# Circuit Overvoltage Protection Using 12V Power Rail from Computer Power Supply?

I'm looking at building a circuit that is powered by the 12V power rail that is commonly provided with a desktop power supply.

What would be a good rule of thumb for what an "overvoltage" scenario would look like if a desktop power supply failed, and you wanted to protect what is powered in that circuit?

What sort of current or voltage peaks would you expect in a failure scenario for a desktop power supply?

For over-voltage protection, the easy answer is using an IC something like a LTC4360 -- but is protection up to 80V sufficient for a desktop power supply failure?

What happens when a power supply fails, that is hooked up to 120V AC source? What starts pouring out of the broken power supply? (Hopefully nothing -- right because it has some kind of over-voltage protection -- but still....?).

Do you go with a more discrete solution and add a physical fuse and a crowbar circuit of some sort (I kind of like the fuse approach personally) -- if so, what kind of parameters?

Maybe an SMD resettable fuse and a zener diode? (Seems like the cheapest way).

I don't have any experience in power supply design -- or in this case, when power supplies fail and what that transient event looks like.

• What are you even trying to protect against? – Matt Young Sep 7 '17 at 20:20
• I'm trying to protect against a "failure" of the power supply. I don't even know what a "failure" would look like... I'm imagining a desktop power supply is a constant 12V going to a constant unknown volts (like 12V goes to 80V when a desktop power supply fails?). Is it not constant when it fails, does the broken power supply sputter and spit out some AC type power wave gone haywire? Should you expect one large transient DC spike, and then the power supply goes silent? Do you design for all of these scenarios, etc? – Leroy105 Sep 7 '17 at 20:28
• You need to study architecture of desktop power supplies first, before generating insane scenarios of catastrophic failures. Keep in mind that there are billions of PCs with ATX12 PSUs, and I don't recall any reports of massive damages. The PSUs just die quietly, that's all. – Ale..chenski Sep 8 '17 at 2:27
• anything's possible, but working pc repair for about 5 years i never saw a malfunctioning psu damage the other components on its way out. transformers lower the AC voltage, and i don't see how those would fail in a way that increases voltage. then again, transformers aren't what you worry about; caps, movs, and diodes are... anyway, don't worry about it. – dandavis Sep 8 '17 at 13:16