Filters for Rectifier Circuits

Author: J.B. Hoag

Wherever a supply of current or voltage of constant strength is needed (wherever a battery has been used), one may use an alternating source, a rectifier, and a filter. It is the last-named element of the complete circuit which we now consider. Filters, as used here, consist of combinations of coils, condensers, and resistors suitable to smooth out the ripples in direct-pulsating currents so as to yield as nearly pure d.c. as possible. By storing energy in the magnetic fields of the coils and in the electric fields of the condensers while the voltage and current are increasing, and by returning it to the load while the voltage and current are decreasing, the " valleys " in the direct-pulsating voltages and currents are filled in. The smaller the " dips " in the direct-pulsating current, i.e., the smaller the per cent ripple, the easier it is to filter. The higher the ripple frequency, i.e., the more humps per cycle of the supply frequency, the easier it is to filter.

In Chapter 5 we have seen that a coil in series with a mixed direct-plus-alternating current will, because of the choking action of the coil, reduce the alternating component the while it passes the direct component. It was also shown in Chapter 5 that a condenser across the line will more or less bypass the alternating component, without loss of the direct component. Further details of filters will be found in Chapter 7.

Some of the common types of filters used in rectifier circuits are shown in Fig. 11 G, wherein use is made of the choking action of series coils and the shunting action of parallel condensers to suppress and to bypass the ripple component of a rectifier while permitting the d.c. component to continue to the output.

Fig. 11 G. Types of filters used in rectifier circuits

Filters a, b, and c are of the choke-input type and d, e, and f are of the condenser-input type. Circuits b and e are called one-section filters and are used when a comparatively large final amount of ripple can be tolerated. The two-section filters c and f are used when better filtering is necessary.