Originally Posted by bobosoft
That's not quite how a bipolar transistor works. The current flowing from the base to the emitter determines the amount of current that can flow from the collector to the emitter. That proportion is called the gain so if you had 1ma through BE and a gain of 1000 (as is typical of darlington) you would allow 1AMP through CE. Calculating the current through BE after your voltage divider and taking into consideration the inductor it has to go through before ground is the math I was referring to.
I wasn't explaining the function of the transistor. I know it's not a voltage device. That's why I said I left the transistor's characteristics out of it. I was focusing on the point you made about the PWM's high state being 5v as opposed to an open state. I see how that could cause an issue and explained why. My board's PWM pin is going from 0 to infinity not 0-5v. If the circuit is connected, then PWM goes from 0-12v. Do you have a link handy to the Intel spec on the 4pin? I can't find it now. I'll look into later. I have to run out for awhile.