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Smoke from the PSU

Yesterday I received two new machines from DOLA on-line auctions [1]. I decided to use the first to replace the hardware for my SE Linux Play Machine [2]. The previous machine I had used for that purpose was a white-box 1.1GHz Celeron and I replaced it with an 800MHz Pentium3 system (which uses only 35W when slightly active and only 28W when the hard disk spins down [3]).

The next step was to get the machine in question ready for it’s next purpose, I was planning to give it to a friend of a friend. A machine of those specs which was made by Compaq would be very useful to me, but when it’s a white-box I’ll just give it away. So I installed new RAM and a new hard drive in it (both of which had been used in another machine a few hours earlier and seemed to be OK) and turned it on. Nothing happened, I was just checking that it was plugged in correctly when I noticed smoke coming from the PSU… It seems strange that the machine in question had run 24*7 for about 6 months and then suddenly started smoking after being moved to a different room and being turned off overnight.

It is possible that the hard drive was broken and shorted out the PSU (the power cables going to the hard drive are thick enough that it could damage the PSU if it had a short-circuit). What I might do in the future is keep an old and otherwise useless machine on hand for testing hard drives so that if something like that happens then it won’t destroy a machine that is useful. Another possibility is that the dust in the PSU contained some metal fragments and that moving the machine to another room caused them to short something out, but there’s not much I can do with that when I get old machines. I might put an air filter in each room that I use for running computers 24*7 to stop such problems getting worse in future though.

I recently watched the TED lecture “5 dangerous things you should let your kids do” [4], so I’m going to offer the broken machine to some of my neighbors if they want to let their children take it apart.

3 comments to Smoke from the PSU

  • Stupid me, I read the headline as “smoke from PSU” and clicked on it in aKregator wondering “OMGWTF is going on in downtown Portland? Did the homeless revolt or something? Why haven’t we fed them to the hungry yet?

  • martin

    I’d been told that PSUs get most stress when started. So maybe it just broke because of that and not because of anything else beeing faulty.

  • etbe

    martin: True, spinning up the hard disk when it’s cold can take a significant amount of energy (more than spinning up a disk that has been parked for a while due to viscosity in the lubricating oil). This problem did occur on a cold morning after the machine had been turned off overnight. Also when it initially starts it has to charge all the capacitors that surround the CPU which may impact the current draw.

    It seems quite unlikely that a machine with a default configuration (no extra PCI cards and only one hard disk) when starting will use as much power as a fully configured machine running under load (think about having two hard drives and a CD-ROM drive doing seeks, three PCI cards doing stuff, and 100% CPU use). So if a machine dies under load when starting with a default configuration it seems most likely that it would also have died in normal operation if it had the maximum set of extras added to it.