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: enter image description here

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 :

enter image description here

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

  • \$\begingroup\$ 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 \$\endgroup\$ Commented Jun 12, 2021 at 20:29

1 Answer 1


The calculus you made is for minimum values resistors.

Use the typical values of parameters and applications. The best CTR seems to be @10mA ...

Some time, you can wire the pin B of 4N25 to ground through a 1k resistor for example. So, the 4n25 is off until the opto effect is ok. It seems also, as stated in comment, that currents are too high (not necessary). Must simulate. Anyway, make R2 higher.

enter image description here Made by myself ... with MicrocapSpectrum Software (v12 free)


https://www.vishay.com/docs/81865/4n324n33.pdf (Darlington, but same thing) https://www.vishay.com/docs/83725/4n25.pdf


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