From: Tony Williams
Subject: Re: Meter movements
Date: Fri, 15 Nov 2002 16:00:16 +0000 (GMT)
NNTP-Posting-Date: Fri, 15 Nov 2002 16:19:50 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article <3DD47F82.14E8608@videotron.ca>,
A E wrote:
> Good old meter movements are a bit before my time but I understand that
> as long as you know the coil's resistance and full deflection current,
> you're golden. I think that with a simple resistor in series you can
> make a positive DC voltmeter out of any movement, and a shunt for a
> current meter.
> So how come my old HP 6284A (for which I don't have schematics, anyone?)
> has this complex long tailed pair driving the meter in differential
> mode? My engineer buddy jokingly said 'they're compensating for the
> friction of the jewel'. I'm still laughing at that one.
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
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.