Needs some RF filtering else it will trigger on the least amount of RF
in the shack. And needs some resistor from SCR gate to ground to
accommodate the rounded knee of the standard zener. Often the zener
samples the supply output while the SCR anode is connected right after
the rectifier so the voltage drop of the fuse is not in a place to
worsen regulation, and that keeps the crowbar from destroying the
remaining pass transistors when it fires from nuisance causes like very
low load or too much RF.
A TL431C programmable zener has a far sharper knee and works at less
than 100 microamps which can be an asset to the crowbar protection
73, Jerry, K0CQ
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.