[time-nuts] Simple AC mains zero-cross detector

Didier Juges shalimr9 at gmail.com
Sun Dec 21 10:06:49 EST 2014


A commend regarding your ZCD. You propose to use a dual 120V primary
transformer to generate the isolated 120V AC needed by your circuit.
Unless specifically designed for that purpose, the isolation between the
two 120V primaries of a common transformer is probably not as good as the
isolation between primary and secondary, which could be a safety hazard.
Since small transformers with a 120V primary and a true 120V secondary are
hard to find, a better way would be to use two "regular step-down"
transformers back to back, like two door bell transformers: 120-24-120. You
would then get double isolation.

Didier KO4BB

On Sun, Dec 21, 2014 at 2:52 AM, Charles Steinmetz <csteinmetz at yandex.com>
> Mike wrote:
>  From a Time-Nut perspective, isn't phase/frequency of the (nominal) 60 Hz
>> all we'd be interested in?  Phase is best measured at a zero crossing as
>> this is the (only) phase measurement point which is independent of
>> amplitude.
> That is the primary interest (as I understand it -- I am not, myself, a
> grid-nut), and the reason the "simple ZCD" circuit uses this approach.  But
> grid-nuts are also interested in perturbations of the grid voltage caused
> by grid sections going offline and coming back, lightning strikes, etc.,
> etc.  (After all, simply monitoring the ebb and flow of the line frequency
> is about as interesting as watching the tide come in and go out, so they
> naturally want some occasional excitement.)  These anomalies can be
> detected by their effect on the zero crossings of the mains voltage, so one
> data collection serves both purposes at the time-nuts level.
> While the ZCD approach is ideal for monitoring the grid phase/frequency,
> and as a bonus provides timing information about grid anomalies, it does
> not capture all of the information about anomalies.  If you are a utility
> concerned about grid security or making sure that "new energy" sources play
> nicely with the grid, you probably want more information about anomalies
> than time-stamped zero crossings provide.  Magnus described a system used
> by utilities to track grid anomalies in greater detail.  My reply agreed
> that zero cross detection is not the tool of choice for utilities with such
> concerns, and noted the different needs of grid-nuts and utilities.
> Grid-nuts are well established, and the vast majority of them use
> time-stamped zero crossings as their data sets.  I was concerned that many
> grid-nuts seem to use non-isolated feeds from the mains that, while "safe
> enough" under normal conditions, are not preferred practice.  I also
> thought that the timing relationship between the ZCD and the actual zero
> cross could be improved and stabilized with a new ZCD.  So, I designed the
> "simple ZCD" circuit to provide an isolated source of very predictably
> timed pulses with fast edges.  I tested it and it proved to be reliable and
> to have very stable timing with respect to the line zero crossings, so I
> published it and announced it on-list with the first message in this thread.
> Since then, the thread has taken on a life of its own and ranged very far
> from the initial, simple proposition of improved zero cross detection.
> There has been a flurry of comments mostly aimed not at whether the "simple
> ZCD" is a good AC mains zero cross detector, but more toward whether zero
> crossings are what grid-nuts should be interested in in the first place.
> Since I am not, myself, a grid-nut, I cannot really speak to what grid-nuts
> "should" be interested in.  I do think that time-stamped zero crossings
> have many significant advantages when one is interested in comparing notes
> with others, and it is comparatively easy data to collect with good
> accuracy -- so, IMO, the choice of grid-nuts to settle on time-stamped zero
> crossings was eminently rational.  The "simple ZCD" has proven to be an
> excellent front end for such a data collection, and is a project within the
> skills of anyone who knows which end of a soldering iron to grip.  I am
> happy to answer any questions that potential builders may have.
> Personally, I think the thread has more than run its course and should be
> laid to rest.  But if it is to continue, please accept as a given that
> grid-nuts decided long ago that time-stamped zero crossings are the
> appropriate data to collect, and focus on the narrow topic of the "simple
> ZCD" as a means for accurately detecting zero crossings of the AC mains.
> Best regards,
> Charles
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