ok the image attached is what his circuit actually is... Click to view full-sized image! Hosted by UGBox Image Store Now as you can see the 5 v and the 12 v are both going into the Pot. Because there is a greater voltage on one side, it means that the voltage at MAXIMUM is going to be 7 V (12 - 5), now the actual applied voltage will be less because of losses in the pot. So in retrospec, i am supprised that the fan is even spinning at all. I can only guess that the fan is just getting enough voltage. this is actually a good example of a case where Millman's, Kirchhoff's or the superposition theorems should be applied to work out the actual voltages applied to the fan. All i can say is that in NO way will the circuit run anywhere near 12volts into the fan. Like i said above, the maximum will be somewhere around 7 volts. Click to view full-sized image! Hosted by UGBox Image Store is the proper voltage divider circuit that the OP should be aiming to build. It will supply the correct voltage range to the fans to get a good silent to full speed range of voltages. Most other circuits (including the OP's circuit) will cause a greater draw from the power supply, and place the PS to have incorrect loadings placed upon it. Hope that clears this up. I personally have a slightly advanced version of the second circuit in my computer (I have switches wired in with LEDs running from 5 volts).