78xx Voltage Regulators

The voltage regulators from the 78xx-series
are found in many analogue power supplies. It seems, then, somewhat
superfluous to say much more about them. But on the other hand, it can be
very useful to highlight the important points, just because they are so
ubiquitous. The 78xx is almost always used ‘bare’, because additional
components are almost unnecessary. In fact, only one additional
component is required, and that is capacitor C2. Based on the
manufacturer’s recommendation, this capacitor should be 220nF in order
to prevent oscillatory behavior. In practice, you will almost always see
that a 100nF capacitor used here. This is a value that does not cause
problems. C1 is the smoothing (reservoir) capacitor, its purpose is to
reduce the ripple of the rectified AC voltage and is not actually
related to voltage regulation.

If the DC voltage is provided by a mains adapter, then this
electrolytic capacitor is usually already part of the adapter, although
the value is rather small sometimes. C2 may only be omitted if C1 is
close to the 78xx and C1 is of good quality (low ESR).
But there is nothing wrong by playing it safe and always fitting C2.
Rule of thumb: always place a 100 nF capacitor on the input as close to
the regulator as is practicable. Strictly speaking, there is no need for
a capacitor on the output. However, a capacitor of at least 100 nF (C3)
ensures much-improved regulation with fast (several microseconds)
changes in load current. In practice, a decoupling-capacitor is placed
close to the power supply pins of many ICs. These can provide the same
function, provided they are placed not too far away.

An electrolytic capacitor (C4) can be added for similar reasons: to
catch slow (and fast, if it is a good capacitor) variations in load
current. There is actually no compelling reason to deal with slow
variations, because the IC is fast enough of regulating these on its
own. Rule of thumb: always place an output capacitor of at least 100 nF
preferably as close as possible to the IC with the greatest current
consumption (read: greatest changes in current consumption). When
building the circuit, it is important to connect the capacitors via the
shortest possible path. So don’t use long wires or make large loops.
Obtain the input voltage for the regulator directly from the connections
of the smoothing capacitor, because the ripple is smallest there.

Finally, a few remarks about the temperature that a 78xx may run at.
As a first approximation, if seem to burn your fingers when touching the
regulator, the temperature of the regulator is above 60°C and a (small)
heatsink is definitely recommended. It is not really a problem when the
IC gets too hot, because it was designed in such a way that it will turn
itself off when the temperature is too high. It doesn’t actually turn
off, but the output current reduces when the temperature increases. When
the internal temperature has reached 150°C, the output current will be
only a little more than half the current delivered at 25°C. That is why
it is possible that the output voltage of the regulator has dropped even
though the output current is less than the rated current for the IC. A
heatsink is the obvious solution. Rule of thumb: you should be able to
touch the regulator (or the heatsink) without burning yourself. If not,
then the heatsink has to be larger.

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