Re: [neonixie-l] Re: Calculating multiplexed nixie's RMS current

Tue, 06 Mar 2012 07:32:59 -0800

Actually the signal was applied to a "thermocouple" which was "one arm" of a bridge circuit. An "identical" thermocouple, which was a "second arm" in the same bridge circuit, then had D.C. applied to it. A null was then achieved across the bridge, and the meter actually measured the D.C. being applied. A very simple and elegant solution to the measurement issue of both wave shape and frequency. RMS is the A.C. voltage, regardless of wave shape, that will produce the ""same heating effect"" in a PURE resistance (think here of an incandescent lamp filament), as an EQUIVALENT amount of D.C. Applies to current as well. As an example a 100V RMS wave shape, will produce the same heating effect, in a pure resistance, as 100V D.C. RMS means "Root Mean Squared" of which there is a mathematical way of calculating it. It actually is a mathematical process. If you want more I'd suggest looking it up on Wikipedia, or other places on the net. Hope that this helps. Ira. 

On 3/6/2012 6:16 AM, GastonP wrote:

Actually there is only a definition of RMS, not subject to
"trueness" :)

http://en.wikipedia.org/wiki/Root_mean_square

AFAIK, the old instruments that gave a true-"true RMS" output measured
the heat generated by the signal when applied to a resistor. That way
the waveform shape did not affect the measurement, and they were able
to measure with the DC component included, something fake-"True RMS"
instruments can't do.
Many of the existing instruments assume sinusoidal signals and thus
are subject to gross errors.

Gaston

On Mar 5, 6:15 am, Nick<[email protected]>  wrote:

On Monday, March 5, 2012 8:46:42 AM UTC, Cobra007 wrote:


Yes, you're right Nick, the Fluke is indeed AC coupled. I didn't
expect that to be honest as it undermines the definition of "true RMS"
but a simple battery test shows 0V RMS :-).

Its not a commonly known problem, even among professional EEs. One of my
DMMs, a Tektronix DMM916, has the option to include/exclude any DC
component as required. I've had "forthright" discussions with some over
what theoretically constitutes true-RMS vs. what they expect/want in
actuality.

Nick


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