Electronics > Projects, Designs, and Technical Stuff
Isolated zero cross detection w/ AC mains
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glentek:
What I used in a 1200W battery charger years ago"

Mains to 2x 33k 1W resistors in series, 400V 1A bridge rectifier, 4N28 opto isolator
beduino:

--- Quote from: glentek on October 06, 2019, 08:38:06 pm ---Mains to 2x 33k 1W resistors in series, 400V 1A bridge rectifier, 4N28 opto isolator

--- End quote ---
Mains 230VAC or 110VAC?
glentek:

--- Quote from: beduino on October 06, 2019, 09:56:51 pm ---
--- Quote from: glentek on October 06, 2019, 08:38:06 pm ---Mains to 2x 33k 1W resistors in series, 400V 1A bridge rectifier, 4N28 opto isolator

--- End quote ---
Mains 230VAC or 110VAC?

--- End quote ---

This was for 240vac.
wraper:

--- Quote from: nctnico on October 05, 2019, 08:31:36 pm ---Otherwise I'd use this:
(Attachment Link)
I've used this circuit in a (one off) design but I still need to test it.

--- End quote ---
It's easier to use optocouplers specially made for AC. Like H11AA1

beduino:

--- Quote from: wraper on October 07, 2019, 12:14:16 am ---It's easier to use optocouplers specially made for AC. Like H11AA1

--- End quote ---
Isn't those 33k resistors too low for 230VAC? It looks like something around 0.4W loses on each of them?

I've used for 230VAC 4 x 100k - (2x 100k in parallel) x 2 in series, so 2x 50k, then we have something like: 0.00115^2*100000 ~ 0.132W losses on each of 100k resistors.
BTw, I've used two PC817 to have configuration similar H11AA1, but in this case I could detect on two pins not only zero crosing but also which part (top/bottom) of sine wave which might be sometimes helpfull in some applications.
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