Logic PSU With Over-Voltage Protection

A simple 5 Volt regulated PSU featuring overvoltage protection. The 5 volt regulated power supply for TTL
and 74LS series integrated circuits, has to be very precise and
tolerant of voltage transients. These IC’s are easily damaged by short
voltage spikes. A fuse will blow when its current rating is exceeded,
but requires several hundred milliseconds to respond. This circuit will
react in a few microseconds, triggered when the output voltage exceeds
the limit of the zener diode. This circuit uses the crowbar method,
where a thyristor is employed and short circuits the supply, causing the
fuse to blow. This will take place in a few microseconds or less, and
so offers much greater protection than an ordinary fuse.

Circuit diagram:

Logic PSU With Over Voltage Protection Circuit

Logic PSU With Over-Voltage Protection Circuit Diagram

If the output voltage exceed 5.6Volt, then the zener diode will
conduct, switching on the thyristor (all in a few microseconds), the
output voltage is therefore reduced to 0 volts and sensitive logic IC’s
will be saved. The fuse will still take a few hundred milliseconds to
blow but this is not important now because the supply to the circuit is
already at zero volts and no damage can be done. The dc input to the
regulator needs to be a few volts higher than the regulator voltage. In
the case of a 5v regulator, I would recommend a transformer with
secondary voltage of 8-10volts ac. By choosing a different regulator and
zener diode, you can build an over voltag trip at any value.

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