Quantcast Figure 3-22.Half-wave/full-wave rectifiers (with capacitor filters)

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3-18 Figure 3-22.—Half-wave/full-wave rectifiers (with capacitor filters). The value of the capacitor is fairly large (several microfarads); it thus presents a relatively low reactance to the pulsating current and stores a substantial charge. The rate of charge for the capacitor is limited only by the relatively low resistance of the conducting diode. The RC charge time of the circuit is, therefore, relatively short. As a result, when the pulsating voltage is first applied to the circuit, the capacitor charges rapidly and almost reaches the peak value of the rectified voltage within the first few cycles. The capacitor attempts to charge to the peak value of the rectified voltage anytime a diode is conducting, and tends to retain its charge when the 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. From the waveforms shown in figure 3-22, you should see that the addition of C1 to the circuit results in an increase in the average value of output voltage (Eavg) and a reduction in the amplitude of the ripple component (Er) 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 figure 3-23, 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.

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