Tuned Rectifier Filters

Sometimes an inductor-input filter is tuned as in Fig. 93.

Fig. 93. Shunt-tuned filter.

The addition of capacitor C₁ increases the effective reactance of the inductor to the fundamental ripple frequency. Both regulation and ripple of this type of filter are improved. The filter is not tuned for the ripple harmonics, so the use of high-Q filter inductors is unnecessary. An increase in effectiveness of the filter inductor of about 3 to 1 can be realized in a single-phase full-wave rectifier circuit. Tuned filters are less effective with three-phase rectifiers because slight phase unbalance introduces low-frequency ripple which the filter does not attenuate.

Filters may be tuned as in Fig. 94, where the filter capacitor C₁ is connected to a tap near the right end of inductor L, and the other filter capacitor C₂ is chosen to give series resonance and hence zero reactance across the load at the fundamental ripple frequency.

Fig. 94. Series-tuned filter.

Because of choke losses, the impedance across R_L is not zero, but the resulting ripple across load resistor R_L can be made lower than without the use of capacitor C₁. Ripple is attenuated more than in the usual inductor-input filter, but regulation is not substantially different.