Sphere478 wrote on 2022-09-27, 19:00:
Ah, they have devices like these in the form of blue discs. They are used in lightening protection. They look like ceramic capacitors.
They serve a similar purpose, overvoltage protection, but in a significantly different way.
These little blue discs are "metal-oxide varistors". Their purpose is to convert excessive energy that create a voltage spike into heat. They require that the current they draw causes the voltage to go down to a reasonable value. While the varistor dissipates the extra power added by lightning, the device can keep functioning. Whenever a varistor dissipates a significant energy spike, it ages. If exposed to a continous overvoltage driven strong enough, it will overheat and possibly catch fire.
Crowbar circuits on the other hand try to imitate a dead short as good as possible, driving the power supply either into short circuit protection or blowing a fuse, definitely disrupting operation. The advantage of creating a dead short is that the heat is created elsewhere, the crowbar itself (if dimensioned correctly) will not detoriate even when it activates, but the supply that is shorted might suffer from crowbar activation. You usually don't care about that supply, though, because the crowbar is supposed to activate only if the supply is already broken and outputs too much voltage.