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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: (skipp isaham)
Date: Wed, 31 Jul 2002 23:31:09 -0700
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. 
>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.
>Gerry, W1VE

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