Relay switching unexpectedly. EMI problem?

I have got a small commercially built relay board with two Songle relays |500x412. The jumper is on JD-VOC.

Switching the relay from two digital pins is all fine. It happily clicks away. Putting a small test load on the power side (a light and a power check plug) all still works ok.

But I have 12V PSU plugged into the power side and as soon as one relay switches to power this on the other relay comes on.

When this occurs I have checked the voltage at pins on the relay board one is high and one is low. Suggesting that interference is not changing the state of the other pin but that the problem is more intrinsic to the board.

So from this I diagnose that the problem is not on the cabling from the Arduino to the relay board. Further (I presume - ready to be told that I am wrong) that it is interference from the power side - or possibly something coming back from the PSU connected to the power side.

Most of the things I have read seem to suggest interference affecting the cabling from the arduino to the relay board. I would appreciate suggestions on how to tackle diagnosing and resolving this.

cabling from Arduino is shielded twisted pair (CAT5). Most of my other cabling is shielded. Unshielded is the mains to relay and relay to PSU.

Thank you for reading through this.

The relays are opto-coupled. So do not connect your arduino stuff to the power side.

The arduino is only connected to the pins GRD,IN1IN2,VCC The power side is 240V on the 240V is a 12VPSU

Do you have a link to the schematic?

On your 2 relay module you need to remove the jumper, use a separate voltage supply, and do not use a ground wire from the relay board to Arduino. Now you will have full optical isolation between the 2 boards.

8 Relay Module shown, but use same connection scheme: |500x82

dlloyd: On your 2 relay module you need to remove the jumper, use a separate voltage supply, and do not use a ground wire from the relay board to Arduino. Now you will have full optical isolation between the 2 boards.

8 Relay Module shown, but use same connection scheme: |500x82

Thank you. I made this change and put a MOV across. This document proved most helpful

http://sound.westhost.com/articles/relays2.htm

Well written and very useful - thank you for the link.