Electronic – Is this opto input current sized wrong and can it have such symptoms

analogoptoelectronics

I have been using 4N26 opto for some time for a trigger input of a module. Vin is fixed 40Hz 0..5V pulse with %50 duty cycle. OUT goes to trigger of a module.

Below is the schematics:

enter image description here

Among thousands of triggering, I'm observing false triggering once in each ten minutes. Since it is very hard to catch that time. I'm suspicious of the opto. Im saying false triggering but Im at the moment not sure since it is hard to catch whether this is really due to pulse trigger problem.

But I noticed that the opto's LED current (If) in my circuit is 8.5mA and Ic for the opto is 5.5mA. But the datasheet gives CTR>20%. CTR = Ic/If

So I guess I designed this thing wrong. I think If should have been at least 5.5*(100/20)= 28mA. Is that correct?

My question is:

Can this be the reason for this random false triggering? Would that be a symptom of an opto if the CTR is drifting? And would replacing 4N26 with 4N35/36 be a remedy? Since 4N35 has CTR>100%

Best Answer

Yes, this definitely could be the reason.

You forgot to include the opto's LED Vf in your calc. Vf typ. is 1.3 V, 1.5 V max. That is a worst case current of 7.4 mA, worse than you thought. The secondary current is around 5.5 mA, way over 20%. The simple fix is to change to an opto with higher CTR.

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