Capacitors & Capacitance

Hi Richard, John has given you a technical and eloquent (as usual) explanation. Your response indicated that you may not have comprehended the explanation. You say "I know capacitors don't allow current to flow through them ". Well if you look at DC you could be considered somewhat correct. If you understand the capacitors mechanisms you may see that AC will 'Pass'. You also say, "they can be charged and discharged and that the time taken to charge and discharge can be calculated. ". After saying those things you still can not think of any uses for capacitors? Think some more, Regards, Tom

People who run motor windings with capacitors in series would be amazed to hear that. While it is true that no particular electron that goes in one side ever makes it out the other side, if you push an electron in one side, a different one comes out the other side. And that is still current.

Charge flows into one sire and out of the other, even though no charge makes it through the insulation between the plates. An electron arriving onto the surface of one plate creates an electric field that repels an electron on the other side of the insulation to leave the other plate and leave the capacitor. Since there are electrons moving through both leads, there is effectively current passing through the capacitor. All that is required to make this current is to force a change in the voltage applied to the capacitor. Once the voltage stops changing, the current stops.

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John Popelish

If it does then it is.

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John Popelish

A bit overstated, I think. I would say that a capacitor is a device that must pass current in order for the voltage across it to change. There are other devices that oppose voltage change by other means than capacitance.

Capacitors are often connected across DC supply rails for the effect you describe. If only small and brief currents are involved, then small capacitors may do (e.g. .1 uf across the power pins of a logic chip). If larger and longer lasting currents are involved, then quite large capacitors ar used (following rectifiers in power supplies, for instance). In both cases, the voltage is certainly more steady with the cap in place than it is without it. This is using caps as something like small rechargeable batteries, except that chemical batteries can supply current with almost no change in their output voltage, while capacitors must always have some voltage change if they are going to pass current.

Again, a bit overstated, but not wrong. Change "the property" to "a property". There are other things that oppose current change that do not involve inductance.

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John Popelish

Hi, Just been reading about capacitors and understand the property of capacitance. The problem is I don't see how this property is of use, many circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they can be charged and discharged and that the time taken to charge and dischage can be calculated. But how are these devices off any real use?

Thanks for ya time guys.

many

can

can

current through a capacitor never happens.

AC will charge and discharge a capacitor, reversr polarity of charge will occur based on the frequency of the AC but no current will pass through a capacitor.

Agin if current can't flow through a capacitor how can voltage or a charge?

Thanks for your time.

Well, that is true sort of true. I expect that is why John used the quotes. If a capacitor is charging or discharging then it appears that current is flowing though it.

You know that capacitors can be charged and discharged, well the charge, Q, stored in a capacitor is:

Q = C*V [1]

Where V is voltage and C is capacitance. Now, if you change the voltage you must also change the charge on the capacitor. This charge has to come from somewhere in the circuit, so charge must be moving. Moving charge is current, so we have some current, but only while the voltage is changing. If the voltage is constant then no current flows.

Current is actually the rate of flow of charge. So if we increase the charge on a capacitor by dQ (where dQ means a small amount of charge) in a time dt (where dt means a short time) then the current which appears to flow through the capacitor during the time, dt, is dQ/dt

so I = dQ/dt [2]

For example if the charge I = C*(dV/dt)

If you use the water flow analogy and think of wires as pipes full of water and voltage as pressure, then a capacitor would be a stretchy rubber sheet blocking a pipe.

Now, if you increase the pressure (voltage) the rubber sheet will stretch and some water (current) will flow in the pipe. Note that as the rubber sheet stretches the water on the other side of the sheet will be pushed down the pipe. No water actually crosses the rubber sheet but as you increase the pressure water flows into one end of the pipe and water flows out the other end. It is different water, but we don't care about that, as far as we are concerned we force water in one end, by increasing the pressure, and water comes out the other end.

If the pressure is constant no water will flow. If you reduce the pressure the rubber will contract and push water back down the pipe.

If you constantly increase and decrease the water pressure water will flow up and down the pipe. No water will actually cross the rubber sheet but water is flowing up and down the pipe.

A capacitor will do a similar thing.

Gareth

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Current *can* flow through a capacitor - but not forever, in the same direction. As it flows, charge is taken from one plate and piled up on the other. The greater the current, and the longer it flows, the more charge is moved, and the greater the potential difference between the plates. If this goes on for too long, the dialetric breaks down.

That should be "dielectric" of course.

Thanks guys

My documentation must be inaccurate as it states that AC can not pass.

many

explanation.

'Pass'.

to

I have two statements I would like to know if they are correct and that my interpretation of them is good.

1.) Capacitance is the property that opposes changes in voltage in a circuit. There fore a capacitor can be used to steady voltage and keep it constant.

2.)Inductance is the property that opposes changes in current in a circuit. There fore a coil can be used to steady current and keep it constant.

Thanks

can

can

Hi, Its been a bit late, but I hope someone will read this. Could anyone tell me, if a capacitor is compared to a rubber sheet connected to a water pump, what could be the possible analogy for an inductor. That example was pretty good to compare and imagine. Thanks

If you imagine that voltage is torque and rotational speed is current, then inductance is something like the inertia of a flywheel. Apply torque and the flywheel steadily increases its rate of rotation (apply voltage across an inductance, and current ramps up). It takes a large spike of torque the other way to bring the rotation to a halt (it takes a large applied reverse voltage to bring an inductive current to zero, quickly).

A nice thing about this analogy is that flywheels turn around an axis while current goes around the magnetic field of an inductance.

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John Popelish

It's used in filtering power to smooth it out.

