Ok, I undertand it now. I will apply the circuit you recommended in next version. But now I still use the old pcb and just asked them to change the resister in solving not enough current issue. One more question, is the data sheet showing the saturation current at the current rate?
One more question. Since our unit is EL817, why can't choose 50% for Ii/Io ratio rather than choosing 300% ratio?No, the output of the opto is not always higher than the input. It depends upon the current transfer ratio as shown in post #50.
For some versions, such as the EL817, you can see that the minimum current gain gives an output current that is 1/2 the input.
Why is that hard to understand? You seem to be focusing on the details without understand the concept.
I believe your confusion is believing that the output always has to be a fixed relation to the input.
That's true when operating in the linear mode but not in the switching mode as is the case here.
In the switching mode you want to overdrive the input by at least a factor of 2 or 3 above the value required by the minimum current gain so that the output is fully saturated in the ON state.
That's true of any bipolar transistor also.
So the output resistor is selected to give a current that meets this criteria.
In my circuit the input current is about 1.7mA and the output current is about 0.6mA through the 5kΩ, insuring that the output is well in saturation when ON.
That's probably more overdrive than is needed by the current gain of the opto you are using but it doesn't hurt anything to do that.