# Can we extract CTR parameter of an optocoupler from its SPICE model this way?

Regarding 4N26 optocoupler, I tried to obtain the following results from the datasheet by LTspice:

So as in the test condition I set the forward current 10mA and I set the Rload such that Vce = 10V.

If my setup is correct, the LTspice simulation outputs 7mA current for 10mA forward current at test condition where Vce = 10V.

So in my case CTR becomes 70%. Is what I am doing correct? Does that mean for the SPICE model of 4N26 the CTR is set 70%? By the way why would they do this test in active region where Vce = 10V? Isnt it better to use this device in saturation region?

• $V_{CE}$ is 10 V until you start to turn on the transistor. You have 5 V across R so you've only 5 V available for the opto-transistor. Would that explain it? – Transistor Jul 2 at 14:29
• Vcc 15V in my case what do you mean by Vce? Isnt it the voltage between C and E terminals? If so I set it to 10V as you see by setting Rload to 713 Ohm. The thing is CTR becomes 70% here. And also when Vce = 10V it means theres no saturation; why would one use it at this non-saturated condition? I dont get the purpose of the test in the datasheet. – cm64 Jul 2 at 14:43
• Sorry, I assumed that your Vcc was 10 V. I can't help you with the Spice model. – Transistor Jul 2 at 16:28

What is the point of doing such measurements? If we know the CTR is not constant, but it will depend on the IF current and Vce voltage.

My test circuit

And CRT versus IF for a different Vce voltage.

• Thanks but Vce=10V or 5V means theres no saturation for the transistor. If we use the opto foe switching does it matter whether the transistor goes into saturation? – cm64 Jul 2 at 15:42
• Yes, it matters. Look at fig2 in the datasheet. – G36 Jul 2 at 15:56
• One last question; if I design such circuit for pulses should Vce be in saturation region during forward current is ON. Or as long as the circuit works Vce does not matter? Im asking because if it was dihital logic with usual transistors we would saturate the transistors for less heat dissipation. – cm64 Jul 2 at 18:58
• Im wondering whether it is necessary to saturate the opto i.e make its Vce small as 0.5V – cm64 Jul 2 at 18:59
• If you are satisfied with the result, then it can work without saturation. But what bothers you about the saturation in optocoupler? – G36 Jul 2 at 19:37

What you did is a valid way to determine the CTR for that SPICE model under specific conditions. But there is no guarantee that the SPICE model is exactly the same as the typical value in the datasheet (which was 50%).

I have designed lots of circuits and my experiene is that SPICE models don't always caputre all the details of a real part. Whereas the datasheet was probably based on actual testing.

• If used for switching or pulse application does it matter to use the opto in saturation region? – cm64 Jul 2 at 14:50