to see the circuit working, but this is how you visualise what is happening.

Here it is:


When an electronics engineer looks at a circuit, he sees the capacitors (and electrolytics) "jumping up and down" or "charging and discharging."

HOW DOES HE SEE THIS ???

1. If a capacitor has the negative lead connected to the 0v rail, it will charge and discharge:


This capacitor does not jump up and down.
It just charges and discharges

2. If a capacitor is NOT connected directly to the 0v rail, it will JUMP UP AND DOWN.


This capacitor jumps up and down

In other words, the positive lead on the capacitor will receive a signal and the capacitor will start to charge. Because the negative lead is not connected to the 0v rail, it will also rise and this will make the whole capacitor "rise" in the circuit.
During this time the voltage across the capacitor will increase and when the signal on the positive lead reduces, the capacitor will "fall"  The voltage on the negative lead can actually go below the 0v rail and that will be a surprise to many readers.

Here is a capacitor (electrolytic) "jumping up and down" to increase the voltage to more than 1.7v to illuminate the LED.

The 1u electrolytic is charged via the 100R and the collector-emitter of the third BC547 when it is turned ON. When the multivibrator changes state, the second transistor is turned ON and the left-lead of the 1u is pulled towards the 0v rail.
This takes the right-lead DOWN and the 1u acts like a battery of about 1v. The right lead goes BELOW the 0v rail by about 1v and the LED see 1.5v on its anode and -1v on its cathode.
This makes a total of 2.5v across the LED. But the LED only allows a voltage of 1.7v to appear across its leads and thus it turns on to produce a bright flash.

The following diagram shows a timing circuit made up of C and two resistors. When the capacitor charges to 2/3 of the voltage on the supply rail, the 555 turns ON and starts to discharge the electrolytic. This capacitor does not "jump up and down" because it is connected to the 0v rail.


 

Here is an oscillator circuit using a capacitor. It is a metal detector that operates at about 100kHz and when a piece of metal is placed near the coil, the frequency of the circuit decreases. 
There are 4 capacitors. The 330p is moving up and down a very small amount because the voltage on the base does not alter very much.
The 470p also moves up and down (sideways) by a large amount because it is across the output of the coil and this is a sinewave with an amplitude of about 16v. The 100n does not move up and down because one lead is connected to the 0v rail.

By knowing how much a capacitor jumps-up-and-down, you can "see" a circuit working.

THE CAPACITOR AS AN INTEGRATOR
(in other words: A Capacitor accumulates pulses)
An Integrator is a "collector of pulses."

An INTEGRATION circuit basically takes a high voltage pulse and produces a lower voltage pulse with a wider duration.
It works like this: The energy of a pulse is the area under the pulse as shown in the diagram and this areas is spread out into a wider format with a lower height.
The shape of the resulting waveform depends on the shape of the pulses. Remember the fact that the incoming waveform charges the capacitor according to the curve above and discharges it when the input is low.
If the input consists of s number of pulses, the result can be a small DC voltage this DC may trigger another circuit.
In many cases an INTEGRATOR circuit is designed to produce and output when a number of pulses in rapid succession are detected, and neglect individual stray pulses.   


A INTEGRATION circuit accumulates the incoming pulses, but the pulses must be the right shape for this to occur.
In the case of circuit A, the square wave has a HIGH equal to the LOW and the capacitor charges during the HIGH and discharges during the LOW. The result if integration is not produced.

In circuit B, the HIGH has a longer duration than the LOW and the capacitor charges during the HIGH and does not have time to fully discharged during the LOW. This means the capacitor will gradually charge more and more after each pulse.

In circuit C, the "steering diode" prevents the capacitor discharging during the LOW and it gradually charges more and more after each pulse, even though the incoming waveform has an equal HIGH and LOW period. 


AN INTEGRATOR CIRCUIT IS A LOW PASS FILTER
In other words, the circuit only allows a low frequency to pass. To put it another way, a high-frequency signal is ATTENUATED or REDUCED.    In the following three diagrams, the incoming frequency is increased and you can see the output amplitude decreases.


