I have a bog standard 80mm case fan on my exhaust which has been running at full speed (don't know what speed as there is no sensor) and is dead noisy. Tried fitting a potentiometer to vary the speed but fan would no run until I got to about about 90% on the potentiometer dial, speed was very hard to judge in the remaining movement. Do I need a special type of potentiometer to control fan speed? Am I better off buying a proper control unit from somebody like CoolerMaster?
I would just get a quieter fan. They are not that expensive in my opinion. If you go this route make sure you get a fan with at least three wires going to the power connector. (I believe the third wire is often the one that controls fan speed - maybe someone with a little more know-how can verify that)
As for the potentiometer - I would just get a proper control unit. Sure it is twenty bucks or so, but they are easy to install, makes it real easy to adjust the fans, and often have extra goodies like temperature gauges or USB ports.
'Course I'm lazy, and ain't into that electrical engineering thing. If you like messing with that stuff then keep working with the potentiometer, you will figure it out.
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Take a look at the ThermalTake Smart Fan II w/ 3 Fan Function.
This comes with a variable speed control you can adjust manually. You'll need to use one of the small knockouts on the back of your case and some automotive silicone to fasten the knob in place. These fans work well and have plenty of rpms in reserve for extreme usage situations. I have three of these installed on my machine and it has made a real difference in my temps.