Transistor Selection for PWM Application

Discussion in 'The Projects Forum' started by aut0pilot101, Apr 23, 2009.

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  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 there was some threshold rating on the datasheet that I missed, 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 experimentation.

    Thanks for reading!
  2. Wendy


    Mar 24, 2008
  3. Dave

    Retired Moderator

    Nov 17, 2003
    This is duplicate. Please post replies in the active thread.

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