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From: Winfield Hill
Subject: Re: Meter movements
Date: 24 Nov 2002 05:50:31 -0800
Organization: Rowland Institute
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org>
X-Newsreader: Direct Read News 4.11
Ken Smith wrote...
>Winfield Hill wrote:
> [... lots snipped ...]
>>. IN __ ____ full scale: R1 = Im/Vin
>>. ----|+ \ R2 | | R1 offscale Imax = Vamp(max)/
>>. | >-+--/\/\--| / |--+----/\/\----, R1+R2+Rmeter
>>. ,--|-_/ | ==== | R3 C2 |
>>. | == +--/\/\--||--+-- gnd
>>. | | C1 |
> This circuit works a lot better with a capacitor across the meter
> and/or one from the op-amps output to its inverting input.
> A capacitor across the meter protects it somewhat from mechanical shock.
> The other thing that the capacitors do is prevent oscillation of the
> op-amp. The coil of the meter movement is inductive enough to make a
> fair phase lag in the feedback at high frequencies. Depending on the
> op-amp you use, this could lead to oscillation.
That's a good suggestion. I haven't encountered the trouble but
it's always a possibility, so nip it in the bud.
> You can also add a series RC combination across R1. This was typically
> done in galvo. pen motor based chart recorders. If you use the right
> values you can put an electrical zero where the mechanical pole is.
> This sharpens up the edge when a step is input.
It likely Mike Monett would like that idea, earlier in this thread
he was looking to upgrade his slow FET VOM from Radio Shack.
Looking at the modified circuit above, R3 serves nearly the same
purpose as C1, at least from the opamp's viewpoint.
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