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A capacitor is defined1 as "a device, the primary purpose of which is to introduce capacitance into an electric circuit." The connecting leads and the metal foil and plates of a capacitor offer resistance and inductance to current flow; also, energy is dissipated in the dielectric. An equivalent circuit of a capacitor is as shown in Fig. 4.

equivalent series circuit of a capacitor
Figure 4. An equivalent series circuit of a capacitor. The resistance and inductance are undesired.

For direct-current, and at the lower communication frequencies and voltages, capacitors with metal-foil electrodes and wax or oil-impregnated paper dielectrics are used. Mica is used for the dielectric of capacitors where the losses must be kept low, or where high voltages are employed. Capacitors with ceramic dielectrics such as titanium dioxide and similar substances are used for small radio capacitors. Air is used as a dielectric for many radio capacitors, particularly if they are to be variable, must have very low loss, or are to be operated at high voltages.

Electrolytic capacitors are used at low frequencies where a polarizing direct voltage is available, as in filters for power supplies.

What is called the energy dissipation factor D of a capacitor is usually defined as the ratio of the energy dissipated to the energy stored per cycle. Numerically,

This is the ratio of the equivalent series effective resistance to the capacitive reactance.

Last Update: 2011-05-30