Change the voltage for a 12v fan?

DennyD

Senior member
Oct 29, 2004
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In the Freebies section of the B/S/T forum, I read the following:

2 80mm RAIDMAX case fans (no screws) - these were a bit too noisy for me so i change the wires around to get them to only output 5v instead of 12v

My question is: how would you rewire a 12v fan to make it run normal of a 5v current? Anyone? Bueller?
 

pcgeek11

Lifer
Jun 12, 2005
21,708
4,739
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Originally posted by: DennyD
In the Freebies section of the B/S/T forum, I read the following:

2 80mm RAIDMAX case fans (no screws) - these were a bit too noisy for me so i change the wires around to get them to only output 5v instead of 12v

My question is: how would you rewire a 12v fan to make it run normal of a 5v current? Anyone? Bueller?

It probably isn't a good idea to try and run a 12 volt fan at 5 volts. It probably wouldn't start up. I think what you are referring to is what is known as the 7 volt fix. Where you supply one lead of the fan with 12 volts and the ground lead with 5 volts the opposing voltages end up running the fan at 7 volts the difference between the two. 12 volts minus 5 volts equals 7 volts driving the fan.

A word of caution here:
1. Some fans will not start reliably with only 7 volts.
2. On some MB's and power supplies back feeding voltage into the 5 volt rail could cause un-good things to happen with the MB or power supply. Rare but yours could be the one that screws up.

I would get a fan bus with a load resistance and regulator circuit built in. Then you could vary the speed of the fan to achieve what you want. Google for a fan bus controller. You could also build your own fan bus cheap and easy. Google that also.

pcgeek11

 

DennyD

Senior member
Oct 29, 2004
224
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0
Maybe it's because I really don't know what I'm talking about, but ultimately this is what I'd like to do: run some fans from a USB port power supply. I put a 12v fan on one and it ran pretty slow; less than half speed.

Am I understanding it correctly when the guy somehow converted a 12v fan to a 5v fan (what a USB supplies) that if I converted 2-3 fans from 12v to 5v that they would run normally, or would I have to actually get some 5v fans for this?

I want to make a custom laptop cooler with some nice bells and whistles... and dammit! The fans are the most important part! :)
 

pcgeek11

Lifer
Jun 12, 2005
21,708
4,739
136
Originally posted by: DennyD
Maybe it's because I really don't know what I'm talking about, but ultimately this is what I'd like to do: run some fans from a USB port power supply. I put a 12v fan on one and it ran pretty slow; less than half speed.

Am I understanding it correctly when the guy somehow converted a 12v fan to a 5v fan (what a USB supplies) that if I converted 2-3 fans from 12v to 5v that they would run normally, or would I have to actually get some 5v fans for this?

I want to make a custom laptop cooler with some nice bells and whistles... and dammit! The fans are the most important part! :)

IF you could locate them use 5 volt fans as 12 volt fans cannot be converted to run on 5 volts reliably. They may or may not start when powered up. The fan specs should state what the minimum starting voltage is. I would recommend getting a laptop cooler if what you want to do is cool the laptop, they usually run from the USB port.

pcgeek