Power for a computer fan

I have a few case fans lying around that I would like to re-purpose, but I don’t know how to power them. I have an unused computer power supply, but that seems like overkill, and I don’t know how to make it turn on without being plugged into a motherboard.

What could I do to provide permant power to a standalone case fan, or switch on a computer PSU that isn’t connected to a computer?

I am terrified of electricity (and kraken, but that’s a different thread), so I won’t directly tell you how to get the PSU working without a motherboard.

But if you look online, you’ll easily find resources on how to test a PSU with a multimeter (side note: spending a few bucks on a PSU tester is a great investment, particularly if you don’t know a multimeter from a multigrain sandwich loaf). To do so, you need to jump certain pins in the PSU cable so it thinks it’s attached. Or so I think it was before I got a tester.

Case fans are generally 12vdc units, but will often run (slower) at lower voltages. So most any dc wall wart transformer could be used to power them. Or even a 9v battery.

A power supply would work too. You need to short one of the lines in the 20-pin connector to ground. IIRC it’s the only green wire, short to any black. Look that up, though.

A PSU is overkill. Case fans are DC powered, so you can run them off of any power supply you might have lying around from old cell phones or other small electronics. They’ll almost always have their voltage output printed on them. Fans are almost always 12V, so a 12V power supply will run the fan(s) at full speed, a 9V supply at about 75% speed, and a 5V power supply (cell phones, etc.) at about 40% speed. You’ll want to connect the red wire of the fan to the positive wire from the power supply, and the black wire to the negative wire. The yellow fan wire is for speed sensing, and should be left unconnected for your purpose.

On preview, I see Gorsnak answered more quickly and concisely. Oh well, I’m tired.

I wouldn’t use a power supply. PCs use a switching power supply, and many switching power supplies need at least a 5 to 10 percent load or else they won’t regulate themselves properly. It is very common for PC power supplies to regulate themselves of of the 5 volt supply, so if all you have are 12 volt fans, the 5 volt supply 5 volts won’t be sufficiently loaded and the power supply may act very strange. It may shut itself down if it detects that it can’t keep the voltages in regulation. It may even destroy itself if it’s not a very well designed power supply (though a good power supply should protect itself from this sort of thing). With some of those el-cheapo Chinese jobs you never quite know what’s inside of them and how well they are designed.

To make an ATX style power supply turn on, you short the PS-ON signal to ground. PS-ON is usually green and there should be a convenient black ground wire right next to it, but I wouldn’t guarantee that every PC power supply out there uses this color scheme. Here’s a pinout of the ATX connector so you can double-check it.

If you use a wall wart to power the fans, make sure your wall wart can supply enough current for all of your fans.

I have plenty of wall warts lying around that can be sacrificed. How do I tell which wire is positive? Do they have a second, colored layer of insulation under the typical black?

Sometimes you can’t tell unless you check it with a voltmeter.

Sometimes the positive wire will either be colored or will have ridges on the insulation to distinguish it from the negative.

There’s no law that says the marked wire is always the positive wire. Double check it with a meter to make sure. If it’s a “tip negative” type of wall wart the marked wire might very well be the negative wire.

Most often though, if one of the wires is marked, it’s the positive wire.