Capacitor

A capacitor (formerly known as a "condenser") is a device that stores electric charge, or, more accurately, consists of two plates which each store an opposite charge. These two plates are conductive and separated by an insulator or dielectric. The charge is stored on the inside of the plates, at the boundary with the dielectric.

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

The above equation is only accurate for values of Q which are much larger than the electron charge e = 1.602·10^-19 C. For example, if a capacitance of 1 pF is charged to a voltage of 1 µV, the equation would predict a charge Q = 10^-19 C, but this is impossible as it is smaller than the charge on a single electron. However, recent experiments and theories (e.g. the fractional quantum Hall (FQH) effect) have suggested the existence of fractional charges.

A capacitor has a capacitance 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), nanofarads (nF) or picofarads (pF).

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

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

The energy (in joules) stored in a capacitor is given by:

Moving a charge Q across a potential difference of V requires an energy QV; here the charge is CV but the energy is not CV², but less (in fact half of that) because while charging the potential difference is not yet equal to the final value; therefore (simple) integration is required to find the formula above.

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

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

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

Electrons cannot pass from one plate of the capacitor to the other. When a voltage is applied to a capacitor, current flows to one plate, charging it, while flowing away from the other plate, charging it oppositely. In the case of a constant voltage (DC) soon an equilibrium is reached, where the charge of the plates corresponds with the applied voltage, and no further current will flow in the circuit. Therefore direct current cannot pass. However, effectively alternating current (AC) can: every change of the voltage gives rise to a further charging or a discharging of the plates and therefore a current. 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:

where:

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

It is called reactance because the capacitor reacts to changes in the voltage.

Thus the reactance is inversely proportional to the frequency. Since DC has a frequency of zero, 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:

where j is the imaginary number.

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

Table of contents

1 Practical capacitors
2 Uses
3 Variable capacitors
4 History

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 (ESR) are further considerations. A typical ESR for most capacitors is between 0.0001 and 0.01 ohm, low values being preferred for high-current applications.

Since capacitors have such a low resistance, they have the capacity to deliver huge currents into short circuits, which can be dangerous. For safety purposes, all large capacitors should be discharged before handling. This is done by placing a small 1 to 10 ohm resistor across the terminals, i.e. shorting through a resistance.

Discrete capacitors of various type are available commercially with capacitances ranging from the pF range to more than a farad, and voltage ratings up to hundreds of volts. In general, the higher the capacitance and voltage rating, the larger the physical size of the capacitor (and usually a higher price as well). Tolerances for discrete capacitors are usually specified as 5 or 10%.

Capacitors can be fabricated in semiconductor integrated circuit devices using metal lines and insulators on a substrate. Such capacitors are used to store analogue signals in switched-capacitor filters, and to store digital data in dynamic random-access memory (DRAM). Unlike discrete capacitors, however, in most fabrication processes, precise tolerances are not possible (15-20% is considered good).

Uses

Capacitors are commonly used in power supplies where they smooth the output of a full or half wave rectifier.

Because capacitors pass AC but block DC signals, they are often used to separate the AC and DC components of a signal. This method is known as AC coupling.

Capacitors are also used in power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volts-amps reactive (var).

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 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.