From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Meter movements
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Date: Mon, 18 Nov 2002 04:59:29 -0800
NNTP-Posting-Date: Mon, 18 Nov 2002 04:59:27 EST
Organization: Bell Sympatico
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 the
> > 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 to
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? Actually,
there might be a 1 meg in series with the input divider already, according to
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 drift,
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 current
due to the back EMF, and effectively place a short circuit across the meter.
This might make the response slow. But it couldn't be as slow as the current