From: Winfield Hill
Subject: Re: Meter movements
Date: 16 Nov 2002 17:59:10 -0800
Organization: Rowland Institute
References: <3DD47F82.14E8608@videotron.ca> <firstname.lastname@example.org> <3DD6F1BE.4F3C0ED0@earthlink.net>
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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.