From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Meter movements
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org>
Date: Sun, 17 Nov 2002 01:31:47 GMT
NNTP-Posting-Date: Sat, 16 Nov 2002 17:31:47 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> Tony wrote...
> > 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 expensive.
> > 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.
> Here's my favorite meter-driving circuit. Range is set by R1,
> for example 4.99k for a 1mA movement with 5V full-scale input.
> The maximum meter current is set by R2, talking into account the
> supply voltages and the opamp's maximum output capability. If
> the input voltage range is too high, e.g. 10V with a 15V supply,
> a pair of attenuating resistors can be used on the input.
> . IN __ ____
> . ----|+ \ R2 | | R1
> . | >--/\/\--| / |--+--/\/\--,
> . ,--|-_/ ==== | |
> . | | gnd
> . '-----------------------'
> >> 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.
> Right, one dares not limit before full scale, and I've observed
> that steps to say 120% of full scale appear to be serious slams,
> but really aren't and the meter is happily totally unaffected.
> - Win
Errr..the meter current is from the opamp output thru R1 and R3 to
ground; no current (of merit) flows to the op amp input.
So the "range" depends on both R1 and R2.
Maybe there is an error (or 2) in the schematic?