Problems sizing series input resistor for this optocoupler

Question

Under the electrical characteristics, the optocoupler 4N26 typical "forward voltage" is given as 1.2V. But for sizing the input resistor Im have hesitations.

In my application as a control signal I will use an Arduino 5V digital output for the opto input. The optocpupler will then switch this SSR which requires 16mA to operate independent of input voltage. Here is the basic diagram:

In a net tutorial I found this information: "The CTR depends on the LED input current (IF) and the CTR decreases from a maximum point when the input current is both increased and decreased. If you look at the below diagram, the top of the curve is around the 6mA point. This is where the chip is most efficient."

In my case I have the black package and below is for the CTR vs Forward current:

Here my questions:

1-) So in my case by looking at the plot above I choose the forward current IF as 4mA and from datasheet the forward voltage is typical 1.2V. So I calculate:

R = (5 - 1.2) / 0.004 = 950 Ohm.

So I conclude 1k.

But in many examples I see much smaller resistor value.

Am I doing something wrong here?

2-)

In some examples they place the resistor in return path. Does that have any significance? Her an example:

edit:

Answer 1

The 4N26 is a really cheap crummy optoisolator and has a guaranteed CTR of only 20% at 25°C. It will be worse at temperature extremes and with LED aging. So you would need a minimum of 80mA, but more like 120mA to be safe on CTR, but that's too much for the optocoupler LED. Note that the curve you show is normalized so CTR drops to more like 12% at 20mA.

4N36 has a more reasonable 100% CTR guaranteed so maybe you can choose 25mA drive but that's still a bit high for the Arduino and for good opto life. Or put a transistor on the SSR side to drive the SSR.

Note that the SSR already includes proper isolation so you may not require anything more than a BJT or MOSFET to drive the SSR if you don't need to keep the 12V power supply isolated. In such a case, I would use something like a 2N4401.

Answer 2

It is because the maximum input current of the LED is 100mA (60mA for M version).

You rather drive the LED around 20-40mA to make sure the opto-transistor is conducting fully.

5-1.2/0.02 = 190 ohm so you can use 180 ohm.

Answer 3

The phototranstor browns out over time, so the CTR ratio falls. You need to supply a consitent current. There are other optos with smaller current also. But your SSR device already has opto isolation, so an open collector BJT can solve this without problem. Open collector output can be used for relativelly long distances, if this is an issue.

EDIT:

You can eliminate the worry about false triggering. The noise could come from Vcc and not from signal. The easiest is to use a BJT. If noise is a concern, then you might install a differential choke and cap, MOV where you suppy the output Vcc. Or viceversa, you add this filter to the input of MCU power supply and left unfiltered for external I/O signals.

^{simulate this circuit – Schematic created using CircuitLab}

But most likely you won't have any issues even if the 12V Vcc goes 5m away, since you have also a low pass filter before and after LDO which supplies the MCU.

^{simulate this circuit}

Professional PLC sytem using same power supply for MCU and IO with choke and opto coupled IO signals.