As the frequency INCREASES, the output amplitude DECREASES


THE CAPACITOR AS A DIFFERENTIATOR
A differentiator only passes HIGH FREQUENCY signals.


The word "passes" doesn't mean the output of the differentiator is the same as the input. It means the output signal from a high frequency will have a good amplitude and the signal can be recognised and can be process further.
A Differentiator only passes high frequency signals because if a low-frequency signal is delivered to the capacitor, it will charge via the resistor and the right-hand lead will remain at 0v. A low-frequency signal is not a square-wave but a sine-wave or an audio wave that rises and falls very slowly and the frequency of the signal is low for the value of capacitor and resistor used in the circuit. The circuit is not acting as a differentiator for sine-waves.
When a square-wave is delivered to the capacitor, the output will peak for a very short period of time then fall, as the capacitor charges.
When the waveform drops to 0v, the charge on the capacitor creates a NEGATIVE voltage on the output and it can go BELOW the 0v rail by an amount almost equal to the amplitude of the square-wave.


The output of a DIFFERENTIATOR
 

What is the purpose of a DIFFERENTIATOR?
Suppose we have a very noisy signal from the electrodes measuring the electrical activity of the heart. This is commonly called an electromyogram signal, as shown in the following charts:

The part of the signal that we want to work-on (and amplify) is the spikes. After differentiation, these spikes have a common height and an additional circuit can detect them to produce a "heart-beat-rate." A lot of the noise has been eliminated.

A DIFFERENTIATOR CIRCUIT IS A HIGH PASS FILTER
In other words it only passes high frequency signals.


As the frequency INCREASES, the output amplitude INCREASES

As the frequency INCREASES, the output amplitude INCREASES. With a high frequency, the voltage on the let-lead of the capacitor is rising quickly and the capacitor does not have time to charge via the resistor R. This means the voltage on the left lead is the same as the right lead and thus the output has the same amplitude as the input.
With a low frequency, the capacitor has time to charge via the resistor and some of the amplitude of the signal is developed across the capacitor and this amount (height) of the signal does not appear on the output. Thus the output has a lower amplitude.
 
A CAPACITOR PRODUCES A SINEWAVE
If you place a capacitor and resistor in series and apply a voltage at the connect instant and remove it at the right time, the waveform produced at the intersection of the two components is very similar to a sinewave. 
This knowledge will help you understand what will appear at the join of a resistor and capacitor when a pulsing, or square wave is delivered. The result is a much-smoother version of the waveform.
That's what the capacitor does. It slows-down the rise and fall of the waveform to produce a smooth output.

A CAPACITOR "JOINS" TWO STAGES
A capacitor can be used to "join" two stages. A STAGE is a transistor and surrounding components to allow the transistor to operate so it is not overloaded.
The signal enters the left-side of the stage and emerges on the right.
The stage might amplify the signal or invert it or change the shape.
This state will be connected to a previous stage and a following stage to amplify the signal more or perform other requirements.
Sometimes one stage can be connected to another. This is called DC coupling (DIRECT COUPLING) but sometimes the stages CANNOT be directly coupled because the output voltage on the previous stage will be large and the input voltage on the stage we are studying must be very small because the transistor can only handle small voltages on the input.
In other words the AMPLITUDE of the voltages (the SIZE of the voltages) must be kept separated.
This can be done with a capacitor.
There are two interesting points to note:
The voltage on the left-lead of the capacitor consists of two components.
The voltage will consists of a DC component. This time the letters DC refer to the words DIRECT CURRENT and really mean the value of voltage that a battery would supply.
That's why we use the letters DC. Because we really mean "battery voltage."  That's why you have to learn ELECTRICAL TERMINOLOGY.
The voltage on the left-lead will also consist of a rising and falling voltage. This is called the AC voltage or waveform.
The  letters AC refer to ALTERNATING CURRENT but we just say the letters "AC" because we really mean "WAVEFORM."
So, the voltage on the left-lead consists of two components (two different values) AC and DC.
The capacitor will block (NOT allow the DC components to pass from the left lead to the right lead) the DC component and allow the AC (the waveform) to pass. 
The resistor is this:
The right-lead on sees the WAVEFORM.
The transistor in the stage we are studying can now amplify this waveform.
That's how an amplifier works.
 
