yanivps:
- (Arduino's GND is not connected to anything)
Then the Arduino will have no power and won't be able to control anything. The GND has to be connected to the negative side of the power supply AND (usually) to signal ground of the device being controlled. In this case, where the optoisolator is acting as an LED, the return path is through the I/O pin.
yanivps:
2) Using a AC-DC buck converter
Note: That's a Switched-Mode Power Supply (SMPS). It rectifies the AC into DC and then uses a buck converter to get the lower voltage.
yanivps:
I am using a single buck converter to power relay coils and to power an arduino.
Then a fault in the relay could apply 220V to the Arduino. Not isolated.
yanivps:
3) Using 2 different AC-DC buck converter to convert 220V AC to 5V DC. From one converter I power relay board. From the other converter I power arduino which is connected to board's VCC (jumper removed) and IN pin
This provides the best isolation, but it's only as good as the isolation in the SMPS.