Do you have room in your control box for a 12 volt power supply for the relays? Also will need a transistor to operate them from the Arduino.
Not quite, I'll have to 3D print another box and work out how I want to go about this next
The little cheap Chinese relays use hemispherical contacts, so alignment can be anything that will allow a contact. And the actual contact point will be a very tiny spot on the hemispheres, so current capacity is very low. My very first Arduino project used two of them switching 12 volt DC. They quickly failed, first open and then the second relay welded closed. Current was 4-5 amps.
Took the cover off with a hack saw. There was something on the contacts that were open. Don't know what it was. The welded were welded. In both cases, the contact alignment was sort of correct.
Proper relays will have either flat or slightly convex contacts with quite a large area of contact. They should be of open construction and may have a transparent case. So you can see the condition of the relay.
Thanks Paul this is good to know
Regarding the 3d printer application,
For starters I would use a relay driver board with opto-isolation. ie the mains side is galvanically isolated from the low voltage arduino or raspberry Pi side.
There are lots of reliable relays out there that can do the job.
I ve been using TE Power Relay 12V OJ-SS-112LM by TE Connectivity for all my mains switching needs (home automation etc) I ve had relays of this type in operation for years now controlling all my house lights, curtains etc with no failure.
Note that this is a 12V coil relay.
Indeed, the one I have standby for this doesn't have opto-isolation so I’ll have to search for another one like you have mentioned. Thank you
It's not the size of the image that's the problem, it's that I cannot see the printing on the top edge of the board properly as it is obscured by the angle of the photo. From what I can see I think the contacts are in the order:
COM - NC - NO
If that's correct then that explains your problem, you are connecting the 2 supplies together when the relay operates. Confirm with a meter what the order of the connections actually is.
Ah, I see what you mean now; I was looking at the diagram at a slightly different way (the pivot in the middle rather than the pivot on the left side). The previous board I was working with had a different orientation
I shall swap the connections around, however the relay might weld itself after some time. I have another board to test this anyway
Updated to correct wiring*
Oh, have you realised that with a 240VAC supply if it happens that the 2 supplies happen to be in anti-phase then then the peak voltage across the contacts is approaching 700V?
That is a good point I didn't think about that, I'm guessing there is no way of minimising this from happening in my configuration?