Logic PSU with Overvoltage Protection
Circuit :  Andy Collinson
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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. 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. I have a simulated transient graph of this over voltage protection circuit in the Design section

Circuit Modification
I have a circuit modification from one of my viewers in Sweden, Ulf Kylenfall. The modified circuit is shown below and the electrolytic capacitor is moved to a position directly after the rectifier bridge and before the fuse.

With the original circuit, the thyristor had to [rapidly] discharge the capacitor in its effort to protect the downstream circuitry. This current surge may otherwise destroy the thyristor depending on what quality that has been used for the capacitor.

With the modified circuit, shown above, the fuse needs to blow in such a short time that the integrated energy dissipation over the thyristor is within its limits. Moving the capacitor to a position before the fuse ensures this. (Provided that a sufficiently fast-blow fuse has been used).

Thank you Ulf, for highlighting this design problem, and your solution to improve the circuit, kind regards Andy Collinson.

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