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Reply-To: "fred bartoli"
From: "fred bartoli"
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org> <3DD6F1BE.4F3C0ED0@earthlink.net> <email@example.com> <3DD7ADF4.2B1E@Spam.Bots> <3DD7C944.7CBC@Spam.Bots> <firstname.lastname@example.org> <3DD8E431.4D57@Spam.Bots>
Subject: Re: Meter movements
Date: Mon, 18 Nov 2002 11:22:48 +0100
X-Newsreader: Microsoft Outlook Express 5.00.2314.1300
Organization: Guest of ProXad - France
NNTP-Posting-Date: 18 Nov 2002 11:20:02 MET
Mike Monett a écrit dans le message :
> Tony Williams wrote:
> > In article <3DD7C944.7CBC@Spam.Bots>,
> > Mike Monett wrote:
> > > Thanks, Speff. I was thinking of retrofitting some VOM's and a FET VOM
> > > from Radio Shack. The FET VOM is nice - 10 meg loading and lots of
> > > ranges, but the movement is something like 16 or 25uA and settles to
> > > final reading very slowly.
> > There might be an RC slug in there as well.
> Yes, that's definitely possible. The response is so slow it is difficult
> adjust the zero pot.
> > > It has a 9V battery, so the LMC662 might be suitable.
> > Those old jfet followers may not be as accurate, but
> > will probably be far more robust than a CMOS opamp,
> > especially when connected to an input voltage without
> > the power turned on.
> > Tony Williams.
> How about a 1 meg in series with the positive input of the LMC622?
> there might be a 1 meg in series with the input divider already, according
> several web sites that talk about FET VOM's. This should help protect the
> device even with power off.
> The reason for looking at the LMC622 retrofit is to improve the zero
> and hopefully speed up the meter response.
> Although I'm not sure what damping factor is provided by the constant
> current feedback. It seems the op amp would counteract any change in
> due to the back EMF, and effectively place a short circuit across the
> This might make the response slow. But it couldn't be as slow as the
Not at all Mike.
The opamp will force constant current into the meter movement. So its
driving impedance is infinite and there will be no damping at all.
You'll have to add the parallel resistor to damp the movement or to provide
a secondary feedback path from the top side of the meter. Don't have time to
find what the best compensation network should be but finding it should be
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