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

>Checking my memory the times
>I wanted to find average voltage I have always used
>an averaging system, and the odd occasions I needed
>real power measurements I always measured instantaneous
>volts and amps and averaged the result.

Good; what you computed, in those instances, was truly the real power.

>So can someone clear up two points, why do I remember
>being taught about RMS=3Daverage (20 years ago now!),
>is this true for sinewaves maybe?

Negatory.  It isn't true for sine waves or ANY other waveform, except
in the trivial case of square waves, in which rms =3D peak =3D average.  =
I
think either your teacher was confused (this does, unfortunately,
happen) or your memory was faulty (hey, 20 years is a long time).

>And if I get voltage samples and use the RMS calc to
>get the overall RMS value of the volts, would this
>be usable to calc power in a load directly??=20

If the load is resistive, yes; and if what you're interested in is
power (not the equivalent rms voltage) then the square-root step is
superfluous.

>What if
>the load was inductive or capacitive?

In that case, integrating the product of  [e(t) * i(t)] over one or
more full cycles gives you a measure of the real (as opposed to
reactive) power delivered to the load.

Dave

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads