Jinx wrote:
>
> A friend of mine has asked me, in desperation, if the list
> has any comments about his circuit.
>
> http://home.clear.net.nz/pages/joecolquitt/12vmotor.html
>
> The problem he's got is that this circuit tested OK on the bench,
> and 2/3rds of the finished units seem to be alright in use, but
> the others fry all 3 FETs short circuit,


I think the FET gate drive circuit needs
a lot more safety... The pull down resistor
is 470k (!!) which is way too high, and
the only mechanism to turn the fets off is
when 555 pin 3 goes low, which is a collector
of its output transistor. So unless the 555s
output transistor is reliably turned on, and
its collector pulled low, the fets can't be
reliably turned off.

I also question the 27v zener diodes to the
fets gates??? This allows commutation or
other spikes to turn on the fets!!

My suggestions; drive the fets from the 555
output, with as low ohm pull down resistor
as possible. 555 will output about 11.5v on
high, set it up for about 9v at the fets
gates. Lose the zeners that attach the fets
gates to the noisy motor!! You want to make
sure that unless the 555 output is pulled hard
high, the fets MUST be off. Your circuit is
almost the opposite of that. :o)

Use a small cap across the gate pull down resistor,
check on the CRO you just want to round the
corners of the squarewave going to the gates.

Your snubber is wimpy for a 30A motor, try 0.22uF
and a 10 ohm 5w resistor. Slightly smaller snubber
across the fets, D-S.

It's pretty vital to decouple the motor PWM as
close as possible to the motor and fets. This is
one of the main hassles with electric vehicles,
you need BIG caps to run a 30A motor and with low
impedances, so commonly used lots of smaller caps,
like 10x 1000uF 25v caps in parallel very close
to the motor and fets. And the caps may get hot...

There is a good web site,
http://www.4qd.co.uk/

They make some impressive high current DC motor
drivers, for electric vehilces etc, and they have
some great info on PWM controller design. If your
friend is going to build 30A controllers this is
a real good place to start. :o)
-Roman

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