Arfa Daily wrote:
"CBFalconer" wrote in message
...
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.