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Isolated mains zero crossing detector: how?
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technix:

--- Quote from: spec on November 02, 2018, 02:54:51 pm ---
--- Quote from: technix on November 02, 2018, 03:20:57 am ---Can this produce a square wave at the output of 6N137 with its logic edges corresponding to the zero crossing of the mains? If so I can feed this output to two timers on one microcontroller, with one timer measuring the mains frequency and the other generating the TRIAC firing pulses.

--- End quote ---

+ technix

Is this what you still want?

Using one 6N137, would you be happy with an isolated square wave, at 5V logic levels, representing the mains sine wave. The edges of the logic level square wave would equate to about 2V of the mains sine wave rising and the same voltage of the mains sine wave falling. Because the zero crossing point is at the maximum dv/dt of the sine wave there is not a load of difference in time between the two.

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Yes it is still what I want. If the application can tolerate the slight imbalanced duty cycle of the square wave it would trigger on both rising and falling edge of the square wave and have a constant delay; if not I can trigger on only the rising edge and implement a software PLL using the MCU timers for finer timing control.
spec:
Good  :)

I have done a circuit: just got to put it in Eagle and will post for you to have a look at.
spec:
Schematic attached.

Hope it works  :)

The circuit switches at about 1V depending on the gate threshold voltage of the MOSFET.

If you want a precise zero crossing detector, within a few millivolts, just say and I will post another circuit- basically just replace the MOSFET with an open drain comparator.
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