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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.

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    \$\begingroup\$ It likely is a tacho output from some hall sensor \$\endgroup\$
    – PlasmaHH
    Commented Feb 2, 2016 at 15:53
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    \$\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\$
    – Bimpelrekkie
    Commented Feb 2, 2016 at 16:00

3 Answers 3

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

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

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    \$\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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