OVER VOLTAGE PROTECTION

[moki]

What is the best way to protect circuits from over voltage. I recently had a case where a 5 volt dc switching power supply failed and went up to 9 volt dc. This fried the circuit boards that it supplied. MOVs might be a bit slow , I think. Any ideas, circuits etc will be appreciated.

[pebe]

A simple way would be to use an SCR crowbar circuit, provided your power supply has some form of current limiting or short circuit protection.

[sunilsuta]

@pebe: I'm not experienced as you pebe.but I have a doubt why can't we use zener as voltage regulator in this case?is it because of SMPS dc is used here?

[pebe]

If a power supply has suddenly increased its output voltage, it must be because its internal voltage regulator has failed in some way. That probably means that its full current capability is now available at its output. If that current is a few tens of milliamps then you could possibly use a zener for protection.

But a 5V SMPS can probably deliver a maximum output of an amp or more. For a zener to give protection it would then have to rated for at least 5W, and for sustained protection would need a heat sink. So the protection would be physically larger than the SMPS, so making a bulky power supply.

If an SCR is used, then once turned on its voltage is quite low and would cause the supply fuse to blow.

[sunilsuta]

Oh thank you very much pebe