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Calculating the RMS Value:
The calculation works like this:
1. Take the waveform and divide it into a “large” number of individual increments
2. For each increment, SQUARE the voltage value.
3. Sum these squared values over all increments and then calculate their MEAN value.
4. Take the square ROOT of this mean.
That’s how you get the Root-Mean-Square value!
Neither this above procedure nor the formula given below can be practically realized with small microcontrollers such as PIC16F72 with its very limited math powers .. it is just not feasible ..
Check out microchip app notes 575 and 660 for floating point support. The code should fit into your controller (if you sort through what you need) to implement what Ianp suggests.
1. Square waves: Like sine waves, square waves are described in terms of period, frequency and amplitude:
Peak amplitude, Vp , and peak-to-peak amplitude, Vpp , are measured as you might expect. However, the rms amplitude, Vrms , is greater than that of a sine wave. Remember that the rms amplitude is the DC voltage which will deliver the same power as the signal. If a square wave supply is connected across a lamp, the current flows first one way and then the other. The current switches direction but its magnitude remains the same. In other words, the square wave delivers its maximum power throughout the cycle so that Vrms is equal to Vp . (If this is confusing, don't worry, the rms amplitude of a square wave is not something you need to think about very often.)
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