Hi all.

Can someone explain me what is the purpose of the resistor in attached circuit

Hello Milosh;

In most transistor circuits (and you can consider an opto coupler a transistor), a resistor in the input is used to control the input current and keep it within acceptable limits for the transistor to operate and produce the expected output. Knowing the output voltage of your circuit at the input of the optocoupler would allow you to calculate that current level.

Maybe this quote can provide more details:

https://learnabout-electronics.org/Semiconductors/opto_52.php

Hope this helps,

Daniel Chartier

Hello dchartier.

Thanks a lot for help.

Yes, this is logic what you send.

But I’m not sure what these bridge is used for. Mean on bridge between A1, and A2 of optocoupler?

BR,

MK