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
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    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

    Moderator

    Mar 24, 2008
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  3. Dave

    Retired Moderator

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

    Dave
     
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