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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Meter movements
References: <3DD47F82.14E8608@videotron.ca> <email@example.com> <3DD6F1BE.4F3C0ED0@earthlink.net> <firstname.lastname@example.org> <3DD7ADF4.2B1E@Spam.Bots> <3DD7C944.7CBC@Spam.Bots> <email@example.com> <3DD8E431.4D57@Spam.Bots> <firstname.lastname@example.org> <email@example.com>
Date: Mon, 18 Nov 2002 14:19:33 -0800
NNTP-Posting-Date: Mon, 18 Nov 2002 14:19:33 EST
Organization: Bell Sympatico
Tony, Fred & Speff:
Spehro Pefhany wrote:
> On Mon, 18 Nov 2002 11:27:59 +0000 (GMT), the renowned Tony Williams
> > So the solution may be to slug it, with a cap across the
> > meter, or across the opamp. This will reduce the dynamic
> > source-impedance.
> Tony has the best simple answer- by putting a capacitor
> across the coil you can change the damping without affecting the
> tempco. A series R-C in parallel with the coil would probably be
> even better if it was properly tuned. Then you get the damping
> even when the power is off.
> Best regards,
> Spehro Pefhany
Thanks for the replies! I tried a cap across the movement some time ago,
but even the largest cap I could find in my junk box provided little or
no damping. This makes sense when you realize the cap has to provide
fairly low impedance at sub-hz frequencies. Also, some resistance would
still be needed to keep the whole thing from making one large LRC
As far as placing a damping resistor across the meter movement, and
suffereing from the loss of accuracy and poor temperature coefficient, I
finally realized the nostalgia of fine mechanical movements holds little
benefit in today's world. For example, there is no way to log the meter
readings and look for trends or do other very desirable analysis on the
Probably the best thing would be to consign the RS meters to the back
shelf, and work on a 6 1/2 digit wireless voltmeter. The LTC2400 series
A/D would provide the resolution, and the LTZ1000 could provide a stable
reference. The whole thing could run off a wall-wart and use one of the
ISM bands for data. External circuitry could be used to measure
resistance, current, or whatever is needed.
The whole thing should cost far less than a conventional Fluke or other
DVM, so multiple units might be quite affordable. Perhaps they could
share the same reference, wireless interface and power supply to further
reduce the cost.
The next trick is to figure out how to synchronize multiple units to
trigger the measurement at the same instant. With separate units at
different locations, there would be some lower limit due to the low data
rates on ISM, but perhaps 1 to 10 ms accuracy might be feasable. This
could satisfy a wide range of applications.
Maybe someone already makes such a device. I should do a web search and
see what they charge and what kind of accuracy is provided.
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