For a 4N25/26 I tried to plot the CTR vs LED Current graph by simulating the following circuit. To obtain results similar to the one in the datasheet I keep Vce constant at 10V and sweep the forward LED current between 0.1mA upto around 50mA.

enter image description here

As you see the peak CTR occurs when the LED current is around 1.8mA.

But in the datasheet here, at Fig. 2 or Fig. 3 the graph looks very different and the peak CTR occurs at 10mA LED current:

enter image description here

CTR plot is basically Ic/If when Vce is kept constant. But I obtain very different plot. Am I interpreting something wrong?

  • \$\begingroup\$ The saturated peak CTR occurs at 10 mA, not the unsaturated CTR (VCE = 10 volts). \$\endgroup\$
    – Andy aka
    Jul 3, 2019 at 13:59
  • \$\begingroup\$ This is a bit confusing to me. In the circuit I use the Vce is kept constant at 10V which means I'm plotting unsaturated CTR as you informed. But the LTspice plot doesn't look like that one either. If mine is unsaturated CTR, its shape must look like their unsaturated CTR.but it doesn't :( \$\endgroup\$
    – cm64
    Jul 3, 2019 at 14:13
  • 1
    \$\begingroup\$ Models don't have data sheets so I trust the real device (and its data sheet) more. Inspect the model is my advice. \$\endgroup\$
    – Andy aka
    Jul 3, 2019 at 15:46
  • \$\begingroup\$ Seems the transistor's gain is not modeled correctly. Simulation says it peaks at Ic = ~1.3mA, whereas the datasheet says it is fairly flat out to ~70mA and then drops off rapidly. I tested a real 4N25 and performance was similar to the datasheet. \$\endgroup\$ Jul 3, 2019 at 17:59
  • \$\begingroup\$ Optocoupler technical data, particularly CTR, often suffers from a wide tolerance, so you may measure quite different values from one exemplar to another, especially when they are from different manufacturing batches. \$\endgroup\$
    – sh-
    Jul 3, 2019 at 19:56

1 Answer 1


It is possible that the SPICE model was created for a particular version of the 4N25 that is manufactured by someone other than Vishay. Note that the results from your SPICE simulation do meet the datasheet specifications so in that sense the simulation is correct. The graphs that are provided in the datasheet are for informational purposes; the specified minimum CTR is 20% and that is the only guarantee you have.


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