Crowbar circuit question

[GeoAVR]

Dear all,

I don't understand how the crowbar circuit achieves short circuit in input rails without damaging the switch it uses.

It seems it short-circuits the input on purpose, but then wont the device (ie. mosfet) will be destroyed?

 

[chuckey]

Crowbars are used on current limited power supplies to drop the output voltage to that of the saturated SCR. .i.e. if its a 5A PSU, a 5A SCR will work well. If the PSU is not current limited, then a fast fuse is used in series with feed to the regulator.When the SCR fires it puts a short circuit across the output of the PSU, but because of the thermal lag and the ability of transistors to handle short term current overloads, the fuse blows before the semiconductors self destruct.
Frank