[R-390] R-390a meters

[Roy Morgan]

At 08:46 PM 8/23/2004 -0700, mparkinson1 wrote:
>How can you really test what the resistance is on the sub 1Ma meter? ... 
>would like to know how to test the internal resistances of the movement.

Matt, and others,

There was a long thread on this some time ago. I now can't find any of the 
posts, but here is a summary of the situation:

1) There is a technique in at least some ARRL Handbooks to measure the 
internal resistance of a DC meter.  Part of the discussion was that the 
method described would lead to some errors.  Those errors may be small with 
a higher voltage battery (such as 6 volts instead of the single cell they 
suggest.)

Be *very* careful with clip leads and such.  Any normal battery will 
destroy a 1 mA or 50uA meter in less time than it takes you to say "OH DARN!"

2) Short method:  Get a voltage source and a resistor estimated to get the 
meter to full scale.  Make up the resistor of mostly fixed and partly 
variable (this reduces the chance of a disaster.)  Set the current to full 
scale and then connect a second variable resistor *across* the meter and 
adjust till it reads *half* scale.  Remove the parallel resistor and 
measure it with an ohmmeter to get the internal resistance of the meter.

3) Refined method: Do as above but put a current meter in series with the 
circuit (a DMM is good here).  Alternately adjust both variables for the 
original full scale circuit current and half scale reading.  This will 
eliminate errors due to changed total current.  The difference may be 
negligible with sensitive meters. If your meter has a full scale current of 
25 mA or so, it will matter a lot.

4) If you know or can measure the full scale current of the meter, and you 
can measure very small voltages with moderate accuracy (e.g. 50 
millivolts):  Set up the series circuit with variable resistor and measure 
the voltage across the meter. Apply Ohm's law to figure the internal 
resistance.

Happy Metering.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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