Can someone explain in very simple terms what a capacitor actually does in a circuit?
Until someone knowledgeable gets here, I’ll just say that they somehow store electricity.
Store charge. One of their main uses is to “flatten” currents, by storing charge when the current is high and releasing it when it’s low. Another one is as a sort of rechargeable “batteries”.
The capacitor has two distinct chargeable parts (each linked to one of its “poles”), separated by an insulator. When you connect a pole to a cable, the other end of this to a battery, the other end of the battery to the other pole via a second cable, the condenser gets charged. Take out the battery, the circuit is open and the charges in the two halves of the condenser stay where they are. Connect a lightbulb where the battery had previously been: the charges see the group “cable plus lightbulb” as an easy path, they move, that’s electric current, so you get light.
That’s the speedy back of the envelope explanation. You can charge the capacitor using sources of electric power other than a battery, and of course you can use its charge to power things more complex than a lightbulb.
A capacitor does two basic things:
- It can store charge, which is used to provide power filtering and power storage.
- If blocks DC, and passes AC (at a rate that is proportional to the frequency), which allows the creation of signal filters (low pass and high pass).
An ordinary wire can be thought of as a pipe full of water: Put some more water in one end, and some water will come out the other. A capacitor is more analogous to a pipe with a flexible rubber barrier in it: If you put a little water in, you’ll still get water out the other end, but the more water you put it, the more the rubber stretches, and the harder it is to put water in. You certainly can’t keep pushing water through indefinitely, and none of the water you put in is ever actually the water that comes out the other side.
I suppose you were probably looking for the nutshell version, but I notice that the Wikipedia entry on the Capacitor explains the functionality of the capacitor far better than any of the texts I had when I was studying electronics:
They’re what makes time travel possible.
At least the ‘flux’ types do.
And, if you want an even more simplified explanation of a capacitor from Wikipedia, their Simplified English page on it gives an even more basic explanation.
When I was a freshman, the people who put together the circuit theory class for computer science majors thought we couldn’t handle such real life concepts, so the diagrams had an integrator in place of a capacitor. Which is what everyone else said. Farads, the unit of capacitance, is actually a unit of distance, which makes sense when you consider that the “wider” a capacitor is the more charge it can store. Not counting the material used and all that other complicated stuff, of course.
Actually, the “narrower” it is… at least as far as plate separation goes.
Like I said, I’m a computer scientist and misedumated.
A capacitor is a device that stores energy in an electric field. The simplest sort of capacitor is just two metal plates next to each other. Apply a voltage and you can “charge” the plates. Remove the voltage and the energy stored in the electric field will be discharged into whatever circuit the capacitor is attached to.
Capacitors are primarily used in one of two ways in a circuit.
- Temporary energy storage.
The first way that capacitors are used is for temporary energy storage. This might be to back up a clock chip so that it doesn’t lose the time when the device it is in gets unplugged. You could use a battery to do the same thing, but a capacitor is cheaper and lighter (though it doesn’t store as much energy as a battery).
When you convert AC to DC, you use capacitors to temporary store energy. You take the AC sine wave, then rectify it with a diode or a full wave bridge (four diodes), after which you end up with electricity that is all flowing in the same direction, but is “bumpy”. To smooth it out, you add a capacitor, which charges up during the high parts of the “bump” and then discharges to keep the voltage level during the lower parts of the “bump”. I probably have you completely confused, since this is one of those things that is really obvious when you draw a picture of it, but is very difficult to describe in words. This site has pretty pictures which should make it a bit more clear:
http://www.sullivan-county.com/ele/basic_ac_rectification.htm
- Frequency dependent impedance
The second way that a capacitor is used is as a frequency dependent impedance, which is typically part of a filter circuit of some sort.
[warning - ugly math stuff]
The voltage and current in a capacitor have an integral/differential sort of relationship.
i = C dv/dt
i is current, dv/dt is the change of voltage with respect to time.
What this formula means is that the more the voltage changes, the more current flows. If the voltage isn’t changing (dv/dt is zero) then no current flows.
Signals like sound and such are sine waves. A sine wave is v = sin (wt), where w = 2 (pi) f, and f is the frequency.
If v - sin(wt), dv/dt is (w)cos(wt).
A cosine wave is just a sine wave shifted by 90 degrees, so what all these fancy equations mean is that if you put a voltage sine wave across a capacitor, the current that flows is going to be scaled by the capacitance and the frequency, and it’s going to be shifted by 90 degrees.
[/ugly math]
The 90 degree phase shift isn’t necessarily desirable, but the frequency dependent current flow is very much desirable. You can take advantage of this frequency dependence to make a low pass filter (a circuit that only passes lower frequencies), a high pass filter (a circuit that only passes higher frequencies) or a band pass filter (a circuit that only passes a narrow band of frequencies).
Filter circuits have about a bizillion and one applications in electronics.
