I had an old computer that didn't work anymore, so I took the CPU fan out to see what I could make with it.

The fan is NMB model number BG0903-B044-VTL, like this. It has three wires coming off of it, red, black, and white. I know that the white wire is usually a speed sensor. Does this wire need to be connected to something in order for the fan to run continuously?

Right now when I apply power to the red and black wires (from a 9-volt battery) the fan will spin very briefly, and then slow down until it stops. The fan's motor only spins in the instant that the power is turned on, but doesn't continue, even though the power is still applied. How can I get the fan to spin continuously?


Well, you do need 12V for the fan to really kick. Check the image on the link you provided again :)

Anyways, feeling adventurous? Here it is:

Instead of fiddling around the motherboard you can splice the fan wiring with a free 4 or 3 pin molex adapter coming of your power supply unit.

There's 2 ways. I'm explaining the more difficult and leaving a note on the easier:

On the fan:

  • Remove the isolation of the end of both the Red and Black wires, thus exposing a little of the metal wire beneath.
  • Bend the tip of the exposed wire, producing a tiny U shape.
  • Clean and isolate the white wire. You won't be needing it.

On the PSU: (assuming a 4 pin molex)

  • Clip the wires on one of the free molex adapters, removing the adapter entirely. These things are becoming useless these days.
  • Isolate the Red and one of the two Black wires. It's the 5V cable and one ground. You won't be needing them.
  • Removing some of the isolation from the end of the remaining yellow and black wires (12V and ground).
  • Bend each of the tips of the exposed metallic wires into a small U shape.


  • Hook up the fan Red wire with the PSU yellow wire.
  • Hook up both black wires
  • Cover each with electrical tape

So you just hooked your first 3-wire fan to a 4-wire molex. What's easier than this?

  • Use a 3-pin Molex instead, hehe. Just don't forget. On your PSU all yellow wires are 12v and red wires are 5v.
  • Go to an electrical shop and buy pins that fit into your molex. In this case you can attach the end of your fan wires to these pins, wrap up in electric tape for extra firmness and simply attach the pins to the molex on the right positions (as above). You saved yourself removing a molex.

Finally, what you have been waiting for: As for our motherboard

Read the instruction manual and check the available connections. You are after a 3-pin connector on the motherboard with the following setup: Signal-12V-Ground. (In this order I believe. You can read signal or CHA_FAN_SPEED on the manual. Mine reads "signal")).

It's harder to connect to the motherboard as you may guess. These connectors are small and it's tough to securely attach your wires to them without a) buying a adapter yourself or b) go Rambo on it and solder the thing.

Have fun!

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  • Yikes! A 3-pin fan plugs into a 4-pin fan header no problem. A 4-pin fan header has the shroud on once side of 3-pins only, this is so the 3-pin fan header can plug right in and have the correct orientation. No splicing needed, but you won't get any fan control either. – hanleyp Oct 18 '09 at 3:38
  • Yup. It was my guess too. But wasn't sure and was already feeling too lazy to get the manual out again and decided to not speak of the 4 pin motherboard connector just in case. – A Dwarf Oct 18 '09 at 4:05
  • Incidentally, All the splicing talk, I thought I made it obvious, was just for the kicks. Just something else he can be aware of. Especially if, as it may be the case, his fan wires aren't attached to an adapter anymore. – A Dwarf Oct 18 '09 at 4:12
  • Good catch, he was trying to power it by a 9V battery, so the wires are most likely exposed. – hanleyp Oct 19 '09 at 12:09

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Wire #3 is just a TACH sensor to tell the computer the fan speed (converted to RPM by the computer) and doesn't need to be hooked up.

The fan in the picture linked is a 12 V, 1.34 A fan. Doing a quick search of the internet, I see that a 9 V battery can supply from 100 mA to hundreds of mA, but not 1340 mA, so you're going to need a power supply that can supply more current to get this fan moving.

I did some tests on a 12 V fan, and although not in their specifications, I could lower the voltage to about 5.5V before it couldn't restart when I stopped it with my hand.

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Look inside the exhaust port of the fan. You will see a small blue or green thermistor. This controls the speed of the fan. The fan starts slowly and as the air coming out of it warms, the fan will increase in speed. As the air again cools, the fan will slow.

This is done so 1), it makes less noise, and 2), it draws less current when it's not needed. Pretty nice feature if you want to cool something using a 12 V battery. I use them to cool the heatsink on Peltier modules. As the demand goes up, the fan increases speed and cooling. The red and black wires are + and - 12 volts DC, the white wire is for the tachometer output. You can ignore the white wire if you don't need feedback from the fan concerning its actual speed.

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  • 1
    I don't seem to recall seeing any temperature controlled fans. I'm under the impression that most fan speeds are controlled by the voltage put through them (on 2/3 wire fans) or PWM (on 4 wire fans) – Journeyman Geek Jun 15 '12 at 5:35
  • Most, yes, but not this one. I was every bit as surprised when I opened up mine to find the thermistor, but there you have it. – javawizard Jul 15 '14 at 3:09

I have this exact same fan, and I had the exact same problem. Sean is right - if you open up the fan (remove the lone screw, then the cover just pops off) you'll see a little blue or green thermistor. Clip it off, then solder the two pins together - from then on, it'll run full blast 100% of the time.

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