"John Williamson" wrote in message
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Arfa Daily wrote:
"CBFalconer" wrote in message
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Arfa Daily wrote:
"CBFalconer" wrote in message
Arfa Daily wrote:
... snip ...
OK. I'm not sure that 'RMS' is the right term to attach to any
value derived from a ragged-arsed waveform, as it is a mathematical
function normally associated with symetrical waveforms, which the
draw by a SMPS may very well not be, but I see what you're saying.

What I am trying to say is that a chip which is designed to produce
an RMS reading from a sine wave, may well produce a meaningful
figure from a non-sinusoidal waveform also, but *only* if it is
still symetrical.
Oh? Try a square wave, for example. Nice and symetrical. You are
over-simplifying.
Explain ?
The integral is peak voltage times current. Simple. Not 0.7 *
peak voltage. Current is also constant for resistive loads, not
proportional to voltage. RMS doesn't work.


OK. Well in that case, I don't think that I was over-simplifying, because
if you have read the whole thread, you will see that it was I who
questioned the validity of attaching an RMS value to a non-sinusoidal
waveform. However, several posters then came back to me with considerable
levels of mathematical proof, to say that RMS was a valid notion for any
waveshape or symmetry factor, the only qualifiers being DC content or
variable cycle periodicity. Although it might not be too clear, that
second paragraph was more of a musing based on that. My original
contention was that a power meter (or whatever) designed to derive and
display an RMS value from a sine wave, would not give a meaningful
reading from non-sinusoidal or non-symmetrical drawing loads, such as a
SMPS may be, for instance. The replies suggested that the waveshape was
immaterial, and that the chipset could very easily still calculate a
meaningful result. I was a little sceptical about this, as it seemed to
fly in the face of what I was taught many years ago in college, but I
bowed to what seemed to be superior knowledge in the field.

Now, you seem to be saying something quite different ? Comments ?

Arfa


Sorry to butt in here, but when I was studying such things, the RMS value
of a current or voltage waveform was calculated by working out the area
inside the curve plotted over a full cycle, which then allowed you to
calculate an equivalent DC value. This involved counting squares on graph
paper of the plotted waveform or similarly counting squares on a
calibrated oscilloscope tube face. The earlier & most of the current cheap
meters that give an alleged RMS reading take a peak reading & apply a
correction factor of 0.707 to it (1 divided by the (near enough) square
root of 2), as that gives the right answer with a clean sine wave, which
is what most of these meters are used to measure. (Mains power round here
is near enough a pure sine wave that you can ignore the error, as it's
less than the accuracy of the meter)
The RMS value of a (theoretical) pure square wave is exactly the same as
the average of the absolute values of the positive & negative peaks, as
the value is either fully positive or fully negative, with, in theory, no
other value being present.

The most (theoretically) accurate way to measure RMS values is to use a
hot wire meter, which doesn't care what the waveform is, it just measures
the heating effect which is more or less frequency independent & includes
any DC offset automatically.


Tciao for Now!

John.

Yes, all agreed, but the shape of the mains waveform is immaterial, unless
you are talking a purely resistive load like a light bulb. The point that I
have been trying to make all along is that when you are trying to measure
power, it's a function of both voltage and current draw, and in the case of
a SMPS, especially one that's in standby mode, the current draw from the
mains supply voltage, is very likely to be anything *but* sinusoidal.

Arfa