THE TUNED CIRCUIT (TANK CIRCUIT)
When a capacitor and coil are connected in parallel and given a short pulse of energy, they send this energy back and forth between the two components. The capacitor is initially charged and it passes this charge to the coil to produce magnetic flux. When the capacitor can no-longer keep increasing the flux from the coil, the magnetic flux start to collapse and produce a voltage in the opposite direction that charges the capacitor in the opposite direction. This continues until the flux can no-longer keep increasing the voltage on the capacitor and then the the capacitor sends out its voltage to produce magnetic flux in the opposite direction (meaning the north and south poles are reversed). When the magnetic flux can no-longer be kept at an increasing level, the flux starts to collapse and produce a voltage in the opposite direction that charges the capacitor in the original direction to repeat the cycle. 

Here is how the initial charging occurs:

Here is how the charge continues around the circuit:

The animation above is about the closest you will get to understanding how the energy transfers from the capacitor to the coil and back to the capacitor aging.
The amplitude will get smaller and smaller due to losses and the switch must be closed for a very short period of time AT THE RIGHT INSTANT for the circuit to work.
However the result is an amazing sinewave:

THE CAPACITOR IN A CIRCUIT
In the following circuit we see three different capacitors in different situations (different applications - different reasons and different outcomes).
This is a practical application for a capacitor and here is the reason why they are used and how the value is chosen.

 

Capacitor 1 is 100n. (0.1u) and is placed near the power rails of the microcontroller (or any IC) to prevent noise on the supply rail entering the chip.
The capacitor also prevent any pulse produced by the chip entering the supply rail and passing to other chips.
These pulses have a very short duration and are classified as high frequency. A 100n capacitor will have an effect on reducing this type of pulse.
This capacitor also produces an effect called: "tightening the power rails" and effectively acts like a miniature battery with very low impedance supplying current to the chip. This effect (improvement) is very noticeable in high frequency circuits and 22n to 100n will make an enormous difference in a circuit operating at 100MHz.

Capacitor 2 is placed on the base of the transistor and when the transistor is turned ON via the 4k7 resistor, it gradually charge. This causes the LED to gradually illuminate.
When the output of the micro is LOW, the capacitor gradually discharged and the LED gradually dims.

Capacitor 3 is placed across the battery to prevent the battery voltage dipping when the circuit requires pulses of current.
These pulses of current can be 2 to 10 times more than the average current taken by the circuit and when the battery gets slightly depleted, it cannot deliver this current without the voltage dropping noticeable.
This dip in voltage will be sent to all the chips in the project and may upset some of the waveforms.
This is especially noticeable in audio circuits where "pulses" or "glitches" passed to the front-end of an amplifier will be amplified and re-sent back to the front-end in a loop called "feedback."
This feedback is detected as "motor-boating" or "putt-putt-putt" from the speaker.

 

HOW A CAPACITOR CHARGES IN A CIRCUIT
Just because a capacitor charges and discharges in a circuit does not mean every capacitor is "acting" the same.  Some capacitors are "jumping up and down," others are charging more than another capacitor and others are producing a negative voltage.  

Here are three capacitors connected to the same output of a 555 where the output is changing from LOW to HIGH very quickly and remaining HIGH for a period of time before returning to LOW. These diagram are not completely accurate but give an idea of the capacitor charging and discharging and the animation highlights the different effects when other components are connected in series with the capacitor. 


ANIMATION 1:
 

