Capacitor
What is a Capacitor?
A capacitor is
a passive two terminal component which stores electric charge. This
component consists of two conductors which are separated by a dielectric
medium. The potential difference when applied across the conductors
polarizes the dipole ions to store the charge in the dielectric medium.
The circuit symbol of a capacitor is shown below:
 | ||||
| The circuit symbol of a capacitor |
The capacitance or the potential storage by the capacitor
is measured in Farads which is symbolized as ‘F’. One Farad is the
capacitance when one coulomb of electric charge is stored in the
conductor on the application of one volt potential difference.
The charge stored in a capacitor is given by
Q = CV
Where Q - charge stored by the capacitor
C - Capacitance value of the capacitor
V - Voltage applied across the capacitor
Note the other formula of current, I = dQ/dt
Taking the derivative with respect to time,
dQ/dt = d(CV)/dt
From the above statement, we can express the equation as
I = C (dV/dt)
As
you turn on the power supply, the current begins to flow through the
capacitor inducing the positive and negative potentials across its
plates. The capacitor continues to charge until the capacitor voltage
equalizes up to the supply voltage which is called as the charging phase
of the capacitor. Once the capacitor is fully charged at the end of
this phase, it gets open circuited for DC. It begins to discharge when
the power of the capacitor is switched off. The charging and discharging
of the capacitor is given by a time constant.
The voltage across the capacitor is given by
Capacitors are widely used in a variety of applications of electronic circuits such as
· store charges such as in a camera flash circuit
· smoothing the output of power supply circuits
· coupling of two stages of a circuit (coupling of an audio stage with a loud speaker)
· filter networks(tone control of an audio system)
· delay applications (as in 555 timer IC controlling the charging and discharging)
· tuning radios to particular frequencies
· phase alteration.
The
conductors offer a series resistance and if the capacitor is
constructed using tubular structure then some inductance is also
induced. The dielectric medium between the plates has an electric field
strength limit and also passes a small amount of leakage current which
results into a Breakdown voltage.
There are different types of capacitors,
they can be fixed or variable. They are categorized into two groups,
polarized or non-polarized. Electrolytic capacitors are polarized. Most
of the low value capacitors are non-polarized. The symbol of capacitors from each group is shown below:
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