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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.

enter image description here

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?

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    \$\begingroup\$ No connection to ground is needed if the fan is not connected to anything else (PWM, tachometer, etc), voltage is relative. Since a single fan is a small load, the current dump on the 5V rail is acceptable. There are 2 issues however. If you swap the wiring and connect anything with the 2 ground wires in it, you will short 5V to ground (if you swap 5V to GND, you need to remove the second GND wire for safety). The second one is fan startup reliability at 7V. \$\endgroup\$
    – Wesley Lee
    Commented Sep 22, 2016 at 21:55
  • \$\begingroup\$ he swapped both ends of the adaptor, so the colours are messed up, but the wiring of the extension plug is correct. \$\endgroup\$ Commented Sep 23, 2016 at 4:55
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    \$\begingroup\$ I run the case fan in my PC from 5V, it's been just fine for 5 years now. \$\endgroup\$ Commented Sep 23, 2016 at 5:00
  • \$\begingroup\$ Ok i did it. It seems to work well. The RPM now is ~900 RPM from 2100 @ 12V. And it's barely audible. This is what i wanted. The sensing pin on the motherboard doesn't work though. I have an RPM meter device which i used to measure the RPM. I will keep this for now, but for the long term i ordered a 12V to 7.5V DC-DC converter as Daniel proposed to make this setup more reliable. Thanks all. \$\endgroup\$
    – KeyC0de
    Commented Sep 23, 2016 at 10:42
  • \$\begingroup\$ One problem with this setup is you may encounter more noise on your line if you have a low load on your 5V line. I would imagine that PSU's are built to only source current not sink it. \$\endgroup\$
    – Voltage Spike
    Commented Sep 23, 2016 at 16:04

4 Answers 4

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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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    \$\begingroup\$ That's what i did, and i'm still using the converter for about a year now. Thanks. \$\endgroup\$
    – KeyC0de
    Commented Apr 16, 2018 at 18:33
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Technically, yeah it should work (the voltage difference is 7v seen by the fan). But I hate to do that sort of thing.

As far as fan startup, I think it'll probably startup at 7v. It'll depend on the fan... I've put various voltages on fans before (from a variable supply) and if I remember correctly, they turned on there.

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No problem here. The fan voltage is differential 7V and exceeds startup voltage.

Just dont try it on an LDO pulling up the 5V output to 12V ( emitter follower) with no load. as Vo will also rise on LDO. Here its no problem.

I guess no 3 pin fans with 3 pin headers and BIOS controlled fan speed eh? ;)

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I have been using this tricks for years (well actually.... decades) on multiple computers and never had problems. The only issue is that normally the rpm will be not reported or reported incorrectly (this is if you try to connect rpm tach cable to the MB). Also never had problems with fans starting at 7V, all though I can see that some super low rpm fans might have a problem with this. I do however had issues with fans starting at 5V.

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  • \$\begingroup\$ That will work in the short term but what happens as the fan gets older or the temperature drops? \$\endgroup\$
    – Gil
    Commented May 14, 2021 at 2:05

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