AC DMM’s always excluded the DC component, if I am not mistaken. For a
mainly
troubleshooting tool (citation needed), that is not a bad choice. After all,
many AC signals
found in circuits have a DC offset. Assuming sinewaves makes the design of
the meter
easier (cheaper).
I would not expect a different behaviour from a DMM that is TRUE RMS. Nice
to have
that AC/DC switch though, on the Tek meters. But I’m still a Fluke only guy
;-)
Frank
From: Nick
Sent: Tuesday, March 06, 2012 4:03 PM
To: [email protected]
Subject: [neonixie-l] Re: Calculating multiplexed nixie's RMS current
Yes, RMS has only one physical definition, but in the case of DMMs the
actual implementation is obfuscated.
"true" RMS in a DMM context is an RMS calculation that does not assume a
sine wave - most cheaper DMMs do indeed assume a sine wave input.
Then there are "true RMS" (and indeed "ordinary" RMS) DMMs that may or may
not include any DC component, or at least in the Tek case, give you the
choice.
Old meters indeed did use to measure the heat produced in a resistor - the
definition of the "RMS value" used was that of the DC voltage that would
give the equivalent heating effect to the signal under inspection.
Nick
On Tuesday, March 6, 2012 2:16:45 PM UTC, 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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