Electronics guide > Diodes II > Filter tips
Filter tipsAlthough we’ve managed to obtain a full-wave rectified d.c. load voltage we still
have the problem that this voltage is not too steady (ideally we would like a fixed
d.c. voltage which doesn’t vary at all). We can reduce the up-and-down variability
of the waves by adding a capacitor to the circuit output. If you remember, a capacitor
stores charges — so we can use it to average out the variation in level of the full-wave
rectified d.c. voltage. Figure 7.12 shows the idea and a possible resultant waveform.
This process is referred to as smoothing and a capacitor used to this effect is
a smoothing capacitor. Sometimes the process is also called filtering.
Figure 7.12 Levelling the rectifier d.c. with the help of a
capacitor: the process is known as smoothing
You should remember that the rate at which a capacitor discharges is dependent
on the value of the capacitor. So, to make sure the stored voltage doesn’t fall
too far in the time between the peaks of the half cycles, the capacitor should be
large enough (typically of the value of thousands of microfarads) to store enough
charge to prevent this happening. Nevertheless a variation in voltage will always
occur, and the extent of this variation is known as the ripple voltage, shown in
Figure 7.13. Ripple voltages of the order of a volt or so are common, superimposed
on the required d.c. voltage, for this type of circuit.
Figure 7.13 The extent to which the d.c. is not exactly linear
is known as the ripple voltage
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