Filters

Filters are utilized in power supplies to diminish the variations in the rectifier output signal. As our goal is to generate a constant dc output voltage, it is essential to eliminate as much of the rectifier output variations as possible.

The net result of using a filter is demonstrated

            The capacitor filter is the most fundamental filter type and the most of commonly utilized. This filter is simply a capacitor linked in parallel along the load resistance, as indicated in the diagram a below. The filtering action is 

Depend on the charge /discharge action of the capacitor .At the time of the positive half-cycle of the input,D₁ shall conduct and the capacitor shall charge quickly as shown in the diagram (a) above. As the input begins to go negative D₁ will off, & the capacitor will gradually discharge via the load resistance in diagram (b).

Since the input from the rectifier drops below the charged voltage of the capacitor, the capacitor behaves as the voltage source for the load. This is the difference among the charge & discharge times of the capacitor that decrease the variation in the rectifier output voltage.

In the circuit the difference among charge & discharge times of the capacitor is caused by two distinct R_C time constant.