Blown power supply from computer what would cause the NTC to fail (violently?) - Page 1
Forum section: electronics / beginners

author: evglabs
author_rank: Contributor
timestamp: May 03, 2020, 10:17:00 pm
content: So, an old computer I have wouldn't turn on and for the heck of it decided to see if I could fix (knowing that it would be a looong shot and mainly just for learning) and found the fuse had blown.So, I replaced the fuse and as expected the new fuse blew. So for the purposes of science! I replaced the fuse with a wire. Then the NTC threw out a big spark and cracked.I fully expected not to fix this, as I said just for learning purposes. But I have a couple of questions...Am I right in assuming that whatever the original failure (disregarding any new damage I caused by messing around with it) would be no further down the circuit than the NTC?And what would be a probable cause for the NTC failure? I'm again assuming that there was a short earlier in the circuit that allowed too much current to flow to the NTC.

author: tunk
author_rank: Super Contributor
timestamp: May 03, 2020, 10:28:24 pm
content: I had a wall wart where the bridge rectifier shorted ...

author: evglabs
author_rank: Contributor
timestamp: May 03, 2020, 11:23:58 pm
content: That's what I was thinking but the multimeter seems to say it's working correctly.

author: Cubdriver
author_rank: Unknown
timestamp: May 04, 2020, 03:55:40 pm
content: Some failure is causing the supply to try to draw too much current - it could be a shorted rectifier, cap, switching transistor, many things.The fuse blew to protect the mains wiring and prevent the device from catching fire - remember - the purpose of a fuse in this sort of application is to protect the wiring feeding the device, and to prevent the device itself from becoming a ball of fire if something internal fails (as has evidently happened here), not to protect the device itself. When you bypassed the fuse with a piece of wire and powered it up again, then the next weakest link in the chain, which in this case is the NTC, failed catastrophically as it is being fed line voltage while likely looking at effectively a dead short in the power supply.-Pat

author: Gyro
author_rank: Super Contributor
timestamp: May 04, 2020, 04:33:56 pm
content: Please donotcontinue your current fault finding strategy by trying to jumper out the NTC! You are heading towards the point of serious consequences - ejection of pieces of device packages, potentially blinding you, and/or catching fire.You are currently causing electrical failure of more and more components, starting from what was probably a relatively minor fault originally.Never bypass a fuse!

author: Kilrah
author_rank: Unknown
timestamp: May 04, 2020, 06:30:20 pm
content: Are you sure it's being run at the correct voltage, and the NTC is an NTC and not a MOV?It's common to have a MOV across the mains to catch transient spikes, but if you connect 230V to a 110V PSU it'll exceed the MOV threshold and cause a dead short on the mains...

author: SeanB
author_rank: Super Contributor
timestamp: May 04, 2020, 08:30:17 pm
content: Switching power supplies, common shorted items are the mains bridge rectifiers, either one or all of them and very common shorted switching transistors, though if you have replaced the fuse with the 100A version, they will have blown themselves open by now, leaving a crater in the case. After that capacitors failing, and then resistors going open circuit or high.

author: evglabs
author_rank: Contributor
timestamp: May 06, 2020, 12:38:20 am
content: Quote from: Gyro on May 04, 2020, 04:33:56 pmThanks, I did this outside and plugged in the power well away from the power supply. I know it was fairly stupid, but heyAnd I'm not doing anymore with it. I just thought outside, away from any brush or anything flammable, might be worth a single test just to see what happens. And I'd never actually run it with the fuse bypassed, that's just too stupid, even for me.

author: evglabs
author_rank: Contributor
timestamp: May 06, 2020, 12:39:05 am
content: Quote from: Kilrah on May 04, 2020, 06:30:20 pmYeah, 120V here in America, and it was running for years with no problem. Just shut it down one day and when I tried to turn it back on, nothing happened. And that markings on the component say it is a NTC.