uC Hobby has a demonstration of a simple way to hook up a small electronic project directly to your PC power supply. WARNING if you do something wrong you can do permanent damage to your computer. Another option is if you have an old computer power supply laying around you could convert it into a lab power supply.
“You can hack some parts you probably have lying around into a cable that will let you power electronic projects from a PC. First let me say that this is dangerous. Your PC power supply should have protection from shorts but you are sure to loose any open files not if, but when you make a mistake on your breadboard. Also the PC power supply can output a great deal of current so things can get out of hand quickly. In fact, the more I think about this, the more I realize it is a bad idea.”
Power Projects from your PC
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An old AT style power supply is great. It doesn’t require a PC to run, as they have a switch, and they can be found very cheaply.
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It’s a bad idea direct from the computer you are using but it’s a great idea to reuse old power supplies.
It’s easy to power on an ATX supply, just ground pin 14
It has a non-standard pinout.
http://pinouts.ru/Power/atxpower_pinout.shtml
On some types of uses you will need some high capacity electrolytics on your project because switchmode supplies do not like load changes.
You can also have your circuit monitor pin 8 and not apply the load until it gets the PWR_OK signal.
+5V_SB is the constant on standby voltage that can only provide 10ma, enough to drive a transistor to turn on the power supply.
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While the idea of using a PC power supply for another purpose is OK, users should be aware of the large amount of current available.
A typical bench supply has adjustable current (e.g. 0.1-2A)limiting making breadboard experimentation relativly safe.
Whereas a typical 500w ATX-PSU 5v output, can supply 35A; enough to turn a minor short circuit into a fire.
Proceed with caution!
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Bill is correct, computer power supplies will supply whatever current the load requires up to their overcurrent or short-circuit cutoff/shutdown. An overcurrent condition in your circuit would may not cause an overcurrent condition in the power supply and your circuit could easily generate enough heat to burst into flames. Always use properly rated fuses to limit current to your projects or use a professional power supply with built-in current limiting.