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

Charles Steinmetz csteinmetz at yandex.com
Sun Dec 21 03:52:31 EST 2014

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

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,


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