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Power supply information

Using PC power supplies in things other than PCs

Before considering using a PC power supply in something other than a PC, you'd better read about overloading, crossloading, and minimum loading in PC PSUs. Grrrrrrr! PC PSUs are awfully picky about what kind of load they're willing to power. "Real" power supplies (those not found in PCs) are so much simpler. If there's anything weird about them then the manufacturer tells you. And usually there's nothing weird. There's one other difference between real PSUs and PC PSUs. The real PSU probably costs three times as much as a PC PSU at the same wattage. Or more. PC PSUs are designed to run just the kind of load you have in a PC. And the crummier ones can only run a certain range of PCs. Real PSUs are general purpose devices which try very hard to supply a dependable voltage regardless of the load. That's why real PSUs (at least the good ones) are much more expensive. PC PSUs are actually very impressive at delivering watts on the cheap and cramming it into a small space at a very low price.

So suppose that you build electronics projects and you would like to use one of those relatively inexpensive, diminutive PC PSUs rather than the larger, more expensive "real" PSUs. As long as you can deal with the PC PSU's persnickety loading requirements then it's easy enough to use one. The ATX power supply design guide gives the general specs. Keep in mind that cheapy PSUs don't always follow them to the letter. And lots of them don't follow the 240VA per rail limit.

The ATX connectors are easy to get. The 4 pin 12 volt, 8 pin EPS 12 volt, main 20 pin, and main 24 pin are all Molex mini-fit jrs. You can get those just about anywhere. The part numbers are on this page. Watch out for the 6 pin PCI-Express. It looks like a 6 pin mini fit jr. but it actually isn't. It's really a special, modified mini-fit jr. which is keyed differently.

ATX PSUs don't just fire up when you turn on the power switch. They have a TTL compatible input named PS_ON which is the green wire on the main power cable. The PSU is powered up only while you hold that input low. It's pulled up inside the PSU to make sure the PSU is off if it's not connected to anything. According to the ATX spec, the PSUs are supposed to be undamaged if you power them up with no load connected. It's never happened to me but I've seen reports from people who test PSUs that the occasional cheap ATX PSU has been known to kill itself if powered on with no load. My personal opinion is that PSUs which are that crappy deserve to die anyway. But if you want to use a real cheapy PSU then you'd better provide some minimum loads.

ATX PSUs also have the PWR_OK signal which is a TTL compatible output that tells you when the 3.3, 5, and 12 volt rails are above the minimum voltage limits. It's the gray wire in the main power cable. That one can be handy for generating reset. The purple wire is the VSB voltage. That one provides 5 volts even when the PSU isn't powered up but it's often under 1 amp. Don't depend on the -5 volt rail unless you've got an older supply. The ATX spec stopped requiring support for -5 years ago so new PSUs rarely come with it. -12 is still always implemented so you can use a three terminal regulator to create -5 if you need it. The positive voltage rails are supposed to be accurate to plus or minus 5% and the negative rails are plus or minus 10%. Some better power supplies guarantee voltages within 3%.

When you read the total wattage output numbers for your PSU, it's best to not take them literally. Many good ATX PSUs can actually deliver their claimed maximums continuously but many cannot. Apparently, many PC PSU makers have a different definition of "continuous" than the rest of us (and "real" PSU makers). It's best to give yourself a "PC derating factor" when pushing the wattage close to the max. You can see the carnage which results from running PC PSUs at full load for extended periods on this page. And those tests only included expensive (for PCs) PSUs. The cheaper ones tend to be much worse.

Other than the things listed above, PC PSUs are pretty normal. And they're cheap. And they're small. And they're picky about loads. But they really are a good way to go for electronics projects.

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Copyright © 2005 through 2007 by Mark Allen