Opto isolated relay

Hi,

I would like to know if I am using an Opto-isolated relay board, in any of the following cases, will I lose the isolation? If one could also explain why, that would be even greater!

1) Using a single 5V power supply. Connect it to JD-VCC (jumper removed) and to GND. Bring out another wire from the same power supply to power up an arduino. Connect arduino's VCC to relay board's VCC and digital pin to IN pin of relay (Arduino's GND is not connected to anything). Will connecting board's VCC to arduino which is powered by the same power supply that powers the relay coils means I lose isolation?

2) Using a AC-DC buck converter to convert 220V AC to 5V DC, from the buck converter bring 2 wires: one to the JD-VCC and one to power an arduino which then connects to relay's board VCC (jumper removed) (same as 1 but instead of power supply I am using a single buck converter to power relay coils and to power an arduino.

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

I have a feeling that only the third option gives isolation while the other two means my arduino is not isolated from the relay coils, but if the reason for that is because the arduino is powered by the same circuit which powers the coils (same power supply / same buck converter circuit) I would argue that even in the third option, the 2 different buck converters are connected to the same AC source (in this case, my house electricity) and therefore, the arduino and the coils are not really isolated.

Would be very interesting to hear your opinion about this one...

Thanks in advance!

The transformer inside a mains powered supply isolates the high voltage side from the low voltage side.

Only option#3 with two separate supplies can give opto isolation.

Mains AC to low voltage DC is just a power supply. A DC/DC buck converter is a device that can change a DC voltage into a lower DC voltage. Leo..

A relay is an isolator as well. Opto relays are kind of double... Opto isn't needed to isolate the switch contacts from the Arduino and is defeated if you don't use a separate supply for them.

yanivps: 1) (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.