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[Amps] meter internal R value, quick and easy.

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Subject: [Amps] meter internal R value, quick and easy.
From: (Gary Schafer)
Date: Thu, 01 Aug 2002 10:48:20 -0400
That won't work exactly right. If the pot in parallel with the meter matches
the meter resistance then the sum of the meter and parallel resistor is 1/2
of what the meter was alone. That will increase the series circuit current
from what it was with just the meter so there will be more circuit current
than exactly 1/2 the original current in the meter and the resistor. If you
set the pot for 1/2 scale  meter reading the pot and parallel resistor will
each have a different current because the total series circuit resistance has
changed and the circuit current is different.

It would work if you had a second meter to monitor the series circuit current
and readjusted the series pot to maintain the same circuit current.

Gary  K4FMX

skipp isaham wrote:

> I was surprised to see that no one came up with
> the classic simple answer regarding the meter
> internal R post...
> -
> -
> Two Potentiometers, a 9 volt battery and one or two
> resistors will do the trick.
> -
> Assume the movement to be 50 uA unless otherwise
> known "in stone" that it is another value.  I have
> surplus meters with full scale 1mA indicated, but
> having micro amp movements.
> -
> One must construct a simple current loop using
> a few simple parts.
> -
> Limit the max current to a "safe value" that will
> not peg (slam) the movement hard (might actually
> bend the meter needle).   Pick a value like 47K min
> as your soft limit value, placed in series with a
> 250K linear pot and the meter movement.
> Observe polarities...
> -
> Adjust the series linear pot for full scale deflection
> of the meter.    Leave the meter reading full
> scale and apply a second pot as described
> just below.
> -
> Place the second pot in parallel with the meter
> and adjust it for half scale on the meter.  A
> 250 ohm pot is probably a good value to use.
> -
> With the 250 ohm pot set at some position
> which drops the meter reading from full, to
> half scale.... the meter and the 250 ohm pot
> share equal current. Thus.... their values are
> equal.  Measure the value across the pot and
> you have your meter movement resistance.
> -
> In step one, you might decrease the value of
> the 47K resistor to obtain full scale meter
> readings on higher current meter movements.
> -
> A 1 milli amp  "mA" movement might use
> a 4700 ohm resistor as your testing safe
> limit value.
> -
> cheers
> skipp
> -
> >Anyone know of a procedure where I can determine the internal resistance
> >without damaging the meter movement?  or, am i SOL? Some are uA meters,
> >some volts and some watts -- so who knows what the intended range was.
> >Thanks,
> >Gerry, W1VE
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