I don’t know if that second part is really in “simple” terms per the OP, so if I’ve completely lost you, just try to tell me where my explanation went off into the weeds for you and I’ll try to do better.
My Dad, an electronics engineer and homebrew hi-fi enthusiast, tried to explain that second use of capacitors to me as a child, as follows:
Capacitors store charge up to a certain amount, but then don’t accept any more. Therefore, if you put a capacitor in a circuit where the current changes direction infrequently, the capacitor will usually be “full” and will not react to any more current. So, capacitors inhibit low frequencies.
But in a circuit where the current changes direction frequently, the capacitor will never get “full”, and will immediately react to changes in direction. That is, capacitors do not inhibit high frequencies. So capacitors are good at allowing high frequencies through, while suppressing low frequencies.
Combine that with the opposite properties of inductors, and you have a means of isolating high frequencies and low frequencies. Feed the low frequencies into a big bass speaker and the high frequencies into a little tweeter, and you have the modern loudspeaker.
I didn’t follow my father’s career path in the end, as you may have guessed, so I don’t know how much of that is half-remembered nonsense, but I think that was the gist of it.
The “somehow” is in an electric field, as opposed to an inductor (just a coil of wire) which also stores electricity, but stores it in a magnetic field.
It’s not that they get “full”, it’s that they have a frequency dependent impedance. Capacitors are backwards from inductors because in a capacitor, i=C dv/dt, and in an inductor v= L di/dt (C and L are capacitance and inductance).
You can make low pass and high pass circuits using capacitors or inductors, you just have to hook them up differently.
I don’t have time to give a better explanation than that, but I’ll try to get on later tonight and give a more thorough explanation (if no one else beats me to it).
I would take issue with this sentence, specifically with the scare quotes. When you charge a capacitor, you’re literally putting charges on the plates. If anything, the quotes belong on “charging” a battery, which isn’t nearly so direct. In fact, the process of putting energy into a battery is called “charging” precisely in analogy to capacitors.
Lots of great inbfo - thanks!!
The wider and longer its plates are, the more capacitance. But, the closer, also the more. So there are two lengths in the numerator and one in the denominator.
But I’d also think the dielectric constant would have some units, so this isn’t clear to me. These things are often a little hard to puzzle out.
That depends on what system of units you’re using. In SI units, the dielectric constant has units, but in Gaussian units, charge is defined differently, and the dielectric constant is dimensionless.
A capacitor is basically a pair of metal plates hooked to wires and separated by a dielectric; the dieletric can be air (simplest capacitor) or other non-conductive materials. The big fancy caps you see in power supplies are a pair of metal foils wrapped up in a cylinder (for maximal, huge surface area) with a layer of some dielectric oil or gel separating them.
If a voltage is applied across a capacitor, the electrons on the positive side (IIRC) will be pushed down the wire into the plate. They accumulate there, creating an electrostatic field, pushing the electrons AWAY from the other plate, down the wire. So for a short time, the current in the wire will happen until the one plate is as full as it can get with electrons (at that voltage).
If the voltage is reversed or lowered, the opposite happens - there are too many electrons in the plate for the voltage, so they rush out of the plate. If the voltage is opposite from original, the electrons accumulate on the opposite plate.
So if you have alternating current, the capacitor will fill up with electrons on one side, and expel them from the other - then reverse the process when the voltage changes; so a capacitor will NOT pass direct current (DC) but does appear to pass some AC. The bigger the capacitor, the more current it will pass at a lower AC frequency. The higher the frequency, the less of a resistance a capacitor appears to be in the circuit.
The electron storage feature is also useful in power supplies, where you see huge caps, from thumb-sized to pop-can sized. First, the AC current is rectified by diodes, so instead of AC we have pulses of DC from zero volts to peak. Connect a capacitor from the power line to ground and the DC will not pass, but as the pulse fills up the cap, it reduces the current flow and the effective voltage. While the feed voltage goes down to zero, the capacitor begins to leak out its eontents, smoothing the output voltage and current.
As an earlier post points out, it’s a reservoir for electrons.
There are some extra features:
a resistor and capacitor in a circuit can create v=1/t decay of the volatage over time, as the stored electrons try to escape the capacitor - as time goes on, the current fows slowe and the voltage drops down. The RC-time feature is used, for example, to esure that the reset line for a computer processor chip goes “on” a while after all other power is ready, so the chips don’t freeze.
An inductor - coil - has just the opposite effect of a capacitor. The coil absorbs energy building a magnetic field as voltage increases, then slows down the loss of voltage as the magnetic field collapses with decreasing current - effectively, a coil/inductor is more resistance (impedance) the higher the frequency. Put a coil and cap in a circuit they have a “sweet spot” where a certain frequency has the lowest resistance; this is how you tune for example, a radio. Connect a tuned circuit to ground, and it can pass all but that tuned frequency - which is how interference can be eliminated.