The capacitor-input filter, also called pi filter due to its shape that looks like the Greek letter pi, is a type of electronic filter. Filter circuits are used to remove unwanted or undesired frequencies from a signal.
A typical capacitor input filter consists of a filter or reservoir capacitor C1, connected across the rectifier output, an inductor L, in series and another filter or smoothing capacitor, C2, connected across the load, RL. A filter of this sort is designed for use at a particular frequency, generally fixed by the AC line frequency and rectifier configuration. When used in this service, filter performance is often characterized by its regulation and ripple.