Capacitors are passive
devices used in electronic circuits to store energy in the form of an electric field. They are the compliment of
inductors, which store energy in the form of a magnetic field. An ideal
capacitor is the equivalent of an open circuit (infinite ohms) for direct currents (DC), and presents an impedance
(reactance) to alternating currents (AC) that depends on the frequency of the current. The reactance (opposition
to current flow) of a capacitor is inversely proportional to the frequency of the current flowing through it.
Capacitors were originally referred to as "condensers" for a reason that goes back to the days of the Leyden Jar
where electric charges were thought to accumulate on plates through a condensation process.
The property of
capacitance that opposes current flow is exploited for the purpose of conducting signals with a higher frequency
component while preventing signals of lower frequency components to pass. A common application of a capacitor is
in an RF (radio frequency) circuit is where a DC bias voltage needs to be blocked from flowing through a circuit
while allowing the RF signal to pass.
When
used in
series (left) or
parallel (right) with its circuit compliment, an inductor, the
inductor-capacitor combination forms a circuit that resonates at a particular frequency that depends on the values
of each component. In the series circuit, the impedance to current flow at the resonant frequency is zero with
ideal components. In parallel circuits (right), the impedance to current flow is infinite with ideal components.
Real-world
capacitors made of physical components exhibit more than just a pure capacitance when present in an AC circuit. A
common circuit simulator model is shown to the right. It includes the actual ideal capacitor with a parallel
resistive component that responds to alternating current. The DC resistive component is in series with the ideal
capacitor, and an inductor is connected in series with the entire assembly and represents the inductance of the
component leads and plates.
Equations (formulas) for combining capacitors in series and parallel are given below. Additional equations are
given for capacitors of various configurations.
As these figures and formulas indicate, capacitance is a
measure of the ability of two surfaces to store an electric charge. Separated and isolated by a dielectric
(insulator), a net positive charge is accumulated on one surface and a net negative charge is stored on the other
surface.
In an ideal capacitor, charge would be stored indefinitely; however, real world capacitors
gradually lose their charge due to leakage currents through the non-ideal dielectric.
Additionally, an
inductive component is present due to metal leads (if present) and characteristics of the plate surfaces. This
inductance, in combination with the capacitance, creates a resonant frequency where the capacitor looks like a
pure resistance.