Undervolting Fans - Make Your Fans Run Quieter and Last Longer

Trying to get your PC to run fanless, but just can't do it? Don't want to run completely fanless?

Well, one solution is to undervolt your fans. 

There are a number of acceptable methods to undervolt your DC fans. Suprisingly, you can get quite a bit of granularity of control with ATX power supplies.

See, another word for "voltage" is "potential difference". In other words, it is the difference between two potentials. A 12VDC fan is usually connected to 12V and 0V, which is a potential difference of 12V. By connection something that uses up energy, that potential is used to create work (spinning the fan). So, any difference can be used to power a fan.

One could connect the positive lead to 112VDC and the negative lead to 100VDC and would end up with a fan that spins at the same speed as one connected to 12VDC and GND/0VDC. But be careful with the electricity in the former situation :)

So, to get 7V, we could connect the +ve lead to 12VDC and the -ve lead to 5VDC, since the potential difference would be 12-5=7VDC. This will not cause issues with a normal power supply, and is relatively easy to wire.

Another possibility is 10VDC, which can be done by connecting the +ve lead to 5VDC and the -ve lead to -5VDC (which does exist as a wire on ATX power supplies). I wouldn't recommend doing this for larger fans, or a large number of fans, as there isn't usually much power on the -5VDC line.

Finally, some fans will run at 5VDC. Some may not. I currently do have a 120mm fan (which runs extremely loud at 12VDC) running at 5VDC. Running a really large fan at a low voltage has the advantage of being able to spin quietly at a low RPM, but still push a lot of air. 

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