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As a part of a system I need to replace a proper opto-coupler with an old one which had insufficient CTR for the given circuit section without changing any other component:

enter image description here

The pulse input is 0..5V pulse train with 50% duty cycle and the frequency of the pulse train is between 10Hz upto 550Hz.

So the forward current If roughly becomes (5V-1.5V)/470 = 7.5mA. And the opto's output current(collector current of the opto) is 12V/2.2k = 5.5mA.

Previously the old opto had min 20% CTR. So If should have been 5 x 7.5mA= 37.5mA but it was only 5.5mA.

So down to overall CTR of 80% is enough for a guarantee operation since If is 5.5mA and output current is 7.5mA.

But for a reason instead of changing R1, R2 or R3; I need to replace the opto-coupler with a proper CTR. I want to use a CNY17F-4XG where its data sheet is given here.

The current transfer ratio versus If is given in the following plot:

enter image description here

What I understand from above, at 25°C where Vce = 5V and in my case where the If is 7.5mA the plot shows a CTR of 190%.

But in my case the ambient temperature can be between -30°C upto +30°C.

I'm stuck at couple of points to estimate the worst case CTR.

  • How can the temperature effect on CTR be interpreted in this case? Like how can I estimate the variation CTR at different temperatures relative to their 25°C.

  • The test results in the datasheet are for where Vce = 5V. Does "Vce = 5V" here mean when the LED of the opto-coupler is off? In my case is Vce = 12V? Again how could we estimate the variation of CTR at 12V relative to their 5V Vce test?

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  • \$\begingroup\$ I had this same exact issue with a Vishay optocoupler not too long ago (TCLT1002). The bottom-most left graph shows a normalized CTR vs Ambient graph. This is a multiplier for the CTR value at 25C. Looks pretty flat from -30 to 30, maybe another 5-10% decrease. As for your Vce, your If is 7.5mA so it will be slightly above the lowest line on the top-right most graph, which also looks pretty flat. It is hard to give an exact answer though (which is why they make the datasheets so confusing! </rant>) \$\endgroup\$ – Stiddily Jul 2 at 12:41
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The datasheet says that at IF = 7.5 mA, the CTR is typically about at least 95 % of that at 10 mA, and that between −30 °C and 30 °C, the CTR is typically about at least 90 % of that at 25 °C:
Isocom CNY17 CTR vs. temperature

Of course, for different IF and VCE values, the relationship might be different.
Vishay's CNY17 datasheet is more informative:
Vishay CNY17 CTR vs. temperature

Your circuit wants to pull the output as high as possible, so you do not care about the CTR when VCE = 12 V, but when VCE ≈ 0.4 V, i.e., when the phototransistor is saturated. Like with 'normal' BJTs, you need more base current (or here: more IF) to make the output saturate.

So, the worst-case unsaturated output current is 7.5 mA × 160 % × 95 % × 90 % = 10.26 mA. Even with a safety factor, this means that an output current of 5.5 mA will saturate the transistor properly.

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