All about Capacitors

Capacitor

A capacitor was formerly known as condenser. It is a device for storing electric charge. Practical capacitors are made using various methods. But in all of them there are at least two conductors separated by an insulation.

A pair of conductors separated by a dielectric or insulator forms a capacitor. If we apply a potential difference or voltage across these two conductors, then a static electric field is formed across the insulator or dielectric. Hence, positive charges are stored in a plate or conductor and negative charges in the other. The unit of capacitance is Farad. It is the ratio of electric charge to the potential difference on each conductor.

Capacitors are widely used in electronic circuits. It is used to block direct current while allowing alternating current to pass. It is also used in filter networks, for smoothing the output of power supplies. It is also used in the resonant circuits.

To increase the capacitance of a capacitor we have to do two things:

01. Increase the area of the conductors by any means and

02. Narrow the separation between them as much as possible.

Operation of Capacitor:

Capacitors are the general model for electric fields. An ideal capacitor is the one which has a constant capacitance C. Capacitance C, can be defined as the ratio of charge on each conductor to the voltage between them.

C = Q/V

where C is capacitance, Q is charge and V stands for voltage.

In fact, charge build up is a continuous process. And sometimes discharge happens. Or a leakage current may flow. Hence we represent capacitance as the derivative of small charge, q with respect to small voltage v.

C = dq/dv.

Energy storage

To 'move' charges between the conductors of a capacitor, work must be done. This work is done by a voltage source. When this external influence is removed the capacitor starts discharging and comes to its equilibrium position. The amount of energy stored in a capacitor is the work done in establishing the electric field. It can be expressed as:

Networks

Capacitors in parallel:

When we connect capacitors in parallel connection then all the capacitors get same applied voltage. Hence their capacitances add up. Actually the plate area of the capacitors are added up in parallel connection.

Capacitors in series:

But in series connection the plates are much more separated. From the diagram above it is very clear that the separation distance is added up. So the total voltage difference is calculated as sum of the inverse of their capacitance. Hence the total series capacitance must be smaller than any other capacitor in the network.

High working voltage is achieved by combining capacitors in series. For example, a high voltage power supply can produce voltage spikes. Series capacitors are used for smoothing the spikes of a high voltage power supply.

Basic Electrical Engineering

Categories