4-25
therefore, the average value of the voltage produced across the capacitor contains a much smaller value of
ripple component (Er) than the value of ripple produced across the choke.
Figure 4-21.—LC choke-input filter.
Now look at figure 4-22 which illustrates a complete cycle of operation for a full-wave rectifier
circuit used to supply the input voltage to the filter. The rectifier voltage is developed across the capacitor
(C1). The ripple voltage at the output of the filter is the alternating component of the input voltage
reduced in amplitude by the filter section. Each time the anode of a diode goes positive with respect to the
cathode, the diode conducts and C1 charges. Conduction occurs twice during each cycle for a full-wave
rectifier. For a 60-hertz supply, this produces a 120-hertz ripple voltage. Although the diodes alternate
(one conducts while the other is nonconducting), the filter input voltage is not steady. As the anode
voltage of the conducting diode increases (on the positive half of the cycle), capacitor C1 charges-the
charge being limited by the impedance of the secondary transformer winding, the diode's forward
(cathode-to-anode) resistance, and the counter electromotive force developed by the choke. During the
nonconducting interval (when the anode voltage drops below the capacitor charge voltage), C1 discharges
through the load resistor (RL). The components in the discharge path have a long time constant; thus, C1
discharges more slowly than it charges.
