>> True RMS requires much more computation.

> Looks like the statement is not true. RMS approach being a "statistic"
> approach does not require that many measurements per sine period as
> the peak-detect polling does to achieve the same precision.
> Considering the fact that PIC24FJxxx got high-speed 17-bit x 17-bit
> hardware multiplier, we may conclude that the total of required
> operations per second would be much less.

Be aware that RMS computation, especially over shortish periods (seconds
rather than minutes) really wants integral cycles. The same real world
things that may cause problems with peak detect MIGHT cause some problems
with whole cycle detect.

Also, once you add in SCR/Triac control then the RMS sampling needs to be
fast enough to not alias the introduced harmonics.

Voltage rate of rise time from zero crossing was suggested. This may have
problems if zero crossing is uncertain. BUT "maximum rate of voltage change"
will occur symmetrically around  zero crossing and may be a quite productive
system. eg maintain N sample memories in FIFO. Determine rate of voltage
change between adjacent samples and store N samples. Push new value and
calculate slope of FIFO. Could be delta V between 1st and last or use all
samples in FIFO by weighting samples in some way. Whatever.
Store maximum value. When delta V goes negative that half cycle is ended.
Average N maxima - maybe with the filter mentioned previously* or whatever.
   (*Vo(n+1) = Vo(n) x (k-1)/k + Vin(n+1)/k).

Vsine = k.delta.V.

May even work :-).

                 Russell
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