Thanks Dave, Ruben and Olin for setting me straight
on the RMS vs average issue. Especially Olin's
square wave example. 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.

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

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?? What if
the load was inductive or capacitive?
-Roman


Olin Lathrop wrote:
>
> > So now i'm confused! Isn't RMS just calculating the
> > AC average??
>
> No.
>
> > 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.
>
> Sorry, but that was wrong.
>
> One way of thinking about RMS is that the RMS of any repeating voltage,
> whether it ever goes negative or not, is the equivalent DC voltage that
> would deliver the same power into a resistor.
>
> For example, consider 1V DC into 1 ohm.  That obviously dumps 1 watt into
> the resistor.  Now consider a 0 to 2 volt square wave.  Half the time the
> voltage is 0, resulting in 0 watts.  The other half of the time the voltage
> is 2 resuting in V**2 / R = 4 watts.  The average is therefore 2 watts.
> Even though both waveforms have the same average voltage, the square wave
> puts twice the power into a resistor.
>
> Note that if we do the square it, average it, square root it thing we get a
> value that can be used to predict the power into a resistive load.  For the
> square wave we have SQRT(AVE(0*0, 2*2)) = SQRT(AVE(0, 4) = SQRT(2) = 1.41
> volts RMS.  Now calculate power = V**2 / R and you get 2 watts which we
> already know is correct.
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, olin@embedinc.com, http://www.embedinc.com
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:
> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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