Electronics guide > Diodes II > Stability built in
Stability built inThe power supplies we’ve seen so far are simple but they do have the drawback
that output voltage is never exact — ripple voltage and to a large extent, load
current requirements mean that a variation in voltage will always occur. In many
practical applications such supplies are adequate, but some applications require
a much more stable power supply voltage.
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 Vin 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.
In summary, the power supply principle is summarised in Figure 7.15 in block
diagram form. From a 230 V a.c. input voltage, a stabilised d.c. output voltage
is produced. This is efficiently done only with the use of diodes in the rectification
and stabilisation stages.
Figure 7.15 A block diagram of a power supply using the circuit
stages we have described
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