Computer Power Supply- How to use for hobby supply?

I got an ATX computer power supply (HEC-250AR-T) that I wanted to use as a fixed voltage power supply at my workbench.
I plugged it in, pressed the on switch, and- nothing. The fan did not
come on and there was no voltage on the output.
What has to be done to get it to work? Is there a jumper that has to be put on one of the wire sets? Is there a web site? Google did not help me; maybe I just did not use the right search words.
Thank you.
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Connect the green wire (PS_ON#) to ground and your ATX power supply will come on.
These power supplies are very sensitive to having their outputs shorted to ground. If you so much as brush one of the DC outputs across a ground, the PS is fried. I have gone through about five this year. There may be a simple fix for this, but since these are nearly free for me I just trash them.
I also have problems using these power supplies for my steppers. I think the energy from switching the steppers on and off gets conducted back through the power supply and affects the 5v logic side of the supply voltage to the stepper driver. My plan is to use the ATX power supply for logic only, and get a nice low voltage high current linear unregulated supply for the steppers. I am driving a CNC milling machine, so when the steppers get extra pulses I loose position and have major problems.
- James B

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Hi Ron.
ATX supplies use a POWER ON signal from the motherboard to turn on. The +5VSB voltage is available whenever the power supply is plugged in (and the switch on the back is turned on), and this powers some logic on the motherboard that monitors the momentary power button on the PC case. When you press that, the motherboard makes the POWER ON signal active, and the power supply starts.
I think the POWER ON signal is at TTL levels, but I do not know if it is active high or low.
Jeff.
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If I were you, I'd steer well clear of computer power supplies for anything except driving a computer.
The big problem with them, other than the fact that they're not adjustable and lack the useful ammeters and voltmeters you find on a laboratory power supply, is that they are switch mode supplies. This means that, rather than using a bog standard isolating transformer, bridge rectifier and some other simple, rugged components, they use a complex mess of high frequency circuits and inductors to regulate the output.
The principle is still a bit of a mystery to me, but it's something to do with applying raw mains (or whatever you're supplying it with) to the inductor and switching it on and off at the correct rate to produce the required output. Some models apparently don't even use an isolating transformer, which means that if they do fail, you might end up with mains voltage on all the output lines. Definitely not good. Of course, if they fail, they might also dump mains into all their internal components, and can sometimes explode quite violently if they go wrong (that's the cheap ones - better models have safety cut off devices)
Another reason they're useless for robotics applications is that they must never be used to drive inductive loads, like any kind of motor, including steppers. This is because the added inductance buggers up the power supplies' internal inductor, and causes bad things to happen. Shorting them out has similarly catastrophic effects.
They do have some advantages, though. The best (and one reason I do plan to use a similar device for some very specialised circuits in my own robot) is that, because of the charge-pump way the inductor works, they can easily produce output voltages greater than their input voltage, or with opposite polarity, and operate with almost any input, ac or dc. There isn't really any other practical way of doing this. Second, they are very small compared to conventional power supplies because they don't need a massive transformer (though, as mentioned above, this can lead to added mains hazards).
The charge-pump effect also means that they could theoretically allow a battery to continue to supply it's specified potential when the emf of the cells themselves has dropped much lower than this value, so the system could run for longer without recharging (though certain types of battery might be permanently damaged by this).
Finally, on PC switch-mode supplies, there is a signal sent back into the supply from the motherboard to shut the supply off immediately if it produces an incorrect output, to prevent damage to the motherboard. If you still plan to use the supply, you'll need to find a way to provide this signal.
Hope this helps.
Tom

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Surely that depends on wether you are stepping up or stepping down. The inhouse design for our equipment at work steps down to 5V and 13V.
Dave
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