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:
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:
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?