The Smoothing Capacitor
We saw in the earlier section that single phase half-wave rectifier produces an output wave every half cycle and that it was not practical to use this kind of circuit to generate a steady DC supply. The full-wave bridge rectifier but, gives us a greater mean DC value (0.637 Vmax) with less superimposed ripple while output waveform is twice that of frequency of the input supply frequency. We can thus increase its average DC output level even higher by connecting a appropriate smoothing capacitor across output of the bridge circuit as given below.
Full-wave Rectifier with Smoothing Capacitor
The smoothing capacitor converts full-wave rippled output of rectifier into the smooth DC output voltage. In general for DC power supply circuits the smoothing capacitor is the Aluminium Electrolytic type which has a capacitance value of 100uF or more with the repeated DC voltage pulses from rectifier charging up capacitor to peak voltage. But, there are two significant parameters to consider while choosing a suitable smoothing capacitor and these are its Working Voltage, which should be higher than the no-load output value of rectifier and its Capacitance Value, which determines amount of ripple which will appear superimposed on top of DC voltage. Too low a value and capacitor has less effect but if the smoothing capacitor is large (parallel capacitors can be used) and load current is not too large, then the output voltage will be almost as smooth as pure DC. As a usual rule of thumb, we are looking to have a ripple voltage of less than 100mV peak to peak.
The maximum ripple voltage present for the Full Wave Rectifier circuit is not determined by the value of the smoothing capacitor only but by frequency and load current, and can be calculated as:
Bridge Rectifier Ripple Voltage
Where: I is DC load current in the amps, ƒ is frequency of the ripple or twice t he input frequency in Hertz, and C is capacitance in Farads.
The basic advantages of a full-wave bridge rectifier is that it has smaller AC ripple value for the given load and a smaller reservoir or smoothing capacitor than the equivalent half-wave rectifier. Hence, the fundamental frequency of ripple voltage is twice that of AC supply frequency (100Hz) where for half-wave rectifier it is exactly equal to supply frequency (50Hz).
The amount of ripple voltage which is superimposed on top of the DC supply voltage by diodes can be virtually removed by adding a improved π-filter (pi-filter) to the output terminals of bridge rectifier. This type of the low-pass filter consists of 2 smoothing capacitors, generally of the same value and an inductance across them to introduce a high impedance path to alternating ripple component. Another much practical and cheaper alternative is to use a three- terminal voltage regulator IC, like a LM78xx for a positive output voltage or LM79xx for a negative output voltage which can reduce ripple by over 70dB (Datasheet) while delivering the constant output current of over 1 amp.
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