4-26
Figure 4-22.—Filtering action of the LC choke-input filter.
The choke (L1) is usually a large value, from 1 to 20 henries, and offers a large inductive reactance
to the 120-hertz ripple component produced by the rectifier. Therefore, the effect that L1 has on the
charging of the capacitor (C1) must be considered. Since L1 is connected in series with the parallel
branch consisting of C1 and RL, a division of the ripple (ac) voltage and the output (dc) voltage occurs.
The greater the impedance of the choke, the less the ripple voltage that appears across C1 and the output.
The dc output voltage is fixed mainly by the dc resistance of the choke.
Now that you have read how the LC choke-input filter functions, it will be discussed with actual
component values applied. For simplicity, the input frequency at the primary of the transformer will be
117 volts 60 hertz. Both half-wave and full-wave rectifier circuits will be used to provide the input to the
filter.
Starting with the half-wave configuration shown in figure 4-23, the basic parameters are: With 117
volts ac rms applied to the T1 primary, 165 volts ac peak is available at the secondary [(117 V)
(1.414)
= 165 V]. You should recall that the ripple frequency of this half-wave rectifier is 60 hertz. Therefore, the
capacitive reactance of C1 is:
