> 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

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