# starting up a salvaged PC power supply



## Spooky Dave (May 12, 2011)

Howdy all,

I've already got another question about the PC salvage process. Hope that's okay! 

So I managed to test a bunch of abandoned PC's from my work and found that most of the power supplies were good. Score! Now that I've removed them from their towers, however, I'm having trouble turning them on. Note: these don't have an on/off switch on the supply. Here's what I've done so far, please somebody tell me if I'm already off base:

Per the comments from another thread on power supplies, I followed one poster's advice to connect the green wire with one of the black (ground) wires. This I've done, but still no juice when I turn it on.

I also read in the power supply tutorial that most (if not all) PC power supplies will not turn on without a current load. The tutorial suggests wiring in a 10 ohm 10W sand resistor to the +5V line in order to provide this current load. My local Radio Shack doesn't have that resistor. I can buy one online (http://www.radioshack.com/product/index.jsp?productId=2062291) but I wondered if there was another way.

How do the rest of you do this? My understanding is that PC power supplies are gold for haunting, but I'm new to them. Down the road I'll have various props, lights, and such to pull current, but I'm looking for how to turn the thing on in the first place for testing and such. Any advice would be greatly appreciated.

Thanks as always, and haunt on!

Dave


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## stagehand1975 (Feb 22, 2010)

Check out this link. There is a ton of info all around about these power supplies.

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


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## HomeyDaClown (Oct 3, 2009)

That usually happens with ATX type PC supplies.

Pretty much everything you need to know is here:

http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

If what you've got is an ATX type, there are some differences here:

http://web2.murraystate.edu/andy.batts/ps/DESIGN_GUIDE.HTM


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## Spooky Dave (May 12, 2011)

Hi again, yeah that first link is the one I've been following, the one where the author suggests using the 10 ohm 10W sand resistor. Sounds like that's the main way to go, then? I'd just wondered, as it sounds like a lot of people use PC power supplies, if that's the main method, or if there are others.


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## stagehand1975 (Feb 22, 2010)

10 ohm is a common size. You can vary Way from it slightly because it is just a dummy load.


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## stagehand1975 (Feb 22, 2010)

Somewhere other than the link we posted is another detailed how to but i dont where it was


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## Spooky Dave (May 12, 2011)

stagehand1975 said:


> 10 ohm is a common size. You can vary Way from it slightly because it is just a dummy load.


Yeah, I can find lots of 10 ohm stuff, but it's all small. 1/4W, or 1/2W at biggest. It's the 10W I can't seem to find anywhere else but that online link, at least not in 10 ohms.

Success, however! I read closer and found that I can run an automotive signal bulb. This I've done, and it turned on the power supply, but that bulb sure heats up in a hurry. I'm thinking I will still need to grab those sand resistors, unless anybody else knows a better solution?

Thanks, one and all.

Dave


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## Jaybo (Mar 2, 2009)

The resistors are the best way. Those suckers heat up also, so mount them to the metal frame inside the PSU to help dissipate the heat. Don't worry about installing a switch, like some of the tutorials show. You can just plug them into a cheap power strip and use the switch on the strip to turn them on and off.


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## Moon Dog (Oct 3, 2006)

http://www.gideontech.com/content/articles/196/1

http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply


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## HomeyDaClown (Oct 3, 2009)

Actually a better solution to using either a bulb or high wattage resistor would be to use a fan. More air flow across a power supply is a good thing, heat is always bad.

The bottom line is that the ATX power supply specification requires a "Minimum Load Requirement" to be met before
the power supply turns on and then again works at full design specs. 

The "Minimum Load Requirement" varies between power supplies and manufacturers but normally falls somewhere between 250 milliamps (¼ Amp) and 1 Amp on the +5 volt rail. You may be able to "Turn On" the supply below this point but the supply will not work at full efficiency since switching supplies like to work under a load. So you may not get full voltage or current out. The 10 Ohm 10 Watt deal is just a cover all hack to put a ½ Amp load on the +5 volt side that generates heat and wastes power. Some ATX supply specs have a very low "Minimum Load Requirement" and may work with a much smaller resistor but resistors all do the same thing, consume power and give off heat. You can usually look up the manufacturer specs on-line to find out what your supply needs.


