Relay protection to kill power to UM2 - but where to put it?

[rotorhead 1]

Earlier this year I setup OctoPrint on a Pi 3b+ for the purpose of additional piece of mind during prints. I don't make a habit of leaving the printer unattended but occasionally it's necessary to be in another room. I also like the idea of many of the other features that OctoPrint adds to printers.

 

I need to work out where to wire in a relay kill-switch - parallel with the power switch or on the main 24v supply line.

My suspicion is that the power switch on the rear of the printer is actually operating a relay/transistor. My plan was to connect a Pi controlled relay in parallel to this switch however I wonder if this might not actually provide the reliable protection I am looking for. The relay is a last resort kill switch in case the printer is about to set on fire but if it's being used to control another part of the printer then could I have a double failure that would mean the relay doesn't actually turn the printer off?

 

Could I have a failure of the temp probe in the hot end and also a failure of something else on the main board that would mean that my relay would not actually turn it all off and the printer would heat on full power indefinitely? If this is the case then I need to think about splicing in to the main 24v power cable and switching that with the relay instead but obviously this is much more complicated and also means that the full 220W will be running through the relay

 

 

Edited August 17, 2020 by rotorhead
make the post less complicated!