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From: email@example.com (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 29 Oct 2002 01:17:24 -0800
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
NNTP-Posting-Date: 29 Oct 2002 09:17:24 GMT
Thanks for the below. I want to minimise complexity as far as
possible, and my strategy will be to start off simple, with series
motor resistance for initial testing. Then if performance or fet
safety shortcomings are recealed, cherry-pick from there on in.
Whatever, I guess i am not alone in finding the progression of ideas
here most stimulating and thought provoking. Thanks guys.
One thought - what's the opinion, now that I have parameters for the
motor, of the value of Spice simulation?
Winfield Hill wrote in message news:...
> Bill Allison wrote...
> > But you've triggered another idea...
> > I could put some resistance in series with the motor and arrange to
> > short it out as power demanded approached 100%. That would soften
> > transients and would still give me 50Hz impulses to overcome stiction
> > and make fine control near zero speed better.
> That prompts me to mention another scheme I find more interesting,
> current-source control. First consider this: Q. How to add an
> electronic fuse or circuit breaker for fault conditions? A. add
> a common 100A 50-millivolt shunt resistor (R = 0.5 milliohms), and
> upon detecting more than say 100mV (200A) for longer than say 3 sec,
> set a fault flip-flop to kill the FET gate drive.
> Q. What other cool thing can I do with this shunt resistor?
> A. Use it with circuitry to turn the power MOSFETs into a say
> 0 to 50A linear current source to run the motor in a programmable
> torque mode when starting. Consider, 0 to 50A would be 0 to 25mV,
> a very managable voltage range. A difference amplifier with 0.1mV
> offset could be used to control the servo; 0.1mV offset creates an
> acceptable uncertainty of only 0.2 amps. And etc.
> - Win
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