OOPS!

"Richard Clark" wrote in message
...
On Tue, 24 Feb 2004 18:28:57 -0600, "Steve Nosko"
wrote:
I assumed Richard's intent here is that you only have to do the
calculation for one full period ...symmetrical, then only one half
period...

And then you offer in contradiction:

However a DC biased periodic shape requires another squaring and root
operation if you capture all the components. It gets a bit more harry

This was a stupid thing to add at this point since it addresses
something which I did not explain, so I'm sure it looks weird. I didn't
even say what was in my mind. Namely, that with the DC + AC you still need
to do one full period of the periodic part.

What I was poorly referring to was this:
If you can break down the signal into component parts such as:
DC
Periodic part #1
Periodic part #2
etc
Then there is a formula for the total RMS which is the square root of
the sum of the squares. It is in one of the papers I linked previously.
That is one way to do an AC+DC situation. Second formula in the Intl Rect
paper:
http://www.irf.com/technical-info/an949/append.htm


[...] There is no such thing as symmetry, except on the
academic page. [...] you don't
make claims to accuracy (admittedly none were offered that I was
responding to) through fudge factors when so many alternatives remove
doubt.

I think is it safe to say that we each determine our own tolerance for
error. Five percent for power is ok for my purposes. SO approximating
waveform functins is just fine.


The simple determination of RMS is the graphical integration of the
area under the curve. There are as many "correction factors" for RMS
as there are shapes, and they all derive from this simple concept.

Here I'll take issue with the ONE WORD "graphical". You can
integrate
if you can describe the function of the wave shape mathematically.

Here, I'll take issue. I believe that the basis for the many
"correction factors" for RMS of various shapes is indeed the mathematical
integration of a function rather than the "Simple concept" of a graphical
solution (if that is what you meant). There is a mathematical integral of a
sine wave.

I think you'd be hard pressed to prove that the average of a sine wave is
2/pi exactly using graph paper. You could certainly say it sure looks close
to 2/pi, but is it exactly pi?... can't say for sure.

Case in point is the phase controlled sine wave made by SCR light
dimmers. I find it hard to believe you can graphicaly come up with the
formula:

Sqr-root[ D/2 + sin[pi(1-D)] cos[pi(1-D)]/2pi

I must also add that the graphical solution and mathematical integration
are different implementations of the same concept. I don't intend to say
that one is wrong and one is right.

Now, for Richard, C. Is the thing coming out of the AC outlet an exact
sine wave...no. It is very noticably flattened on the top by all the power
supplies drawing peak currents near the peak. There must be other
corruptins as well. However, if I assume it is a sine wave, will my
calculations come out very close, I believe Yes.

Now in all fairness, I won't dispute that you can use the graphical
method to find the RMS to any desired accuracy, just not exact...without
being able to integrate the waveform.


Of course, but it is eminently doubtful if you can actually express it
mathematically. Far more here own o'scopes than works of multiple
regression.

Graphical analysis is first year engineering stuff out of
drafting class.

Integration is first year engineering stuff out of calc class. At least
it was for me. I am still amazed that I remember doing tripple integrals
then, and thinking...Gee, this ani't so hard after all!. Couldn't do one to
save my life now.


[...] you simply measure the caloric result and ignore
shape altogether.

I always thought that the common method of measuring RF power was pretty
cool! The Thermistor or bolometer. Here you balance a bridge with DC or
low freq AC. It heats the thermistor to the correct resistance. Then, when
you add RF power, the thing heats up more and changes resistance. So, you
remove some DC power to get back to the correct resistance and that amount
is easy to figure. That is how much RF you put in. Cool. I think it is
correct to say that you absolutely cannot measure power *directly*. You
must measure something else which is affected/caused by the power...comment?


[..] migrated from Power to some other consideration

Not trying to migrate. I just think its a cool method.



Yikes! Not sure where you went on that last bit, Richard C...
Now, I ask. Do the power meters on the outside of our houses take all
those
factors into consideration
And, like me, Richard Clark makes a really long answer that says...Yes.

Got me!
I have NO idea how the watt-hour meter works...and don't want to try
to understand at this point.

73,
--
Steve N, K,9;d, c. i My email has no u's.