The output from any of the rectifier circuit is not purely dc but also has some ac components called ripples, along it. The ripples are maximum in the single phase half-wave rectifier and being reduced in the full-wave rectifier and being reduced further with the increase in the number of phases.
Such supply is not useful for driving sophisticated electronic devices/circuits. Of course, for a circuit such as battery charger the pulsating nature of supply available from a rectifier is no great detriment as long as the dc level provided results in charging of battery. But for supply circuits to radio or tape-recorder the pulsating dc results in 50 – (or 100) Hz signal appearing in the output, thereby making the operation of the overall circuit poor. For such applications, as well as for many more, the output dc developed will have to be much steady or smoother than that of the pulsating dc obtained directly from half-wave or full-wave rectifier circuits.
Hence, it becomes essential to reduce the ripples from the pulsating dc supply available from rectifier circuits to the minimum. This is achieved by using a filter or smoothing circuit which removes (or filters out) the ac components and allows only the dc component to reach the load. Obviously, a filter circuit should be placed between, the rectifier and the load, as shown in figure.
A filter circuit is a device that converts pulsating output of a rectifier into a steady dc level.
A filter is generally a combination of inductors L and Capacitors C. The filtering action of L and C depends upon the facts that an inductor allows only dc and a capacitor allows ac only to pass. So a suitable L and C network can effectively filter out (or remove) the ac components from the rectified output.
Commonly used types of filter circuits are:
(e) R-C filter