Anyone have experience PWM-controlling a DC brushless 12V fan?  Right
now I have experimented with between 120Hz and 60Hz.  I'm at 60Hz right
now and I can hear knocking at mid-to-lower duty-cycles (40-25% area)
and that's pretty undesirable of course.  If I up the frequency my
mid-range knocking is eliminated, but the low end knocking (around 25%)
is increased and there's an obvious descrease in translated power to the
fan.  Curious if there is a sweet spot or if it varies fan-to-fan enough
that I just have to find a best estimate.

Also, how much is the speed of the fet coming into play?  Due to the
capacitive nature if the gate, will this cause problems at such a low
frequency or can I simply through it out?  I'm driving it directly w/a
simple PIC18F2320, no isolation as of yet.  This is my first real
playing w/PWM so I'm still trying to get used to all the variables.

Thanks!

Nick

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