fan POT

trying to figure what POT i should get and what wattage. I have a 25 ohm( dunno wattage) but I burned out part of the POT(lower portion, maybe 0-5 ohm). It controls 2 120 MM YS-techs 7.92W each, im putting a full 12 volts to the POT and its wired as a rheostat.

Standard pots are carbon-resistive and not designed for
high power dissipation.
If you are using these - you are lucky you did not un-solder
your connections with the heat.
If the pot you are using is a quality wire-wound one it should
be labelled with it's wattage.

The problem with using a rehostat is that the smallest end
of the wiper range may have to carry the full current.
W=VI
16 = 12*I
Current is 1.33Amp .
A 25 ohm resistor needs to be 33 Watt. Really huge.

In the real world - if your minimum fan voltage is 7Volt.
IF the current is the same 1.3Amp (big if)
W=VI
W=5*1.3
we get 6 watts being dissapated from the resistor.

I would not leave the computer alone with a 6 watt variable
resistor fitted. Maybe 10 watts.

I suggest you use either a guaranteed resistor system
or one of those nice voltage regulators to save chance of fire.

I built a simple fan controller using a 2N3904, and a 100KOHM pot. Works fairly good, but is not totaly accurate, and does have a 600mA current limitation. You can use an LM317, a resistor and a pot to create a precision voltage regulator.

I think the diagram for the regulator that uses the bi-polar transistor can be found on the main overclockers.com site, but the link for the one using the 317 was down last I checked, but I did a bit of research in my solid state electronics book and found a schematic for a voltage regulator.

If you want i can draw it out at school using Electronics Workbench and then email you the schematic.

I am using a 250 Ohm POT made by PEC controlling a Panaflow 120mm fan, and it works perfect and never even gets warm.