I'm picking up a few (3) 2-pin 12v fans to put into a isolation box I am making for my present computer. I want all the fans to run simultaneously but want to be control them via an external pot. The problem lies in how to control them all at once.

I haven't been able to find a single variable resistor that could control all three together which means I might have to wire them in parallel/series. I know the impedence would start changing drastically, but is it feasible if I use an external 12v power supply?

Use this config. You will want to have the pot plug in from positive to negative on the ends that the resistance does not change. The connector that the resistance changes should be connected to your fans positive connectors. ground connections should be connected directly to ground. This is a parallel set up. If you set this up in series, each fan after the first will have less power and will spin slower.

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