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The 12 volts input is
stepped down to 5 volts
by the regulator.
If the regulator goes
faulty, 12 volts could
be applied to the load,
damaging it.
However, if the
regulator output goes
above 5.1 volts, then
the zener diode
conducts.
Current flowing through
the resistor provides a
voltage across it, which
triggers the gate of the
thyristor.
The large current
through the thyristor
blows the fuse and
protects the load.
Crude but effective.
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