> -----Original Message-----
> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On
Behalf
> Of Olin Lathrop
> Sent: 19 March 2010 17:34
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] measuring 220V mains
> 
> Dario Greggio wrote:
> > Take 20 samples/sine
> > for each sample
> > take previous sum
> > divide by 20
> > subtract this (sum/20) from previous sum
> > add new sample to sum
> 
> That makes no sense for any reasonable interpretation of "measure
mains
> voltage".  Remember this is a AC signal.  This description above is
not
> totally clear, but it looks like you're box filtering the signal,
sortof,
> I
> guess.  Box filtering a AC signal with a aperture width a multiple of
the
> period will always yield 0.  In other words, the sum of any
consecutive 20
> samples (one whole period) will be 0.
> 
> Again, it's too early to jump into the implementation since there
isn't
> yet
> a clear definition of what exactly you want.  On top of that this
> implementation appears broken, at first glance at the level of detail
of
> what I think the above description is saying anyway.

Looking back through the posts in this topic I think there may be a
fundamental disconnect between Dwaynes measurement technique and what
Dario is hoping to achieve.  Dwayne is rectifying and smoothing the AC,
so a box filter could be useful to get the mean DC value and reject any
ripple.  

I may be wrong but I get the impression that Dario is effectively half
wave rectifying the AC waveform by clipping the negative side to ground
with a diode since there was some discussion on the dangers of not using
the PICs protection diodes for this.  Since that will give one half
cycle of positive input followed by one half cycle of 0 volts it's not
surprising his box filter isn't giving zero (would be Vpeak/PI I
think?).  A schematic would certainly reduce confusion here.

Regards

Mike

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