From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Meter movements
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org> <3DD6F1BE.4F3C0ED0@earthlink.net>
Date: Sun, 17 Nov 2002 08:49:35 GMT
NNTP-Posting-Date: Sun, 17 Nov 2002 00:49:35 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Winfield Hill wrote:
> Robert Baer wrote...
> > Winfield Hill wrote:
> >> Tony 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. ...
> >> 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.
> >> Vin __ ____ Im
> >> ----|+ \ R2 | | -> R1
> >> | >--/\/\--| / |--+--/\/\--,
> >> ,--|-_/ ==== | |
> >> | | gnd
> >> '-----------------------'
> > 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?
> No, it's fine. Thanks to the opamp feedback loop, R1 effectively
> defines a current source for the meter. Im = Vin/R1 Then the
> job of R2 plus the meter resistance is to use up enough voltage
> so the opamp's output-voltage limit also defines a maximum meter
> current, Im(max) = Vo(max)/(R2 + Rm + R1). Works like a charm.
> - Win
Thanks for the complete explaination.