1445326735900-0

Diodes II
We’ve already seen a device capable of stabilising or regulating
power supplies — the zener diode. Figure 7.14 shows the
simple zener circuit we first saw last chapter. You’ll remember
that the zener diode is reverse biased and maintains a more
or less constant voltage across it, even as the input voltage
V in
changes. If such a zener circuit is used at the output of
a smoothed power supply (say, that of Figure 7.12) then the
resultant stabilised power supply will have an output voltage
which is much more stable, with a much reduced ripple
voltage.
Figure 7.14 A simple zener circuit which can be used with a smoothing circuit to
give a greatly reduced ripple voltage
Zener stabilising circuits are suitable when currents of no
more than about 50 mA or so are required from the power
supply. Above this it’s more usual to build power supplies using
stabilising integrated circuits (ICs), specially made for the
purpose. Such ICs, commonly called voltage regulators, have
diodes and other semiconductors within their bodies which
provide the stabilising stage of the power supply. Voltage
regulators give an accurate and constant output voltage with
extremely small ripple voltages, even with large variations in
load current and input voltage. ICs are produced which can
provide load current up to about 5 A.
161