I have a project with a 4 module optocoupler relay, controlled by an esp32 Arduino.
I wired the project following the instructions to isolate the esp32 MCU and the relay, powering the relay with an external 5v psu from JD-Vcc and Ground, and then the VCC to the MCU.
I understand that having separate Grounds isolates the high voltage circuit from the MCU and thus protects it.
However, I fail to understand how this works since a common ground should always be there for a circuit to work, right?
What's working as a common ground here? or why does it work without one?
The other thing I don't understand is why the VCC, even when you are powering the relay's JD-Vcc and Ground using an external PSU
Why do I need to connect also VCC to the MCU?
Thank you in advance for the help