> http://bit.ly/1Q11zbg
>=20
> Currently I'm using 2 1N5822 schottkys in series.  I'd think these
> would be fast enough, but it doesn't help.

Hi Neil,

one point of using a Schottky is that its Vf is lower than that of the
FET's body diode, which is why it conducts first and shunts crap
away from the FET. By using two in series you've raised the Vf

It's difficult to estimate what the actual outcome will be because
Vf changes with current, but I suspect that the 2 x 1N5822 will
not be very effective. Just one would be but I doubt it would take
the strain for long

I make a product which is quite similar to yours, using a PWMed
3A 12V DC motor. I have a logic FET (AOD454A, 40V 20A)
driven by a 16F628, and an SK84L diode (40V 8A) across d-s.

The PIC which does the driving also monitors, with the internal
comparator, the voltage across a 0.1 sense resistor. There's a 5V1
g-s zener too

Never had a problem with this arrangement. OTOH, I tried to
help someone who insisted protection components were a 'waste
of money' and who spent more time and money repairing blown
FETs than necessary when he left them out or made up his own
physics

Joe


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