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From: firstname.lastname@example.org (Bill Allison)
Subject: Re: Help - Power mosfets - difficult load
Date: 28 Oct 2002 01:28:46 -0800
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org> <3DB81206.E28CA99F@rica.net> <email@example.com> <3DB963BE.939694C2@rica.net> <firstname.lastname@example.org> <3DBC2B63.146AA440@rica.net>
NNTP-Posting-Date: 28 Oct 2002 09:28:46 GMT
Useful as always. Comments below. Also, I've come across a post on
another forum which states that my particular starter motor's static
inductance is 70uH. Unable to verify accuracy of that, and the same
poster also stated that resistance is 40mOhm, which conflicts with the
32mOhm I have from two other sources. And don't know how useful to
know given the complex scenario of stator and multiple, commutator
John Popelish wrote in message news:<3DBC2B63.146AA440@rica.net>...
> Bill Allison wrote:
> > I'd much appreciate your comments on the thread with Winfield re
> > switching speeds. My cheap little scope, by comparison to what you
> > guys have access to, has a usable response to 40Mhz-ish and a level
> > response only to 20Mhz, so maybe that sets a limit to how fast I
> > should attempt to drive the gate, if I'm to commission this by
> > assessing transient amplitude with scope whilst reducing safety
> > resistor initially in series with motor?
> Your scope can very nicely show sub microsecond transitions, and this
> is what you should shoot for.
> > > and use some hefty bypass capacitors between
> > > the non switched end of the schottky and the source of the fet.
> > How hefty do you think? And low value low inductance caps in parallel?
> This all depends on the length and inductance of the loop back to the
> battery. The shorter these leads and the smaller the loop area, the
> less capacitance you can get by with. Remember that during the time
> that the battery lead inductance limits the current, this bank of
> capacitors has to carry up to the full motor current. During the on
> transient they keep the supply voltage from collapsing, but during the
> turn off transient, they must absorb the motor current that is flowing
> through the battery lead inductance. I would probably try a few
> microfarads of low inductance film capacitors in parallel with a
> couple high value electrolytic caps like the Panasonic T-UP series.
> The 100,000 uf 16 v ECE-T1CP104FA is listed in the Digikey catalog as
> having an 11.36 amp ripple current rating and a .02 ohm ESR (at 120
> Hz). They don't list a an effective series inductance.
> I would also probably make a multi conductor battery cable that had
> several conductors for the positive and negative leads, with the whole
> bundle braided to interweave the two currents to lower the inductance,
The battery leads will be so short as to make that impracticable
> unless the battery were right next to the control circuits. Making
> the control circuit to be bolted directly to the battery would be even
It will be as nearly so as makes no odds.
>and eliminate most of the supply side bypass requirement.
But still need some? battery self-inductance?
>One of those side bolted GM batteries would make this easier. The
battery is a conventional lead acid car battery - connectors are lugs
at opposite ends.
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