[R-390] Another ballast question

[Roy Morgan]

At 09:36 AM 1/4/2006, you wrote:
>... My ohmmeter uses a 9V supply so there shouldn't be a problem
>doing this, right?  Of course, 9VDC isn't the same as 9V RMS so some
>conversion must be made to ensure 9VDC isn't too much for 12V RMS, but 
>this shouldn't be an overvoltage situation, should it?

Barry,

Ohmmeter circuits are quite simple usually. If this is an old style VOM, 
such as the Simpson 360, it works like this:

The test leads, the meter, a range resistor and the battery are all in 
series.  With the test leads shorted, the meter reads full scale, 
calibrated at zero ohms. If a resistor the same value as the range resistor 
is at the test leads, the thing reads half scale.  So the current through 
the test leads depends on the range resistor - set by the ohms scale 
selected.  The open circuit voltage might well be the 9 volt battery 
voltage, but will drop when the leads have a resistor connected to 
them.  The current available (max with the leads shorted) depends on the 
range resistor selected.  The range resistors are chosen depending on the 
sensitivity of the meter movement. In a Simpson 260, I think this is some 
50 microamperes. The tubes you test will never light up.

VTVM's and digital DMM's work on similar principles.

Roy

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