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Electricity > Capacitors

A capacitor is a device that can store electrical energy between a pair of plates. The capacitance (the amount of charge stored) is measured in farads but because the farad is a very large unit, in school science applications, it is usually expressed in microfarads, (µF) nanofarads (nF)or picofarads (pF) Microfarads are a millionth of a farad, nanofarads are a billionth and picofarads a trillionth of a farad.

The plates within a capacitor are separated from each other by an insulating medium known as the dielectric. The value of the capacitor is dependant upon the area of the places, the distance between them and the effectiveness if the dielectric. There are many types of capacitor available with each type providing properties designed for the intended application.

Capacitors store electrical energy for periods after they are disconnected from their charging circuit and so are used to provide temporary power if batteries are changed for example in home computers. This stored power can be dangerous and you should be aware of the risks when opening electrical appliances that contain capacitors. A camera flash capacitor charged from a small 1.5V AA battery can hold up to 300 volts for a long time after it is disconnected. This kind of charge is capable of delivering a painful and possibly fatal shock if you were to come into contact with the terminals.

Capacitors in CRT televisions hold much more power. Repairs to mains appliances and battery powered devices which contain capacitors should be attempted only by trained professionals.

Capacitors should not be charged beyond their ratings. When using Locktronics type capacitors, make sure they are only being used with batteries or labpacks locked-down to the desired voltage. Connecting capacitors to a higher voltage than is intended for their use can lead to damage or in some cases a bursting of the case where the dielectric has been over-heated by the high current.

The standard circuit diagram symbols for capacitors are below:

Fig 1: Capacitor symbols
Fig 1:
Capacitor symbols

Electrolytic capacitors are also known as polarised capacitors and as such, need to be connected the correct way round. One of the leads should read '+' and it is this lead you should connect to a positive connector of the circuit.

Unpolarised capacitors (non-electrolytic) can be connected either way round and usually have high voltage ratings.

Variable capacitors are sometimes used in radio tuning circuits and have very small capacitance values.