28-12-2017, 09:40 AM
The circuit should be fine as-is! The resistor and capacitor between gate and cathode are to stop 'nuisance tripping' and also to guarantee that the Zener diode is well round its 'knee' before the SCR fires. Some Zeners are a bit soggy and could pass several microamps at well below their Zener voltage - which could make the SCR fire prematurely.
There's nothing really limiting the Zener current if the input goes overvoltage. But hopefully the SCR will fire within a couple of microseconds anyway. Let's face it - if the Zener fails short-circuit, it's only going to give the SCR more of a kick, and protect the load even more...
Be sure to use a chunky SCR that doesn't fail open-circuit before the fuse blows. You need one rated to pass maybe 5x the fuse rating. A more thorough analysis would compare the I-squared-T rating for the fuse and SCR.
There's nothing really limiting the Zener current if the input goes overvoltage. But hopefully the SCR will fire within a couple of microseconds anyway. Let's face it - if the Zener fails short-circuit, it's only going to give the SCR more of a kick, and protect the load even more...
Be sure to use a chunky SCR that doesn't fail open-circuit before the fuse blows. You need one rated to pass maybe 5x the fuse rating. A more thorough analysis would compare the I-squared-T rating for the fuse and SCR.
