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Over Voltage Protection

Schematic of a typical crow bar or over voltage circuit.

Crow Bar Circuit

This circuit can be added to any power supply that does not have it already. The value of the SCR should be whatever the supply is capable of producing as maximum output current. The way this works is sort of like this: The zener diode monitors the output voltage of the power supply.

When the voltage gets to 15 volts, the Zener diode conducts. In turn this fires the SCR and therefore short circuits the output of the supply. This then in turn, draws so much current that it destroys the AC input fuse. That is why it is so very important to use only the minimum rated fuse to get the job done. All this happens so fast that it stops any over voltage from reaching the equipment that is powered by the power supply.

The way we figure out what the value of the AC input fuse should be, sort of goes like this: As a rule of thumb, I divide the output maximum current by 4 and that is what I use for the value of the AC input fuse.

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