Most power supplies create a constant output voltage. Even variable supplies are adjusted to some fixed value and then left. The output of a rectifier however is not constant. There is a huge ripple voltage, which must be smoothed or filtered.

Filtering generally requires reactive components. Capacitors placed across the load will tend to reduce the ripple voltage, while inductors placed in series with the load will tend to reduce the ripple current.

Capacitive Filter

Half-Wave Rectifier (the capacitor is missing from the sketch)	Approximation Formulas

Full-Wave Rectifier	Approximation Formulas

The required size of the filter capacitor (in mfd) can be approximated by:

Where:

• C is in mfd.
• I_Load is in ma.
• RC discharge is assumed.

A closer approximation can be made by using the transformer Schade curves. In any case, ripple voltage increases as the load current increases.

Care must be exercised when selecting filter capacitors. Power supplies generally use large electrolytic capacitors as filters. It is very important to never exceed the voltage rating or to reverse the polarity of electrolytic capacitors.

Although the voltage across the capacitor is relatively constant, the current through it is not. The ripple currents are typically 2 – 3 times the DC load current. The capacitor supplies the load current when the rectifier circuit current falls. It also accepts the excess rectifier current when it exceeds the load current demands. The current in the filter capacitor is therefore an alternating current (AC) since it reverses direction.