This circuit can be added to any power supply design that does not have additional circuit protection.

When the voltage exceeds either 10V or 30V, depending on the voltage switch setting via the front panel, the zener diode ZD1 conducts and fires the gate of the SCR. This provides a short-circuit over the output voltage and in turn will blow the AC input fuse of the powersupply you wish to protect.
For the "Bench Top Power Supply" elsewhere listed on the Circuits page, you need two of the little circuits shown above, one for the 0-6V setting and one for the 0-30V setting. Zener for 1 to 6V is 7.2V and a 30V zener for the other one. The 2N4441 can be substituted with an ECG5442, NTE5442, or other equivalent.

It is very important therefore to use the correct value for the AC fuse.
To calculate this value, as a rule of thumb, take the maximum output current and divide by four.
Example: if your output current is 10 Amps, your input AC fuse is 250mA (or 1/4A).

This crow-bar circuit is good for up to 50V and 8.0 amp using the 2N4441 SCR.
The C106Y will do the same for up to 30V and 4.0 amps.
Adjust the SCR as necessary for your project.

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