Redundant bed thermistor safety

[p8blr]

I've got a large bed that's heated in three zones because I'm using (3) phases of 120VAC from a 480VAC transformer. