Selecting Transistor for PWM Application

Discussion in 'General Electronics Chat' started by aut0pilot101, Apr 23, 2009.

  1. aut0pilot101

    Thread Starter New Member

    Feb 23, 2009
    I'm trying to control a 12V .21 Amp fan using PWM from a microcontroller using logic level voltages. The problem I've run into is selecting the right transistor for the job.

    I tried the AO4437 already, but without success. Here's what happened (BTW, I'd really appreciate it if somebody could explain this behavior to me):

    When I grounded the AO4437's gate pin, the fan would turn on, which makes sense because it is a P-Channel FET. When I tied the gate to a logic level high (+5V), I expected the fan to stop, but it only slowed down. I decided to experiment by tying the FET's gate to +12V, and, sure enough, the fan stopped completely (this is the behavior I want at +5V!).

    Shouldn't I have been able to turn the gate off with +5V? What was my error? I thought at first that I missed some threshold rating on the datasheet, but I've read through that thing so many times, I'm starting to doubt my sanity.

    Anyway, I've decided to try the Si7882DP next. What do you guys think? Ordinarily, I would just buy one and try it out myself, but I'm running out of time for aimless experimentation.

    Thanks in advance!
  2. Wendy


    Mar 24, 2008
  3. jpanhalt


    Jan 18, 2008
    Is the P-channel mosfet S connected to the +5V and its drain connected to the motor? A schematic of your set up would certainly help.

  4. AchMED

    Active Member

    Aug 5, 2008
    Your mosfet is only rated for 12Vds, for switching an inductive load off of 12V you really should use 20Vds or 30Vds FET and check with a scope that no Vds switching transients are exceeding your fets Vds. Clamp the drain source voltage if necessary. Use a freewheeling diode across the fan.

    Why dont you just use an N-channel on the low side to switch your load. You should be able to get a cheap logic level FET that can be driven off your I'm guessing micro. Depending on your current requirments you may be able to use a cheap 2n700x or nts4001.I've driven both those FETS off a PIC directly with just an 82 ohm gate resistor at sub 100ns transitions.