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## ctmal (Oct 17, 2009)

Here's what I use. Not sure if it's what your looking for though.


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## Spooky Dave (May 12, 2011)

Hi Homey,

That all makes good sense to me. Since I have all these old PC towers anyway, it seems I could just grab one of the extra fans there. I'll have to let you know how it goes.

That automotive signal light was mighty hard to solder too, and it heats up quicker than a 3-dollar pistol.


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## HomeyDaClown (Oct 3, 2009)

Let everyone know how the fans work out and how you wired them up.

A cooler power supply can always push a little more current than a hot one.


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## Spooky Dave (May 12, 2011)

Okay, so I followed Homey's tip and tried hooking up one of the PC fans from the same tower I salvaged the power supply. It didn't work for more than half a second. As best my research can tell, one fan draws around 1/4 amp to 1/2 amp. 

So, I figured it wasn't drawing enough current. By way of clarification, this was a fan that plugged into the motherboard. It had a black, red, and white wire. The red indicates that it needs 5V. 

I then pulled the other big fan I could find from the tower, the one that cools the whole tower (it sits just below the power supply). This particular fan has a black, white, and yellow wire (yellow = 12V). I figured hey, maybe this one would pull enough juice. I wired it up to the 5V line, and no dice. Same result, only half a second or so with power before the PS shut off. I still hadn't reached a minimum load.

Then I figured hey, worst I can do is blow something up, so I hooked up both fans at once. Voila! That did the trick. The power supply stayed on and I was able to test the voltage on all the lines with my multimeter. Everything behaved as it should.

As for wiring, I admit I didn't know what to do with the white wires. In my research on PC power supply schematics, white stands for -5V, but it doesn't exist in power supplies any more. Sure enough, this particular power supply didn't have white wires. The white wires on the fans plugged into the motherboard. Again, they're all from the same tower.

So, I connected the red wire from the 5V fan to a red wire on the PS, then I connected the yellow wire from the 12V fan to the same red wire on the PS, then I twisted the two black wires (ground) with one of the black wires on the PS. I left the white wires completely alone, and everything seemed to work.

This seems to be a viable option for power supplies, and instead of needing to find a way to dissipate heat, I now have additional cooling. And best thing of all, I didn't blow anything up! 

Thanks, Homey

p.s. CTMAL, sorry, but I couldn't open those files as I wasn't a member of the forum...


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## ctmal (Oct 17, 2009)

In case there's any interest, here's the files. This is a board that you can simply plug the ATX connector from the power supply to the board and it gives you a 3.3v connector, a 5v connector and a 12v connector. There is also a switch connector. If you don't want to add a switch(i.e. just want it to turn on as soon as the power supply is plugged in) you'd just need to connect a wire accross the switch connector.

Umm...I'm not seeing a way to attach files...if there's any interest let me know and I'll post the file on my website so you can download it.


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## ctmal (Oct 17, 2009)

I went ahead and uploaded the file to my website.

Here it is.


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## Spooky Dave (May 12, 2011)

Thanks ctmal, downloaded the files. I may have some additional questions for you down the road, as that sounds mighty handy! 

Dave


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## ctmal (Oct 17, 2009)

Spooky Dave said:


> Thanks ctmal, downloaded the files. I may have some additional questions for you down the road, as that sounds mighty handy!
> 
> Dave


No problem. Just let me know how I can help.


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## Spooky Dave (May 12, 2011)

One more update for anybody following this thread. I was mistaken when I said that my power supply didn't have any white wires. It had one, and only one. It was sort of lost in the shuffle, and I noticed it only once I started sorting out the colors and grouping them accordingly. 

I tried hooking up only one fan, using the white wire this time, and it only ran for half a second. So I think it still needs the two fans, and the two fans don't seem to require the white wire to run. Somebody please feel free to chime in if they have a more informed answer. Thanks!


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