delirium happy

Just keep on trying till you run out of cake

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delirium happy
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And I also meant to say but forgot:

The old non-working PSU which I took out of here carries a big warning on it saying "do not dismantle! trained service personel only! doing so may cause destruction of the known universe! etc!" (or words to that effect). Can anyone tell me if this is just because most people would be likely to fuck things up if they did take one apart, or if it's because they contain 96MF capacitors which hold their charge for approximately umpteen-zillion years, and would thoroughly electrocute me if I went anywhere near them? My scientific curiosity is telling me to open the thing up to see what's in there and see if i can get any idea as to how it works, but my caution is telling me that I really don't feel like getting electrocuted. So what gives?

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By the way, even though I *knew* you don't live in London, I am relieved that you are all right.

I've had a shock from a dismantled PSU that had been left for a day or two - it wasn't enough to blow me across the room, but it certainly made sure I was awake and left me with a tingling in my hand for a minute or so.

It's a switched-mode power supply. Chops the mains at kilohertz frequencies in order to keep a zillion farad[0] capacitor charged, and regulates that DC down to a variety of friendly voltages.

In other words, the main dangerousness comes from a reasonably beefy capacitor or two, which is liable to remain charged for a while after being unplugged, although not if you try to run a computer off it while unplugged. There are also the secondary risk factors of live mains (when plugged in) and spinny fan blades[1].

I've taken the things apart many times (usually to replace the fan in an otherwise fully functional PSU - those AT style ones are getting hard to come by), and have in fact built a bench power supply (with the various voltages available on standard 4mm terminals) out of one[2]. Never been zapped by the caps, but have treated them with appropriate wariness, and have shorted them out with a screwdriver at the first opportunity[3].

I say take it apart. High voltage DC shocks hurt, but are unlikely to do you lasting damage unless you've got a heart condition or are liable to hit your head or fall off something when it happens. Needless to say it's not my fault if you prove me wrong :)

As for likely causes of failure, you're looking for electrolytic capacitors that are bulging or have ruptured and oozed brown goo, toast that used to be semiconductor, or crappy solder joints with no tolerance for thermal expansion.



[0] For small values of 'zillion'

[1] Readers may scoff at that. They probably have shortish hair.

[2] The trick here is a) faking the ATX switching and b) providing a dummy load big enough for the regulation to work properly, these things aren't designed to run unloaded. a) is trivial, unless the donor PSU is from an old Dell and contradicts every ATX specification you can find on the web. b) is best achieved by a couple of beefy power resistors across the 12V, 5V and (depending on fussyness) 3.3V lines. Cooling left as an excercise for the metalworker.

[3] Standard procedure when working on CRTs is to discharge the anode by shorting it to chassis ground with a pair of beefy screwdrivers, as tought to me by TV repairmen. Probably not good for the caps, but once or twice won't hurt, and they're more replacable.

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I agree with Kim - I'm no electrician, a total dummy compared to Kim - but I've taken apart PSU's a few times, but always kept clear of large looking caps - you should be OK if you keep your hands well away from them....

What people say. Be careful. If you're going to have to touch something test it with the back of your hand first (the hand will close up if you get a shock, which will pull it away if you use the back, but grip harder if you use the front).

And enjoy yourself :-)

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