----- Original Message -----
From: Gary Neal <gln103@MAIL.PSU.EDU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 08 October 2001 14:21
Subject: [PIC]: Mosfet speed control of 12V DC motor


> Guys,
>
>         I've been trying forever to get a speed control circuit to work
for
> driving a 12V DC motor from a battery using MOSFETS.  So far all I've
> managed to do is blow stuff up.  Here's what I'm trying.
>         Using single 12V battery to try to drive DC motor with ~30A max
current
> using 3 IRLZ44N mosfets in parallel and controlling them with a
> PIC16F870.  Have mosfets on huge heat sink (even though they haven't run
> long enough yet to get hot).
>         Using an LM7805 to get +5V for the PIC.  Have caps on both input
and
> output of 7805.  Source of mosfets is tied directly to ground.  Drain goes
> through motor then to +12v.  PIC outputs PWM signal to gate of
> mosfets.  Have 220 ohm resistor in series with this PWM signal and have a
> 22kohm pulldown resistor to ground on this line.
>         Everything works fine with motor disconnected.  Get good switching
of
> mosfets.  But, when the motor is connected things get all screwed up.
When
> I try to adjust the duty cycle, it basically shuts down the PIC.  Have to
> disconnect power and reconnect before it will start again.
>         I put oscope on the +5v bus.  It swings quite a bit when the
mosfets
> switch on/off.  I'm guessing the PIC is going into brownout or something
> like that.
>         Does anyone have a simple solution to this problem?  Or, a fairly
simple
> drive circuit that will accomplish what I want.  I've searched the net and
> can't find a good example.
>
> TIA,
>
> Gary Neal
>
########################

Try :

1:Check motor actually works ( straight across batt)
2: Put Schottky across motor, cathode to Positive. This should clamp voltage
to rail.

   Regards
           Jonathan





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