Read the text. Capacitors are devices which have the ability to store electrical energy, and are of vital importance to electronic circuits

Capacitors

Capacitors are devices which have the ability to store electrical energy, and are of vital importance to electronic circuits. Other important features of capacitors include their ability to change the phase relationship between the current and the voltage in an AC circuit, and also the fact that their reactance alters with the frequency of an alternating supply.

PRINCIPLES OF OPERATION

A capacitor consists of two conductors, known as plates or electrodes, separated by an insulator, known as the dielectric. The electrical energy stored by the capacitor is retained in the dielectric material. Dielectric materials employed include air, impregnated paper, plastic, mica and ceramics.

The mechanism of charge storage is described in the following. The molecules of the dielectric materials may be regarded as being the electrostatic equivalent of bar magnets, but having positive and negative “electric poles”. In the discharged state, the “poles” of the molecules cancel each other out and no energy is stored by the capacitor. When voltage is applied between the plates of the capacitor, the electric force on the molecules causes then to align with the electric field so that they point in the same direction.

At this instant of time, the plate connected to the positive pole of the supply has a deficiency of electrons, and the plate connected to the negative pole contains a surplus of electrons. When the supply is disconnected, the dielectric molecules remain in the state of strain and energy is stored in the dielectric. Alter the source of supply has been disconnected from the capacitor, a potential difference can be measured between the terminals of the capacitor. This p.d. is retained for a period of time varying between a few minutes and several days, depending on the leakage resistance of the dielectric. If the dielectric leakage resistance is high, then the p.d. decays at a very low rate; a low value of leakage resistance allows the charge to leak away more rapidly.

When working on electronic circuits, readers are advised to ensure that capacitors with large values of capacitance are fully discharged; otherwise there is a risk of electric shock from the stored charge. Most capacitors can be safely discharged by connecting a resistance of about Ik between their terminals for a short period of time. Also, when replacing faulty capacitors, the maintenance engineer should check that the replacement capacitors have the correct voltage rating: when operating beyond their voltage rating, there is a risk of them exploding due to the possibility of gas being generated inside the capacitor.

UNITS OF CAPACITANCE

The ability of a capacitor to store electric charge is known as its capacitance, symbol C. The unit of capacitance is the farad, which has the symbol F.

The farad is a very large unit of capacitance, in practice; units of the microfarad (mF), the nanofarad (pF) are used.

The majority of capacitors in use have capacitance values less than about 0.01 mF, but a few will be found having very large values in power supplies and similar circuits.