You should read up on ELI the ICE man.

let me give you an example. lets assume that you have the need for a negative pulse from a circuit that only has a common and + supply of voltage. using a 100Uf capacitor to test with, connect the - side to your + input of your DM ( multimeter Voltage Scale). the - lead of your DM to the common side of lets say a 9 volt battery which would be the - terminal in this example. using a simple thing like a double throw switch, connect the + side of the capacitor to the center pole of the switch, one side of the switch to common and the other side to the + terminal on the 9 v battery. watch you DM as you switch from one side to the other. you will noticed that you get a momentary pulse of + voltage when the center pole of the switch is connected to the + terminal of the Battery. now switching the center pole of the switch to the common side (which would be the - side of your battery) yields a - voltage in your meter. if you were to short the capacitor with a jumper and repeat the action of connecting the center pole of the switch to + terminal of the battery and then the common side (- terminal of the batter), you will see only a

• reading on your meter and then only a 0 zero, you will not see a - voltage develop. just think of a battery's terminals being switched to give reverse polarity. the capacitor is in effect a cell (battery being 2 or more cells);

-- now lets apply this to a simple class A amplifier. i will only talk about the basics here. imagen using a resistor from the + terminal of your 9 volt battery to supply current to the collector of a transistor. now when the circuit is properly biasing the transistor (setting is current state), you can measure the voltage at the collector and lets assume that it is for now 50% of the supply voltage, in this case using your 9 volt batter = 4.5 volts. if you were to connect a speaker to this same point, the speak would push to one side and stay there due to the DC , there for you would only get a half cycle movement in the speaker. this is not good. if you were to now decouple the connection with a capacitor of lets say around 1000 uf or more (incase you want to experiment with this), the variation of voltage at the collector and resistor will give you a

• and - (true -) voltages. When the DC stabilizes, the cap will become fully charged and thus no current will no longer flow leaving the speaker cone at it's natural resting place. So you can simply think of capacitors in circuits like alternate the direction of flow to give you real AC voltage effects, and also caps are used to suppress the variations voltages and for making time constant effect circuits etc. ( the list goes on) if you would look at a LM555 timer and some examples, you could a good idea how they could be applied in timers, oscillators (tone generators) etc. you mite want to get your hands on one of those 101 kits. they are very intuitive for introducing people to electronics.

I hope long winded message didn't bore you. :)

An inductor is a bit like a heavy object on wheels. The current in the inductor is analogous to the speed of the object. The force applied to the object is analogous to the voltage applied to the inductor.

If you apply a force to the heavy object on wheels it will slowly accelerate. Similarly, if you apply a voltage to an inductor the current in the inductor will ramp up smoothly.

If you stop pushing the heavy object it will slow down due to friction. Similarly if you remove the voltage from an inductor current will continue to flow through it but the current will decrease due to resistance in the circuit (for a perfect frictionless object or a perfect inductor with no resistance the motion or current will continue).

If you try to stop your heavy object quickly when it is moving fast the inertia of the object resists this change. Similarly if you try to stop the current in an inductor quickly the inductance opposes this change. This is why you need to be careful when you switch inductive loads like relays and motors.

The equations for a heavy object are:

F = ma

Where F = Force, m = mass, a = acceleration (rate of change of speed)

E = 1/2*m*(v^2)

Where E = Kinetic Energy, m = mass, v = velocity

The equations for an inductor are:

V = L*dI/dt

Where V = Voltage, L = inductance, dI/dt = rate of change of current

E = 1/2*L*(I^2)

Where E = Energy stored in the inductor, L = Inductance, I = current

If you think of inductance as mass, voltage as force and current as velocity the equations are the same.

Gareth.

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An inductor is like a water pump without a motor, it is driven by the water that is pushed through it.

It needs a pressure and a current to get going, and then it keeps on running, pumping water, until the resistance slows down the current.

This is used in cars for creating a spark, or to create the starting spark in a flouroscent tube.

Voltage is used to get the current running in a coil, and then suddenly the connection is cut off, the current still pumps through the coil and where is that current going to go? It has nowhere to go so the voltage increases until a spark jumps over to ground, and that spark starts the car or the fluoroscent tube.

Inductors have a resistance against quick changes in the current, capacitors have a resistance against quick changes in the voltage.

The resistance changes with frequency, so inductors have little resistance at low frequencies and high resistance at high frequencies.

There is a diagram over these factors which I think is very useful but I can only find it in a pdf file from a swedish company.

This pdf is written in swedish but that doesn't matter because it is only the diagram we need. In the index find the word " Induktanser" and click on it, then scroll down one page, there is the diagram. Zoom in to see the details. It is on page 33 in the pdf file.

You can see how the horizontal scale is the frequency scale, the vertical scale is resistance, diagonally you see inductance and capacitance.

This diagram tells you what resistance a certain inductance or capacitance has at a certain frequency.

For example, we want to know what inductor is needed for a loudspeaker filter, it should have a resistance of 10 Ohms at 200Hz.

We go into the diagram from the horisontal 10 Ohm line, follow it to the (vertical) 200Hz line, there is our working point. From there, follow the diagonal line down left towards the border of the diagram and you see the value 10uH.

So, we need a 10uH coil for this purpose.

I wish there was a better way to find such a diagram, better than to have to download a pdf file in swedish and find the diagram. If anybody knows about such diagrams in other places on the web, tell us about it.

These diagrams are useful because you only need your eyes to focus and follow lines, there is no need to do calculations or touch anything, I have this diagram in front of me all the time at the work bench, and use it very often.

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Roger J.

What is ELI and ICE ?

many