Rectifier Filters

In the usual rectifier unit, the direct component is desired and the alternating components are unwanted. The rectifier filter is used to suppress the alternating components.

The battery and the generators of Fig. 12 acting together give the full-wave rectified voltage of equation 2. As an approximation, the internal resistance of the rectifier Z_i can be neglected. Each source may be considered separately, and the magnitude of the current that each source forces through the load R_L can be calculated separately.

The low-pass filter of Fig. 12 is known as a choke-input filter. If the first inductor is omitted, it becomes what is called a condenser-input filter. Choke-input filters are assumed in the preceding paragraph. If the usual condenser-input filter is used, then the rectified (or distorted) voltage wave is not as shown in the preceding figures, and equations 1 and 2 do not apply. With condenser-input filters, no simple equation is available, because the shape of the rectifier output wave depends on the constants of the filter and on the magnitude of the load resistance.¹⁰ For these reasons, a graphical solution¹¹ is usually employed for studying rectifier operation with condenser-input filters.

Figure 11. A full-wave bridge rectifier.

Figure 12. Equivalent circuit of a full-wave rectifier, filter, and load.

When high-vacuum tubes are used, the filter may have either choke or condenser inputs, but, with large gas tubes, choke-input filters are usually employed. The condenser-input filter draws a high peak current, and this may damage gas tubes.