Fan voltage question

[stigweed]

You can run a fan at 12, 7 or 5 Volts depending on how you wire it up and assuming the fan will happily run at such speeds. Now. At 7 volts, the +5V line gets conencted to the negative terminal on your fan.

Normally conventional current would flow from +12 or +5 down to 0 through you components. Now if you run at 7 volts, the conventional current goes from +12 to +5. Surely this means that current is going the wrong way up the 5V line? Is this bad for PSUs? I've never seen it affect the way a pc functions I just don't feel good about it. can someone explain?

[Allan]

A friend of mine who is in the electronics business assured me that it wouldn't hurt the PSU at all... But I can't really remember the reasons he gave!

[stigweed]

WEll that's good cos I just started doing it anyway. 120mm fans are SOO Loud at 12V

[Aedan]

Originally posted by stigweed
Now if you run at 7 volts, the conventional current goes from +12 to +5. Surely this means that current is going the wrong way up the 5V line? Is this bad for PSUs

Ahh. Now, lets say your fan uses 0.1A when running at 7 volts. What happens? Well, the power flows from the +12V line, through the fan, and into the 5V line. However, there's a wacking great motherboard chewing up 25A worth of power on the 5V line. The fan will be providing 0.1A worth of that 25A, and the PSU will provide the other 24.9A on the 5V line.

In other words, as long as there's a significantly larger load between 5V and 0V, then putting a fan between 12V and 5V won't really make any difference.

Áedán

[stigweed]

Ah I was thinking about that. I wasn't sure what would require that kind of current on the 5V line.

I thought it'd be you that came through on this kind of question, Áedán.