[R-390] Re: R390 panel meter resistance

[Roy Morgan]

At 09:03 AM 5/6/03 -0700, Bob Tetrault wrote:
>Not only is it published in the reference noted, but it has been the method
>published in almost every ARRL Handbook I've ever read, from '64 when I was
>first ticketed to the present.


Bob and other Meter Measurers,

I just happened to have a 1989 ARRL Handbook handy and despite no luck with 
the index I found the method on page 25-2.  It differs from the method 
outlined by Mr. Li as follows:

  Mr Li suggests:
"...adjust the pot to get full
deflection of the R390 meter, and note the reading on your 2 mA meter.
Remove your R390 meter, and substitute a 10-turn precision 500 or 5000
ohm pot, and adjust it alone to get the ORIGINAL READING on your 2 mA
meter."

In the 1989 Handbook method, the circuit has
1) a series meter able to measure the full scale current of the meter under 
test
2) a series resistor adjusted for full deflection of the meter under test
3) a parallel resistor adjusted for half deflection of the meter under test
4) the series resistor adjusted to return the total circuit current to the 
original value.

Steps 3 and 4 are done alternately to arrive at both half deflection and 
original total circuit current.

The resistor in parallel with the meter is then measured and taken as equal 
to the meter resistance.  A 1.5 volt battery is mentioned as an example 
with no discussion of the effects of different source voltages.

It seems to me that this method accounts for all significant variables and 
effects, assuming the battery voltage is constant enough.

Roy



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