Full-Wave Rectifier with Filter

The same principles as for the half-wave rectifier apply here, although of course you expect to see a ripple with twice the frequency as for the half-wave rectifier; moreover, two diodes are conductive at a time, so the voltage to which the capacitor charges corresponds to the peak voltage minus twice the diode threshold voltage. Another very important condition, namely the surge current in the capacitor filter will be considered in this section, but naturally, this applies to the half-wave rectifiers as well.   



Before switch J1 in the figure below is closed, the filter capacitor is uncharged (as long as it had not been charged just before and the switch had been open for less than a time constant so that the capacitor could not completely discharge). At the instant the switch is closed, voltage is connected to the bridge and the capacitor appears as a short. This produces an initial surge of current through the two forward-biased diodes. The worst-case situation occurs when the switch is closed at a peak of the secondary voltage and a maximum surge current is produced. The surge current could destroy the diodes, which is why a surge-limiting resistor is sometimes connected at the output in series with the RCL circuit (here between the positive output terminal of the bridge and the positive terminal of C1). The value of this resistor should be small compared to RL and the diodes must have a forward current rating such that they can withstand the momentary surge of current. The following graph of the output current, input voltage and output voltage of the rectifier with an RC filter illustrates this problem.  


Ripple Voltage and Ripple Factor:

For a given input frequency, the output frequency of a full-wave rectified voltage has less ripple than a half-wave voltage for the same load resistance and capacitor values. This is because the capacitor discharges less during the shorter interval between full-wave pulses. However, the same calculation principles apply and they will be repeated here so that you recall them. The variation in the output due to charging and discharging is called the ripple voltage. The smaller the ripple, the better the filtering action. The ripple factor is an indication of the effectiveness of the filter and is defined as


where Vr is the rms ripple voltage and Vdc is the dc (average) value of the filter's output voltage, as illustrated in the following picture. The lower the ripple factor, the better the filter. The ripple factor can be lowered by increasing the value of the filter capacitor.



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