I am trying to create an AC zero crossing detection circuit using a 4N35 optocoupler. The circuit is very simple I connect a bridge rectifier (DF15005S-T) to the AC mains. One of the sides of the rectifier is connected to two 50k 1W resistors in parallel.

The rectified DC is connected to a 4N35 and pin 4 is connected to a 10k pull up resistor and an ESP32 pin. Pin 5 is connected to ground.

When I turn the AC on I get a constant high signal on the ESP32. If I switch out the Optocoupler to an 4N25 everything works perfectly. I was reading the datasheets to understand why this wouldn't work with the 4N35 but wasn't able to figure it out.

Anyone understands why this circuit works with the 4N25 and not the 4N25 and what modifications I need to do to make it work with the 4N35?

 

Hello,

Have a look at the CTR values of the optocouplers.
The 4N25 is less sensitive as the 4N35.

Bertus

 

Hello,

Have a look at the CTR values of the optocouplers.
The 4N25 is less sensitive as the 4N35.

Bertus

I saw this on the sheet but can’t quite figure out how this translates to the results I’m seeing. With the 35 I just see a constant high voltage and nothing else at all.

 

Try removing one of the 50k resistors. or connecting the two resistors in series.

 

Thanks everyone. It looks like I had a problem with multiple broken componnets. I had 2 busted 4N35s so made me think that it was the wiring but just tried with a brand new 4N35 and it worked as expected.