Electronic – Undervolting PC Fan from 12V to 7V

fangroundpower supply

I have a 3 pin 12 V Fan, rated at 2000 RPM max. Its nominal voltage is 12 V which will make it spin at full throttle. That's what is happening when I plug it in the motherboard. However I don't like that, because the fan is making a lot of noise. I would like to lower its speed to about half. I have figured out I should supply it with 7 V instead. There are methods online here, here and even a famous video on YouTube here demonstrating the procedure.

I've read a lot about this and I'm unsure about whether this is safe. I use a modern and pretty good and reliable 80 Plus PSU. So I'm skeptical now and I want to ask here to understand better. What we have to do is pretty much shown on the picture below.

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Instead of supplying the fan with 12 V, using the yellow 12 V pin and the black ground pin from the Molex cable, we instead supply it with 12 V for VCC and 5 V for "ground". So the fan sees voltage difference of 12-5=7 V, but shouldn't there be a path to 0 V ground? This is the part where I'm confused.

Is this secure, is this possible?

Best Answer

This is not a great solution. The 5V rail is not intended to sink current in most multi-output power supplies, and it may not be able to do that effectively. It may do weird things like cause the 5V line to float up to a higher voltage or even damage something.

A more solid solution would be to use a DC-DC converter to step the 12V down efficiently. You can get them cheap on Amazon.

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