a better one is at
http://www.radio-electronics.com/info/circ...rowbar.php

At least it has some chance of working properly. I'm not convinced the triac in the one from Wikipedia will ever see a trigger, aside from occasional noise spikes.

As they explain, their small cap swallows those small spikes that occasionally come up, and might false-trigger the thyristor, but the SCR, presumably a sensitive-gate type, and, viewing the zener diode as a fixed voltage drop, allows the current through, hence the voltage across, the anode resistor to trigger the thyristor.

That's all available from a Google search on crowbar circuit.

RE