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So one day, the PSU in the machine I was working with decided to blow up. Yes, I heard a pop, and a strong burnt smell.

So what happened? What caused it? How can I prevent it?

The situation:

  • The PC was NOT running, it was turned OFF
  • It was connected to a power outlet
  • The PSU was turned on
  • There was this USB fan attached, and it was kept running
  • The PSU was more than 5 years old
  • USB Legacy support has been turned on in the BIOS to provide power to USB socket even when machine is turned off

So, I was afraid that the whole motherboard was fried, I went to buy a new PSU to try things out. As soon as I fitted the new PSU, everything is fine again.

I noticed that no cooling fans were running while the PC was off. CPU and Case Fans were off obviously. However the PSU's own cooling fan was not on.

Could it be that, the tiny USB fan, that draws electricity from the PSU, overheat the PSU since the cooling fan was off?

It sounds bizarre but that is the only reason I could think of. If so, would it be a bad idea to keep USB devices powered and running while having the machine turned off?


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closed as off topic by RobM, voretaq7, Iain, Scott Pack, MDMarra Dec 8 '11 at 18:43

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The relevant bit there is that the PSU was 5 years old. Did it overheat the PSU? Probably not. Did the fact that a 5 year old PSU was running eventually cause it to fail? Yes. PSUs aren't built to run eternally, especially not in a consumer grade machine.

Also... why not just buy a traditional fan? That seems way overkill to run a PC (albeit in mostly shut off mode) just to power a fan.

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+1 for Buy A Real Fan. These USB toys often draw more power from the USB Bus than the USB Specification allows -- While I doubt it would kill your PSU it could eventually damage the USB controller's power circuitry. Remember: USB is a communication bus that happens to provide power, not a wall socket that happens to provide communication. – voretaq7 Dec 8 '11 at 17:47
@voretaq7 Ahem: – Chris S Dec 10 '11 at 3:59
@ChrisS OMG WANT! – voretaq7 Dec 10 '11 at 6:06
@ChrisS Oh yes... I must have it. – Driftpeasant Dec 10 '11 at 15:53

PSU's have an average smaller lifetime span then hard disks. And we all know how much hard disks like to blow up due to moving parts, etc.

Various random things like humidity, too much dryness, stressful power drains, or unstopped surges can all make a PSU go "POP".

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Even in "legacy" mode... power should not be applied to USB when the PC is off. That's probably some sort of bad design by the motherboard manufacturer and/or power-supply manufacturer. More than likely... the power-supply failed (due to surge... cheap components... bad design... who knows) and since the 5v line was still getting juice... that bit got fried. Honestly... in my experience... if the PSU dies... and ends up cooking anything on the motherboard... I would throw the motherboard away & start from scratch. I've never seen a motherboard not misbehave afterwards. (random reboots/lock-ups/glitches etc...)

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Many MBs keep power to USB devices for Wake on USB Dev (keyboard "power" buttons, USB NICs, etc). – Chris S Dec 8 '11 at 16:10
I defer to your knowledge on that one. I forgot about keyboard "power-buttons" ... I never use 'em. – TheCompWiz Dec 8 '11 at 16:23

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