True 7 volt fan mode?

Ive read around that you can drop the voltages on a fan from 12 volts to 7 by connecting the positive lead from the fan to the yellow 12 volt line of the power supply and the ground from the fan to the red 5 volt line of the power supply. Is this correct?

Can this damage the power supply or put strain on it. It seems that the fan will spin opposite direction if you give power to the ground lead and ground the positive lead. So it seems like what would happen if you connected both a 5 volt and a 12 volt is you are getting a "simulated" 7 volt like "effect". But only an effect not a true 7 volts. The 5 volt lead on the ground looks like it would just try and spin the blade the opposite direction (altho loosing the battle with the 12 volt) slowing it down to a certain extent. You are not truely supplying 7 volts.

If i am wrong about this can someone elaberate why this works and if it is safe for the power supply.

^ the first bit is correct, yes.

All the fan sees is the potential difference between the two wires, which is 12-5=7v. It couldn't care less if the 5v was at 2v or 2000v, as long as the 12v line is 7v above what the 5v line is then all the fan will see is 7v across it's motor and will spin more slowly.

It is safe for the PSU as long as you are drawing current from the 5v line. If you are drawing less current from it than the fan then the 5v line voltage will increase as regulators don't like working backwards. As most motherboards use about 15A anyway unless you run 10 fans (tornados) on 7v you shouldn't see any problems.

Related to this, is there some way to combine the 5V and 12V lines to (probably unsafely) pump fans up to 17V? The ground is common between the two I think, so I haven't been able to find an easy way to do it. It's not as simple as connecting two batteries in series anymore.

If you use two power supplies and put the ground of one into the 5v of another then the 12V of the 1st supply will be 17V above the 2nd supply's ground. Best not let the two touch cases though

yep you can.

12v to positive on fan and ground to 3.3 or -5 or -12, whatever your appetite for power is.


now you go and fry some fans on 24v you bad boy!!

Before you all go load down your -5 or -12V line, make sure it can handle the load. Check your amps fellows... While the +12V can supply several amps, the -12 thing has to suck 'em all up, and -12V line is usually on a pretty short leash.
So, forcing the -12V line to eat more than stated for that PS will burn the -12V line. Remember that the MB also wants some -12V juice.

and I think video cards use the -5 line...I bought a PSU once for a server and the PSU was wrong, I spliced in all the wires to work but there was no -5v line. The computer would work fine and boot but the only way to get a video signal was to use a video card that was incredibly old (and PCI...so it might just be the AGP slot that uses -5v).

'Thought this might help



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