The smaller capacitor charges to a higher voltage than the larger capacitor and at no time is any lead of a capacitor taken to below the 0v rail. (See the next animations for a negative-effect).
This effect takes a lot to understand and MANY electronics engineers get it wrong.
The explanation is as follows: A capacitor has the amazing ability to change a charge consisting of a high voltage with a small current into a low voltage and high current.
Firstly we will explain what we mean: We can charge a huge electrolytic, such as 100,000u to a voltage of say 200v over a period of 5 minutes with a small current such as 200mA.
We can now get a spanner and short the two terminals and get an enormous spark that welds the spanner to the contacts.
This welding process is due to the enormous current.
But we only charged the electrolytic with 200mA.
The electrolytic has changed the energy it has received from a value of 200v and 200mA into a few volts and 100 amps.
That's what ANY capacitor can do in ANY circuit and it all depends on the value of the surrounding components.
If we take the animation above, the output of the 555 delivers a voltage and a current to the two capacitors in series. Since they are in series, the current through the top capacitor must be the same as the bottom capacitor.
So, we cannot talk about different current-values.
This is the only way we can explain it: Water is flowing out pin3 of the 555 and half of it flows into the small top capacitor to fill it very quickly while the other half flows into the large electrolytic and it rises on very slightly. 
When pin3 goes LOW, it is exactly the same as connecting the top capacitor to the 0v rail.
The charge flows out of the capacitors at the same rate as they were charged. In other words, both capacitors will discharge smoothly but the discharge-time may be a lot quicker than the charge-time.
 

ANIMATION 2:
The following animation shows the electrolytic charging via the output of the 555. The output actually rises very quickly and this animation would be correct if the output rises slowly.
The point to note is this: The negative lead of the electro drops below the 0v rail when the output goes LOW and the electro is discharged via the resistor. 

ANIMATION 3:
When a diode is connected in series with the electrolytic, it gets fully charged on the first cycle and the negative lead goes below the 0v rail when the output of the 555 goes LOW. The electrolytic remains charged and does not get discharged by the diode. 

 
The electro does not discharge

SPLIT RAIL
Here is another circuit that requires a special way of thinking to "see" how the electrolytics work.

Some circuits need a voltage that is identified as 0v voltage at a particular connection and a positive voltage and a negative voltage.
The reason for this is the output of the circuit has a voltage that is half-way between the voltage on the top rails and the bottom rail and it produces a signal that rises almost as high at the top rail and almost as low as the bottom rail.
This means the amplifying circuit, such as an op-amp or AMPLIFIER, requires a voltage that is equally POSITIVE and NEGATIVE, with the earth, or chassis or neutral classified as having zero voltage. In other words it is the reference point (0v) for all the other voltages.

As far as the output is concerned, the two 470u electrolytics are connected in PARALLEL and prevent the bottom lead of the speaker moving, when a signal is delivered to the top lead.
The do not prevent the bottom lead moving like a fixed connection but limit the movement or restrict the movement just like a 2 ohm resistor.
A 100Hz, the two 470u electrolytics have a resistance (impedance) of slightly less than 2 ohms and at a higher frequency this value is a lot less. That means most of the signal will appear across the speaker.


Producing a SPLIT-RAIL SUPPLY


The electros charging and discharging

When the signal from the amplifier rises to a HIGH, the speaker sees nearly all the amplitude. Both the 470u electros try to hold the bottom lead of the speaker in a fixed position but one of them charges and the other discharges slightly. This action is reversed when the signal moves in the opposite direction.
You can see one of the electrolytics is charging and this is easy to understand. But the other electro discharges at the same rate and it produces the same effect on the lead of the speaker in trying to keep it from moving.
An electrolytic is exactly the same as a shock absorber containing oil and the oil is passed from one cylinder to another via a very small hole. You can press very slightly on the shock absorber and it will open or close completely. This is the action of the 4k7 charging the electro over a long period of time.
But if you push very hard on the shock absorber, it will only move very slowly. This is the action of the signal (via the speaker) on BOTH electros.   
It does not matter if you are charging the electro or discharging it. The same "resistance to movement"  or resistance to charging/discharging, applies.
Because the positive rail are classified as "unmovable" the positive end of the top electro DOES NOT MOVE and the negative of the bottom electro DOES NOT MOVE.
Because these ends do not move, the top electro can be taken to the negative rail and it will have the same effect.


The electros can be placed on the negative rail
to deliver the same effect as creating a Split-Rail

The end result of the 2 x 470u electrolytics is exactly the same as placing them in parallel to produce 1,000u. The diagram above shows the circuit re-arranged with the two electros on the negative rail.
The amplifier will produce an output swing of +4.5v to -4.5v and charge/discharge the two electros so they will produce exactly the same result as in the first circuit. 

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:



 

17/7/2014