Multiple optocouplers current draw concern (Due)

Other Hardware General Electronics
1

Hi. I'm doing a project where I need an Arduino Due to press multiple buttons on a video game controller. I will be connecting the outputs to these buttons through optocouplers, but I'm unsure if this could end up drawing too much current. As many as 14 optocouplers could be activated at the same time, and I'm not sure if that will exceed the 130mA max on the Due I/O.

I also need to drive a small OLED display and maybe shift registers too.

Looking at the optocoupler datasheets it's a bit overwhelming. Is "IF forward current" the information I'm looking for? And if so these should be able to do the job, right?

https://www.mouser.dk/Optoelectronics/Optocouplers-Photocouplers/Transistor-Output-Optocouplers/_/N-6x5jkZ1yzvvqx?P=1z0yye2Z1y965r7Z1yopnxo&Keyword=optocoupler&Ns=Pricing|0&FS=True

Those do have a pretty high fall time compared to some of the others I was looking at. Not a problem in this project, but I would like some that are faster if possible.

2

You can use a much lower current than 10mA.

You might only need say 1mA per opto isolator, it depends on how much current the load needs (probably a small current through a pull-up on the controller) and the Current Transfer Ratio of the optoisolator you choose.

3

Hi John. Thank you for answering my question. I guess my understanding of optocouplers was a bit off. I was under the impression that the input side of the optocoupler would never be affected by the load on the transistor side.
But that is what you are saying, that the the higher the load on the output the more current will drawn from the input, is that right?

These were the ones I looked at initially:

If they don't draw much more current than the other ones I'll go for these instead due to their improved fall time (for future projects).

4

Konsolkongen:
But that is what you are saying, that the the higher the load on the output the more current will drawn from the input, is that right?

Close, but no cigar.
The higher the load on the output the more LED current is needed to saturate (turn fully ON) the opto transistor.

If the circuit of the game controller has a pull up current of say 100uA, then the transistor must be able to sink that.
If the opto coupler has a minimum CTR of 50%, than a LED current of >=200uA is needed.
More LED current isn't making any difference to an already fully conducting opto transistor.

Opto LEDs have a Vf of about 1.2volt at low current.
A 2k2 resistor would give a LED current of about 1mA, and is likely more than enough.
4k7 or even 10k could also work.
Leo..

5

Okay that does make a lot more sense, I was pretty sure that the two sides of the opto was supposed to be completely isolated :smiley:

I'll go with the Toshiba ones. Thank you both for helping me out :slight_smile: