Filtering with a capacitor

Rectified output is a string of half-sine pulses: one-way, but not steady. Circuits that smooth it into nearly pure dc are called filters, because they seem to filter out the ac ripple.

The usual filter is a capacitor across the output terminals, in parallel with RL. An inductor in series with RL can do the same job.

While the rectified voltage rises the capacitor charges. With no load it would stay charged at the peak voltage.

With a load, the capacitor discharges through RL between pulses and its voltage sags, then the next pulse recharges it to the peak.

How fast the voltage sags depends inversely on the time constant, the product C × RL. A large time constant means a small sag.

To make the time constant large, C must be large, so capacitor input filters use large capacitors.

The filtered output stays near the peak of the rectified voltage. This capacitor input filter is the type most widely used in power supplies.