A capacitor consists of two metal plates separated by a small gap. When ‘uncharged’ each metal plate contain as many electrons as protons, so there are no overall electric fields between the plates. We can ‘charge up’ the capacitor by dragging some electrons out of one plate and into the other. Once this is done there will be a difference in potential between the plates — i.e. there will be a voltage between them. For any particular capacitor we can say that dragging a charge, Q, from one plate to the other will produce a voltage between the plates of
where C is the capacitance of the capacitor. The value of this capacitance depends upon the plate areas, spacing, shapes, etc, and the properties of any material stuffed in the gap. Physics books tend to quote the equation
as giving the capacitance of a given capacitor where A is the area of the plates, d, is their spacing, and is the dielectric constant of the material in the gap.
However, electronic engineers don't take this relationship too seriously. This is because capacitors come in various shapes & sizes — some ‘layered’, some ‘swiss-roll’, as shown in figure 4.3. Equation 12 is only strictly correct for ‘parallel plate’ capacitors with a tiny plate separation. For the others it only gives a rough idea of the right value.
To measure the value of a capacitor we can proceed as follows:-
- 1) Make sure the capacitor is discharged by connecting its leads together for a moment.
- 2) Connect the capacitor to a circuit which applies a known current, I, for a set time, T.
- 3) Measure the voltage ‘across’ (i.e. between the plates of) the capacitor.
Since charge = current × time we can know that . Having measured the voltage produced by the charge we can calculate the capacitance
This approach will tell us the correct value no matter how the capacitance was made.
Capacitors are used in electronic circuits almost as often as resistors. If you look in a component catalogue you'll find a bewildering array of different capacitor types listed. This can be very confusing, but we can simplify things by dividing capacitors into two basic types — ‘normal’ and electrolytic. Normal capacitors use a simple insulating solid, typically a plastic, in the gap between the plates. Electrolytic capacitors are made by squeezing an electrolyte, usually a squidgy gunge, in between the metal plates. You may have heard of electrolytes elsewhere. They're the kinds of stuff used to make batteries or accumulators. As a result, they seem a daft choice for a capacitor since they conduct electricity. How can we expect to make an electrolytic capacitor to hold a charge if current can flow across the gap? The solution (a pun there if you look for it!) is that the manufacturer then deliberately applies a steady voltage to the capacitor to ‘form’ or polarise it.
At first the applied current flows through the capacitor. This current produces electrochemical reactions at the two plate-electrolyte boundaries. At one boundary this has no noticeable effect. At the other it produces a tiny layer of insulating material. Once this layer has been formed it blocks any further current flow. Now the electrolyte (being a conductor)behaves as if it were a part of one of the metal plates. The result is a capacitor with a very tiny gap of insulator, formed by electrochemical action.
Electrolytic gaps formed in this way can be much thinner than we can make using conventional capacitors. As a result, we can buy electrolytic capacitors with capacitance values thousands of times higher than non-electrolytic ones of similar physical size & cost. This is very useful in applications where we need very large amounts of capacitance. The bad news is that electrolytic capacitors aren't very stable! Their capacitance tends to fall if they're left on a shelf unused for a long time, and slowly rises again when they're first used. Their capacitance also changes with temperature, etc. The electrolytic gunge is also a lousy conductor compared to metal, so there's an unwanted resistance hidden inside the capacitor. Worst of all, if you connect the electrolytic capacitor to a steady voltage which is the other way around to that used to form it, you'll eventually eat away the electrolytic layer and destroy the capacitor altogether, sometimes with quite a big bang! Despite these warnings, modern electrolytic caps are pretty good provided you don't use them the wrong way around. But it's a good idea to avoid using them if you can find a normal capacitor with a high enough value.
Apart from the normal/electrolytic decision, the most important other feature of a capacitor is its breakdown voltage. This is the voltage which is high enough to produce a spark between the plates & damage the material in the gap. You can't measure this easily without blowing up the capacitor, so manufacturers check this by blowing up a few & printing the highest ‘safe’ value in the data sheet!
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.