# How to determine the wires on a computer power supply



## Darkmaster (May 9, 2009)

I have a couple computer power supply units. What voltages are all the wires?
Thanks


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## bw1 (Apr 12, 2008)

Get a volt ohm meter and start testing the wires. The black wires are most usually the negative and other colored wires are the positive. The power supply I'm looking at right now provides a +5 and +12 volts and a -5 and -12 volt output. The positive voltages usually have a high current output the negatave outputs usually have a very low current output. There should be a lable on the powersupply that tells what the voltage and current outputs are. Then all you have to do is get your meter out and find out which wires are what voltage


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## hedg12 (Jul 6, 2008)

Most I've seen are the same - black is ground, orange is +3 volts, red is +5 volts, and yellow is +12 volts. White's -5 & blue's -12, if you need them. I've only seen one out of hundreds that wasn't this way, & it was pretty old.


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## fritz42_male (May 5, 2009)

Computer power supplies are complex units and many will not start up unless they have a certain amount of load.

They are what is called a switching power supply and have very high voltages internally that can kill - unless you know exactly what you are doing then stay away.

That said there are some excellent online sources for modifying the units. Try here:

http://web2.murraystate.edu/andy.batts/ps/powersupply.htm

Alternatively, there are some excellent sources for power supplies. Try dealextreme.com


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## Hobie14T (Apr 8, 2009)

Thanks for the link Fritz.


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## psyko99 (Jan 11, 2008)

Try this site:
http://pinouts.ru/pin_Power.shtml

Also, check out this how-to on Wiki-How:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply


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## joshua17ss2 (Jan 10, 2008)

all you should need to do to get power from a computer PS, is short the green wire to a ground (black) 
ive used power supply's to run control boards and leds for a long time, and they work perfectly. 
you can just install a switch between those 2 wires and be able to switch it on and off


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## Dead Things (Apr 4, 2009)

Since many solenoids, wiper motors, leds, etc run off of dc power, and can have varying volt requirements, although 12 seems to be common, how many items can powered from a computer ps? Is it a matter of watts? I know that the cps can have a lot of amps , up to 12 if I'm not mistaken.


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## hedg12 (Jul 6, 2008)

It's a matter of how many amps at what voltage. (So yes, it's a matter of watts.) Most PC power supplies will list the amperage rating for each voltage on its label. As far as how many devices you can run from one, it depends on the devices you're running.


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## Dead Things (Apr 4, 2009)

hedg12 said:


> It's a matter of how many amps at what voltage. (So yes, it's a matter of watts.) Most PC power supplies will list the amperage rating for each voltage on its label. As far as how many devices you can run from one, it depends on the devices you're running.


I was thinking of running 12 LED spots, a couple of sets of LED eyes and a microcontroller with 4 servos off the 5V leg and 6 or so 12V solenoids and/or a couple of wiper motors off the 12V leg. Is there a way to calculate this? (stupid question, I'm sure there is)


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## Allen H (Feb 13, 2010)

Most LED spots are 20 ma (milliamps) so 12 of them would take 240ma
the eyes if you have 3 sets 120ma
Im unsure of what controller you are using so there is no way to know the rest
The wiper motors sound feasible and it sounds fine to me.
Sounds like it will go to me. but you may want to protect the microcontroller with a resistor. Use the number of LEDs your using to tell you how many amps you have left and then find out what the microcontroller needs. do the quick math and add a resistor if you need to. thats how I would do it.


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## hedg12 (Jul 6, 2008)

Dead Things said:


> I was thinking of running 12 LED spots, a couple of sets of LED eyes and a microcontroller with 4 servos off the 5V leg and 6 or so 12V solenoids and/or a couple of wiper motors off the 12V leg. Is there a way to calculate this? (stupid question, I'm sure there is)


Without knowing the current draw of each item you're connecting, we can't say with any certainty if it will work or not. I'm fairly sure you'll be OK with the LEDs and microcontroller, but some solenoids draw a fair amount of current. I've never measured the current draw of a wiper motor, but according to Scary Terry they can draw 14 or 15 amps at stall. That's more itself than a 300 watt PC power supply puts out. Under normal conditions, one probably draws no more than 3 -4 amps. A 5 amp 12 volt fuse in line with the wiper motor will protect your power supply in case something goes horribly wrong with your prop and the motor gets in a bind.


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## Dead Things (Apr 4, 2009)

Thanks folks, appreciate the help. Do I add up the watts of all the items I want to use, 3V, 5V and 12V and compare that to the CPS (300 watts)? I might put a fuse in line on all the legs just to be safe.


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## hedg12 (Jul 6, 2008)

You're better off adding up the amps drawn at each voltage, and comparing them to the ratings for those voltages on your power supply.


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## randyaz (May 26, 2006)

Deadthings, here's some good reading with the equations to determine the numbers your looking for. You can use a multimeter (one that has amperage function) in series with your circuit to find the current drawn on each leg. Then do the math to see what you can run on 300W.

http://science.howstuffworks.com/question501.htm


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## Dead Things (Apr 4, 2009)

Appreciate the help, guys


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