Crowbar circuit - zener "biasing" resistor.

[Diabolical Artificer]

Thanks all. Misread the circuit. The zener's died on an old breadboard, I suspect the tracks insulation has about had it.

Thanks for the explanation Mark, good idea to use a lamp instead of fuse.

How does one work out the value of R1? In the Art of Electronics crowbar circuit, which is more or less identical R1 is 68 ohm. Big disparity between 68r and 1k. Or is it a case or suck and see?

Andy.