Question

Why do we need filters in a power supply?

Answer 1

A rectifier-half-wave or full-wave-outputs a pulsating dc which is not directly usable in most electronic circuits. These circuits require something closer to pure dc as produced by batteries. Unlike the pure dc waveform of a battery, a rectifier output has an ac ripple riding on a dc waveform.

The circuit used in a dc power supply to remove the ripple is called a filter. A filter circuit can produce a very smooth waveform that approximates the waveform produced by a battery. The most common technique used for filtering is a capacitor connected across the output of a rectifier.