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Thread: Industrial Electronics Problem [OT]
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face BY : Mark Willis email (remove spam text)



Russell McMahon wrote:
{Quote hidden}

 Sure, good explanation; I'd thought that even with a FAST dv/dt the
zener/Transzorb on the cable end would catch the spike sooner than if
the zener was at the gate end, due to higher voltage across it
(Transzorb is what I'd use - I think of them as the "Same device",
transzorb's a "Rough hewn" Zener from a black box level, really.  And
sure takes higher wattage!)  With my headache yesterday, I wasn't
getting how the Zener can be driven higher with a fast spike on the
outside of the resistor, finally with less headache today it dawns on
me:  Duh!:  That gate's an ACTIVE output, Mark.  Not tri-stated, which I
do a lot.  Trying to drive that active output 'past the stops', should
do a GOOD job of pulse width stretching, your "the resistor takes the
brunt" finally clued me in! <G>

 For production stuff, the gate end is the way to go - I do custom
few-at-a-time stuff, some of my stuff HAS to have huge "herking"
protection to prevent any failures if I can;  I'd guess Schottky's on
the gate end with a Transzorb ("cheap P6KE6.8", 600W, 88 cents or so,
beats a service call any day!) on probably EACH end of the cable, and
perhaps a Zener at the FET's gate as a backup?  I'd rather have some
cheap redundancy, than drive 300 miles to replace a $1 part, or trust
"Joe Ten-Thumbs" there who's more likely to use Acid core solder than
not, to solder in replacement parts, and definitely more likely to melt
the parts soldered in & still end up with cold solder joints, than not.
Motivates me to put everything in connectorized modules, it's hard to
plug the Red DB-25M into a Green DB-09F socket!  He'll probably use a
hammer <G>

 ("Joe" does what he does really well, it's nice and technical and
incomprehensible to me - he just doesn't "Grok" electronics at all;  I
wouldn't ask my Doctor to solder for me, he doesn't ask me to diagnose
medical problems for him, same with "Joe" here, the world's a safer
place if he doesn't solder...)

 Different markets, different techniques!  Consumer appliances are
different than industrial embedded stuff <G>

> >  Also, Kelly, one question:  Your web page doesn't give relative
> >quantities made of the various units, some sort of "The failure rate
> <snipped>
> He implied that there were probably about 1000 all up - he has not
> been given the exact figure I think.
>
> regards
>
>             Russell McMahon

 Missed that number - Thanks, Russell!  Still would be good to know (in
other circumstances) if failure rate was "uniformish" or not, a minor
design change is the solution here, though, though the more I think
about it the more uncomfortable I am with my ASSUMPTION that ground
loops weren't the culprit;  I'd be tempted (in addition to definitely a
series resistor at that gate!) to consider going Optoisolated at the end
of the driven cable, and have the optoisolator drive that FET, just
because any ground loop that changes the Voltage at the ground end of
the FET could change whether the FET's driven hard or not, potentially.
I'm probably being paranoid, though;  I definitely don't think that's
the problem we're against here <G>

 Mark

<37386874.4ECE2005@nwlink.com> 7bit

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