From: A E
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Subject: Re: Meter movements
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org>
Date: Fri, 15 Nov 2002 13:27:22 -0500
NNTP-Posting-Date: Fri, 15 Nov 2002 13:25:40 EST
Tony Williams wrote:
> The copper in the coil of a moving coil meter
> has a very high (positive) temperature sensitivity.
> This affects the simple I= V/R calculation,
> especially with sensitive meters. It can be
> compensated for mechanically, but that can be very
Yeah, that makes sense. There's an awful lot of tiny wire in there, so I
suppose any effect like that is magnified.
But I never thought of a small meter like that (it's 1 by 2 inches) to have
any accuracy worth worrying about tempco for...
> The cheaper (and better) route is to drive the
> meter with a constant current, such as that obtained
> from the collectors of a long tail pair (with emitter
> degeneration) or an opamp. This also allows a less
> sensitive meter to be used, cheaper and more robust.
> Gain switching is also easier.
> > I'd expect maybe to have current limiting so the needle doesn't go
> > beyond the range, doesn't slam, etc, but when I switch ranges the needle
> > slams. When I go to a low range, the needle slams...
> > So I don't get it. Why make it complex? Esp if it doesn't seem to do
> > anything? Am I missing something?
> Meters can withstand 1.5x to 2x current overload
> without damage and (knowing HP) there probably is
> current limiting in there, but to that level.
There probably is. I haven't reverse engineered the supply's schematic to
100%, but now that I know more about why the meter is driven like that, I feel
better. It's fun to know things!