I have an old 12V cooler master fan that I am converting into a fume extractor for soldering and am connecting it to a standard 12v computer power supply. I am trying to wire it and am confused about one thing.

On the fan's label it has a coding for the different wire colors (red, black, and white) and it says:

red +

black -

white FG

I have looked up FG on google all over the place but can't seem to find anything that says what it stands for. I was thinking that it might be "Ground" but then what would the black negative wire be? Maybe I just have a fundamental misunderstanding (pretty new to all of this electronics stuff)

I did connect the black wire on the fan to a black wire on my power supply and the red wire on the fan to a yellow wire on the power supply (white wire connected to nothing) and it turned on and seemed to be working fine but I want to make sure that this is actually a correct wiring and isn't going to cause any problems in the future.

  • 5
    \$\begingroup\$ It likely is a tacho output from some hall sensor \$\endgroup\$
    – PlasmaHH
    Commented Feb 2, 2016 at 15:53
  • 2
    \$\begingroup\$ ..which is used to determine how fast the fan is rotating. You can just leave the FG wire unconnected. That will not cause any problems. \$\endgroup\$ Commented Feb 2, 2016 at 16:00

3 Answers 3


FG stands for Frequency Generator (or Feedback Generator), it has an output with a frequency proportional to the fans speed. It is used by the CPU to determine the fans speed.

Some (older) fans have an extra winding internally and the FG signal is a sinusoid with both amplitude and frequency proportional to fan speed.

Modern fans almost exclusively use a Hall-Effect sensor and the signal is an open-collector square-wave signal where the frequency is proportional to fan speed. Peak voltage is determined by the magnitude of the power supply that feeds the pull-up resistor.

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    \$\begingroup\$ great! so just to be clear, if i'm using this just as a fan to pull soldering fumes out, i can safely leave the FG wire unconnected? \$\endgroup\$
    – celeriko
    Commented Feb 2, 2016 at 15:59
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    \$\begingroup\$ @celeriko yes. Leave it disconnected. \$\endgroup\$
    – Steve G
    Commented Feb 2, 2016 at 16:01

Be careful. I seen AC-DC power suppliers labeling "FG" for the frame ground. That means, attaching to the metal of the device.


I'm pretty sure that FG here stands for "floating ground", which is an isolated ground for the analog circuits inside the power supply.

  • 2
    \$\begingroup\$ It's a fan, not a power supply. See the accepted answer. \$\endgroup\$
    – Transistor
    Commented May 10, 2020 at 7:12

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