A very simple form of over-voltage protection uses an approach called “voltage clamping”. In its simplest form it can be provided by using a Zener diode placed across the output of the regulated power supply. With the Zener diode voltage chosen to be slightly above the maximum rail voltage, under normal conditions it will not conduct. If the voltage rises too high, then it will start to conduct, clamping the voltage at a value slightly above the rail voltage.
If a higher current capability is needed for the regulated power supply then a Zener diode with a transistor buffer can be used. This will increase the current capability over the simple Zener diode circuit, by a factor equal to the current gain of the transistor. As a power transistor is required for this circuit, the likely current gain levels will be low - possibly 20 - 50.
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