4-19
rectifier output falls to zero. (The capacitor cannot discharge immediately.) The capacitor slowly
discharges through the load resistance (RL) during the time the rectifier is nonconducting.
The rate of discharge of the capacitor is determined by the value of capacitance and the value of the
load resistance. If the capacitance and load-resistance values are large, the RC discharge time for the
circuit is relatively long.
A comparison of the waveforms shown in figure 4-16 (view A and view B) illustrates that the
addition of C1 to the circuit results in an increase in the average of the output voltage (Eavg) and a
reduction in the amplitude of the ripple component (Er) which is normally present across the load
resistance.
Now, let's consider a complete cycle of operation using a half-wave rectifier, a capacitive filter (C1),
and a load resistor (RL). As shown in view A of figure 4-17, the capacitive filter (C1) is assumed to be
large enough to ensure a small reactance to the pulsating rectified current. The resistance of RL is assumed
to be much greater than the reactance of C1 at the input frequency. When the circuit is energized, the
diode conducts on the positive half cycle and current flows through the circuit, allowing C1 to charge. C1
will charge to approximately the peak value of the input voltage. (The charge is less than the peak value
because of the voltage drop across the diode (D1)). In view A of the figure, the charge on C1 is indicated
by the heavy solid line on the waveform. As illustrated in view B, the diode cannot conduct on the
negative half cycle because the anode of D1 is negative with respect to the cathode. During this interval,
C1 discharges through the load resistor (RL). The discharge of C1 produces the downward slope as
indicated by the solid line on the waveform in view B. In contrast to the abrupt fall of the applied ac
voltage from peak value to zero, the voltage across C1 (and thus across RL) during the discharge period
gradually decreases until the time of the next half cycle of rectifier operation. Keep in mind that for good
filtering, the filter capacitor should charge up as fast as possible and discharge as little as possible.
Figure 4-17A.—Capacitor filter circuit (positive and negative half cycles). POSITIVE HALF-CYCLE
