# Optocoupler 4N25 cut and saturation for zero crossing detector circuit

This is my first time working with an optocoupler and I'm trying to find the right resistors for a zero crossing detector circuit that has a 4N25 optocoupler, but the values I've calculated are not giving the expected result for the circuit.

For the 4N25, I've used a forward voltage of 1.3V, forward current of 50mA,and a CTR of at least 20%.

The Sine Source is 12Vac. This is the circuit:

For R2:

 If VCE = 0.2V, and IcMax = 50mA
R2 = 9.8V/IcMax
R2 = 196 ohm


With CTR of 20%, the forward current is: If = Ic - Ic.0,2

If = 40mA


Then for R1: R1 = (Vp - 1.4V - Vf)/If

R1 = 356.75 ohm


But I'm not getting the cut-saturation state as output in the VCE as it goes from 8.5V to 10V :

Are the calculations wrong?, is there something else I should consider ?

• Make R2 much bigger than R1 when the voltages are about the same for in/out so that CTR <1 can amplify to saturation when hFE reduces greatly. 50 mA in is too much. you can do 5mA in and 0.5 mA output Jun 12, 2021 at 20:29