> I've noticed that a lot of the "case modder" types are using the +5 and
> +12 volts from their computer switching power supplies as 7 volts (12 to
> 5) to drive their 12 volt fans at lower than normal speeds.
>
> ...
>
> It appears there are diodes in series with the outputs, so as far as I
> can tell, the current must be flowing through other loads on the +5
> rail, and thus this technique is dependant on having a (heavily) loaded
> +5 rail to prevent pushing the +5 out of regulation.

Yes, the +5V supply is unlikely capable of sinking current, only sourcing
it.  Therefore all current running from the +12V supply onto the +5V
supply has to be sunk by the +5V circuitry.  As long as the current draw
of the +5V circuitry exceeds the fan current plus the power supply minimum
required output current, it should work.

However, I still think it's a bad idea.  Fans cost money, and I'm sure
someone went thru the calculations to determine the placement and number
of fans.  Some newer systems have speed controlled fans based on
temperature.  Messing with the fans on those systems is a very bad idea.
Why do you want to reduce the fan speed?  It won't save much power.  Is
noise the issue?

If you know you're running well below the power supply's maximum capacity
and well below the system's maximum ambient operating temperature, then
you probably don't need as much forced air.  In that case you could just
unplug one of several case fans, or put resistors in series with some
fans.  By the way, never mess with the fan in the power supply or those
mounted on the CPU.


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