March 28, 2011


          There are many kinds of capacitors, but they all do the same thing: store electrons. The simplest capacitor is two conductor separated by an insulating material called dielectric. Like this:

The dielectric can be paper, plastic film, mica, glass, ceramic, air or vacuum. The plates can be aluminium discs, aluminium foil or a thin film of metal applied to opposite sides of a solid dielectric. The conductor-dielectric-conductor sandwich can be rolled into a cylinder or left flat.

How to Make a Capacitor?

You can make a capacitor from two sheet of aluminium foil and one sheet of waxed paper. Fold the paper around one foil sheet and stack the sheet like this:

Be sure the foil sheets don't thouch! Press the contacts of a 9-volt battery briefly to the expossed ends of the foil sheets. Then thouch the probes of a high-impedance multimeter to the foil sheet. The meter will indicate a small voltage for a few seconds. The voltage will then fall to zero.

Charging a Capacitor

The minus side of our homade capacitor is charged with electrons almost immediately. Since resistors limit current you can slow down the charging time by placing a resistor between the capacitor and the 9-volt battery:

Discharging a Capacitor

The electrons in a charged capacitor will gradually leak through yhe dielectric until both plate have an equal charge. The capacitor is then discharged. The capacitor can be discharged very quickly by connecting its plate together. Or it can be discharged more slowly by connecting a resistor across it:

Capacitor Symbols


Capacitor can store a chaege for a considerable time after the power to them has been switched off. This charge can be dangerous! A large electrolytic charged to only 5 or 10 volts can melt the tip of a screwdriver placed across its terminals! High voltage capacitors like those used in television sets and photoflash units can store a lethal charge! Never touch the leads of such a capacitor. At the very least the jolt can throw you across a room!

How Capacitor are Used?

Parallel Circuit
Often capacitors are connected in parallel like this. The total capacitance is the sum of the individual capacitance.

Series Circuit
Sometimes capacitors are connected in series like this. The total capacitance is the product of the two capacitances divided by their sum.

Three or more capacitors in series? Here's the formula:

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