[time-nuts] Line Frequency

Chris Albertson albertson.chris at gmail.com
Sun Feb 9 14:44:45 EST 2014


All you need to monitor line frequency is a small transformer connected to
a computer's DCD pin on a serial port.   You can get fancier and use
comparator, opto isolation and so on but a simple 5V transformer is enough.
 This takes advantage of the electronics inside the RS232 port that is
designed to handle positive and negative voltages up to about +/- 12 volts.


Then the linux pulse per second driver will time stamp and log every cycle
with the internal clock.  It is accurate to a couple microseconds.

Don't worry about line noise because that is what you are measuring and it
averages out after a few cycles.   You might try a >60Hz RC filter to
remove noise but I think to do any better you will have to treat the signal
as if it were audio frequency.  So scale it to 1 volt peak to peak and read
it with an audio interface and then use an FFT.     But the transformer on
the DCD pin of any normal computer with the Linux PPS driver works.  People
are doing just fine with the simple transformer and time stamping the
transitions.   And then it is just pure luck that Linux will already do
this out of the box.


On Sun, Feb 9, 2014 at 5:53 AM, M. Simon <msimon6808 at yahoo.com> wrote:

> This probably came up during the recent discussion of Line Frequency
> Monitoring but I may have missed it.
>
>
> Does any one have a circuit (tested - operational) for monitoring line
> frequency? I'd like something that checks zero crossing so that it is
> relatively insensitive to line voltage variations.
>
>
> Simon
>
>
> Engineering is the art of making what you want from what you can get at a
> profit.
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-- 

Chris Albertson
Redondo Beach, California


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