Capacitor

A capacitor is a device that stores electric charge. It consists of two conductive plates separated by an insulator or dielectric. The charge is stored on the inside of the plates, at the boundary with the insulator or dielectric.

The capacitor's capacitance (C) is a measure of how much voltage (V) appears across the plates for a charge (Q) stored in it:

C = Q / V

A capacitor has a capacitance value of one farad when one coulomb of charge causes a potential difference of one volt across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (μF) or picofarads (pF).

When the voltage across a capacitor changes, the capacitor will be charged or discharged. The associated current is given by

i = C dV/dt

where i is the current flowing in the conventional direction, and dV/dt is the time derivative of voltage.

The capacitance of a parallel plate capacitor is approximately equal to the following:

C = ε₀ × ε_r × A / D

where C is the capacitance in farads, ε₀ is the electrostatic permittivity of vacuum or free space, ε_r is the dielectric constant or relative permittivity of the insulator used, A is the area of the each of the two plates, and D is the distance between the plates.

In a tuned circuit such as a radio receiver, the frequency selected is a function of the inductance (L) and the capacitance (C) in series, and is given by

f = 1/(2π√(LC))

This is the frequency at which resonance occurs in a series RLC circuit.

In semiconductor integrated circuit devices these capacitors are made up of metal lines and insulators on a substrate. They store charges that can be made to represent binary logic levels (such as high versus low). This is the principle used in dynamic RAM (DRAM), which stores bits in periodically-refreshed capacitors.

Electrons cannot pass from one plate of the capacitor to the other, therefore direct current (DC) cannot pass. However, effectively alternating current (AC) can; the amount of "resistance" of a capacitor to AC is known as capacitive reactance, and varies depending on the AC frequency. Capacitive reactance is given by this formula:

X_C = 1 / (2π × f × C)

where:

X_C = capacitive reactance, measured in ohms
f = frequency of AC in hertz
C = capacitance in farads

It is called reactance because it reacts to change in the value of the current.

Thus the reactance is inversely proportional to the frequency; since DC corresponds to AC with zero frequency, the formula confirms that capacitors completely block direct current; for high frequency alternating currents the reactance is small enough to be considered as zero in approximate analyses.

The impedance of a capacitor is given by:

Z = -j/(2π × f × C)

Hence, capacitive reactance is the negative imaginary component of impedance.

Practical capacitors

Capacitors are often classified according to the material used as the dielectric. The following types of dielectric are used.

ceramic (low values up to about 1μF)
polystyrene (usually in the picofarad range)
polyester (from about 1nF to 1μF)
polypropylene (low-loss, high voltage, resistant to breakdown)
tantalum (compact, low-voltage devices up to about 100μF)
electrolytic (high-power, compact but lossy, in the 1μF-1000μF range)
air-gap

Important properties of capacitors, apart from the capacitance, are the maximum working voltage and the amount of energy lost in the dielectric. For high-power capacitors the maximum ripple current and equivalent series resistance are further considerations.

Variable capacitors

There are two distinct types of variable capacitors.

Those that use a mechanical construction to change the distance between the plates, or the surface of the area of the overlapping plates. These devices are called tuning capacitors or simply "variable capacitors", and are used in telecommunication equipment for tuning and frequency control.
Those that use the the fact that the thickness of the depletion layer of a diode varies with the DC voltage across the diode. These diodes are called variable capacitance diodes, varactors or varicaps. Any diode exhibits this effect, but devices specifically sold as varactors have a large junction area and a doping profile specifically designed to maximize capacitance.

History

The Leyden jar, the first form of capacitor, was invented at Leiden University in the Netherlands. It was a glass jar coated inside and out with metal. The inner coating was connected to a rod that passed through the lid and ended in a metal ball.

See also:Electricity, Electronics, Inductor