3-pin PC case fan -- power from alternative source?

[Jonathan Gilbert]

I have a fairly nice fan in my possession, a BitFenix Spectre Pro 230mm. It is a standard PC case fan, using the 3-pin connector that allows the system to sense the fan speed. My understanding of the pinout is that the first two pins are ground and +12VDC, and the third pin is connected to a Hall effect sensor to feed back information about the fan's rate. I have an application where I want to power this fan without a full PC power supply & motherboard (which is where the 3-pin connectors are typically found, of course). I have a wall wart that puts out 12VDC and is rated for well over the expected current draw of this fan. I have verified with a voltmeter that it is functioning just fine and putting out the expected 12VDC. But, when I hook up the wall wart to the pins on the fan's wire, it does not turn at all. In fact, doubting myself, I tried every possible polarity of every possible pair of pins, and got no results with any of them. After doing this, I plugged it into an actual motherboard and the fan immediately span up.

What am I doing wrong? Is it possible the fan's circuit is designed in such a way that something has to be connected to the Hall effect sensor or it won't operate? Is the motherboard giving the span a special extra spike of voltage just to kick things off that the wall wart wouldn't be doing? What do I need to do to power a 3-pin case fan from a non-standard (but properly-rated) power source?

 

[kellys_eye]

Sounds as if that fan is operating on a PWM (pulse signal) from the mobo. Unless you replicate it you should just purchase a simple 12V fan without the 'third wire' - far simpler and cheaper.

 

[HellasTechn]

Tipically a black wire is the ground and a red wire is the +12v for the fan then a yellow or blue is the feedback to the motherboard. if that doesnt work then most probably its what Kellys_eye suggested.

One other thing is that you dont really need a wall wart. Your PC PSU will supply +12V on any yellow wire and you can grounf it through any black wire. In other words get a simple 12V fan and connect it on any yellow and black wires of your PSU.

 

[Harald Kapp]

As far as I can see a 3 pin pwm controlled fan would be a most unusual beast. Typically pwm control is reserved for 4 pin fans.

I have verified with a voltmeter that it is functioning just fine and putting out the expected 12VDC

Does it output 12 V when loaded by the fan, too? I suspect that the output voltage of the wall wart dowesn't reach the required amplitude when loaded by the fan.

Is it possible the fan's circuit is designed in such a way that something has to be connected to the Hall effect sensor or it won't operate?

The number of possibilities is endless. Likely? No!
Unfortunately the Bitfenix homepage doesn't provide much information.

 

[WHONOES]

The fan could be using a brushless motor which is quite common these days. Normally the fan unit will have all the control circuitry built in. It sounds as if yours may indeed need some sort of pulse train on the extra lead with some sort of PWM. You could try driving it with a function generator set to square wave and a few hundred Hz and see what happens.

 

[Harald Kapp]

From what little information is available on the web, it doesn't look like this fan requires any special signals. It seems to be a common 3 pin fan.

 

[Alec_t]

Perhaps the fan speed is set by a voltage (or average voltage, if PWM derived) on the third pin.

 

[WHONOES]

You are right Harald. I've just had a look on their website and as you say there is not a whole load of information available. It does seem that the fan requires just the 2 connections and current consumption is reasonable at 380mA and will start turning on as little as 5V. I suggest that Mr Gilbert connects the fan to his wall wart power supply whilst monitoring its output volts to see if it holds up at power on.