Roman Black <fastvid@EZY.NET.AU> wrote...

>Edson Brusque wrote:
>
>> > As to RMS vs averaging, that's what the original question was about.=
  I
>> just
>> > didn't know if doing an RMS calculation had any benifits over just
>> averaging the
>> > input signal (better noise immunity, more accurate, more linear, =
etc....).
>>=20
>>     RMS would give you a *more number.
>>=20
>>     Let's say you have a dimming application, where your circuit is
>> connected to mains power @ 220V and you're firing the triac at 90=BA. =
You're
>> measuring the voltage going to the load. If you average your =
measuring, it
>> will say 110V. RMS will say about 155V. Now, if you rectify and =
"regulate"
>> the energy going to the load (with full-cycle rectifying, 4 diodes and=
 a
>> capacitor) and measure it with a voltmeter, it will give you a value =
very
>> close to that 155V.
>
>
>This is wrong, with 90' switching there is exactly
>half the voltage average and half the rms voltage.
>(and how do you make that "degrees" symbol??) :o)

Half the power and half the average absolute voltage, but not half the
rms voltage: you get 0.707 x the original rms voltage.

>So now i'm confused! Isn't RMS just calculating the
>AC average??=20

Absolutely not.

>I was taught that the only point to
>squaring this and then doing a square root is that
>this is only math way of making negative values
>positive for averaging.

I don't think he understood the significance of rms, then.

>As programmers we have more power than mathematicians,
>we are not limited to:
>1. square it
>2. average it
>3. unsquare it

>We can just do the following:
>1. convert - values to +
>2. average it

These two processes produce TOTALLY different results.  The first
calculates the rms value of an AC signal, the second calculates the
average absolute voltage.

>Or have I missed something?? Does anyone think we
>MUST do square and square-root calcs in our PICs
>and why??

You do if you want to actually calculate rms; if all you want to find
is the average of the absolute value, then no.

Several companies make analog ICs that process an input waveform and
produce a DC output voltage proportional to the rms value of the
input; the Analog Devices AD736 is one of these, and if you go to
their website and read the datasheet for that part it has a simple,
but good discussion of the math.

Dave

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