[time-nuts] Mains frequency

Bill Dailey docdailey at gmail.com
Sat Nov 16 17:08:20 EST 2013


There is no higher purpose actually.  I just fiddle.  This is how I relax.


On Sat, Nov 16, 2013 at 3:47 PM, Chris Albertson
<albertson.chris at gmail.com>wrote:

> No, I meant the purpose of the whole thing.  Why are you measuring power
> frequency?  Not why are you using a PIC.    How will the data be used, what
> is the question driving the measurement?
>
>
> On Sat, Nov 16, 2013 at 11:23 AM, Bill Dailey <docdailey at gmail.com> wrote:
>
> > My purpose is to do it with a picpet.  That's it.  So, that eliminates a
> > bunch of the options.  I can decouple the measurements from the pc clock
> > that way.
> >
> > Doc
> >
> > Sent from mobile
> >
> > > On Nov 16, 2013, at 11:26 AM, Chris Albertson <
> albertson.chris at gmail.com>
> > wrote:
> > >
> > > The signal is 120 volts.  You hardly need to amplify it.  Clip it with
> a
> > > diode to +- 9 volts so as not to blow up your serial port.  But I'd
> use a
> > > transformer for safety. The zero crossing detectors are built into the
> > > RS232 interface.    You take advantage of the RS232 spec which has a
> DCD
> > > pin input of about +-9 volts that is already set up to find a leading
> > edge
> > > of a pulse and cause a very low latency interrupt.  The system software
> > > already will capture the time all inside a kernel level interrupt
> > handler.
> > >
> > > The jitter turns out to be on the order of a single digit microseconds.
> > > Good enough for measuring a 60Hz signal.
> > >
> > > I guess if you want to see transients depends on the purpose of the
> > > experiment.  Are you looking at local AC power quality or wanting to
> > > measure the grid.  The grid is well monitored, just use FNET and you
> get
> > > real-time data for all of North America.   I think the reason for
> > measuring
> > > it yourself is to see local power quality and things load switching
> > inside
> > > your own building, that's transients.
> > >
> > >
> > >
> > > The other way to measure AC with zero added equipment is to treat it as
> > an
> > > audio signal and after reducing it to 1 volt run it into an audio
> > interface
> > > And then use FFT.   This will let you see very small spikes and noise.
> > It
> > > depends again on your purpose for doing this.
> > >
> > >
> > >
> > >
> > > On Sat, Nov 16, 2013 at 1:18 AM, Magnus Danielson <
> > > magnus at rubidium.dyndns.org> wrote:
> > >
> > >>> On 11/16/2013 09:52 AM, Chris Albertson wrote:
> > >>> Your method tosses out a lot of data.  You can't see transients.
> >  Ideally
> > >>> rather then record a 1 second average you'd record the time of EVERY
> > zero
> > >>> crossing.  It sounds like a lot of data but not really.   You only
> > record
> > >>> 32 bits 60 times each second.  That is 240 bytes per second.
> > >> But you want it filtered to avoid the transients. Those are really not
> > >> that interesting when you measure the grid.
> > >>
> > >> Also, if you use the event trigger method you probably want to use an
> > >> amplifier to increase the slew-rate such that noise does not convert
> > >> into time jitter.
> > >>
> > >> Cheers,
> > >> Magnus
> > >> _______________________________________________
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> > >
> > >
> > >
> > > --
> > >
> > > Chris Albertson
> > > Redondo Beach, California
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>
>
> --
>
> Chris Albertson
> Redondo Beach, California
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>



-- 
Doc

Bill Dailey
KXØO


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