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

Bob Camp kb8tq at n1k.org
Tue Dec 16 21:17:45 EST 2014


Indeed looking at the AC line is a Time Nut sort of thing to do. It was one of the first things I did with an old Beckman counter back in the 1960’s. Yes I realize that the AC line is a very noisy signal and that this may not be needed:

The same limiter / noise shaper stuff that works for a DMTD is equally at home processing a 60 Hz sine wave. The rise time, bandwidth, and progressive stage gain issues are the same. The Collins paper on hard limiters does indeed apply here. You *could* make a 60 Hz chain that got down into << 1 us sort of resolution. 

Again, just because you can does not mean you should. 


> On Dec 16, 2014, at 8:58 PM, Dave M <dgminala at mediacombb.net> wrote:
> Charles Steinmetz wrote:
>> Every so often, the subject of logging the zero-crossings of the AC
>> mains comes up.  There are any number of ways to couple the AC mains
>> to logic circuitry (coupling with very high value resistors,
>> capacitor coupling, and optical isolation have been mentioned).  A
>> simple AC mains ZCD that is transformer isolated and gives excellent
>> results, is posted at ko4bb.com:
>> <http://www.ko4bb.com/manuals/download.php?file=05_GPS_Timing/Simple_AC_Mains_Zero_Crossing_Detector.pdf>
>> The ZCD uses a small, dual-primary power transformer, two
>> transistors, and a few diodes, resistors, and capacitors.  It
>> produces a ~100uS logic-level pulse at every positive zero-cross, the
>> leading edge of which is predictably and stably related to the AC
>> mains zero-cross.
>> Best regards,
>> Charles
> I'm not trying to downplay the circuit in the link above, but I want to offer another possible solution to Zero-Crossing needs.
> Here's an Idea For Design from EDN magazine that I've used a couple times in non-time-nut circuits, and I must say that it works beautifully.  I have no measurements that would satisfy a time-nut's curiosity, so if someone wants to Spice it or otherwise tear it apart, please do..
> My use for the circuit was in a spot welder control; the output was used to sync and cycle a counter-driven trigger for an alternistor, all of which controlled the number of power line cycles that the welder transformer received for the weld. It worked well for me until I sold the whole contraption.  Don't know whatever happened to it after the guy moved away from the area; never heard from him again.  I hope it's still working.
> http://electronicdesign.com/analog/differential-line-receivers-function-analog-zero-crossing-detectors
> Dave M 
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