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Isolated zero cross detection w/ AC mains

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Ian.M:
Here's an idea that gives you one pulse per AC cycle on the falling zero crossing.  It doesn't give you the rising zero crossing, but that's not a problem if you have a MCU doing the phase control.  It only presents a 1 Meg load to the line, but stores charge to slam the optocoupler LED with a 50mA pulse for a nice sharp output rising edge.



Its also got input filtering to remove fast transients, but the phase shift (lag) due to the filtering is approximately compensated for by triggering early, at about +13V.   Its setup for 240V, 50Hz operation.  The filtering would need to be tweaked for 60Hz and/or 120V operation to maintain the compensation.

LTspice sim attached.

eliocor:
https://web.archive.org/web/20150226043505/http://www.dextrel.net/diyzerocrosser.htm

coppice:

--- Quote from: eliocor on October 08, 2019, 08:45:07 am ---https://web.archive.org/web/20150226043505/http://www.dextrel.net/diyzerocrosser.htm

--- End quote ---
That circuit looks OK, but "all components can be low voltage SMD" in the features list is wrong. Use low voltage 220k resistors for R1 and R2 and you'll be in trouble, especially with 240V mains. Most small SMD resistors are only rated for 50V. You need to either use high voltage resistors, or split the 220k into multiple smaller resistances, and make sure you lay out the board to keep them from arcing across in a surge test.

wraper:

--- Quote from: coppice on October 08, 2019, 11:06:34 am ---
--- Quote from: eliocor on October 08, 2019, 08:45:07 am ---https://web.archive.org/web/20150226043505/http://www.dextrel.net/diyzerocrosser.htm

--- End quote ---
That circuit looks OK, but "all components can be low voltage SMD" in the features list is wrong. Use low voltage 220k resistors for R1 and R2 and you'll be in trouble, especially with 240V mains. Most small SMD resistors are only rated for 50V. You need to either use high voltage resistors, or split the 220k into multiple smaller resistances, and make sure you lay out the board to keep them from arcing across in a surge test.

--- End quote ---
If you go with 1206 size for those, you should be fine with general purpose resistors.

coppice:

--- Quote from: wraper on October 08, 2019, 11:32:42 am ---
--- Quote from: coppice on October 08, 2019, 11:06:34 am ---
--- Quote from: eliocor on October 08, 2019, 08:45:07 am ---https://web.archive.org/web/20150226043505/http://www.dextrel.net/diyzerocrosser.htm

--- End quote ---
That circuit looks OK, but "all components can be low voltage SMD" in the features list is wrong. Use low voltage 220k resistors for R1 and R2 and you'll be in trouble, especially with 240V mains. Most small SMD resistors are only rated for 50V. You need to either use high voltage resistors, or split the 220k into multiple smaller resistances, and make sure you lay out the board to keep them from arcing across in a surge test.

--- End quote ---
If you go with 1206 size for those, you should be fine with general purpose resistors.

--- End quote ---
Even 1206 resistors are only rated for 200V. Two of those gives you a 400V rating. The peak of a 264V sine wave (upper bound of 240V mains) is 373V, so they are marginal best, with no allowance for harmonic peaks or surges.

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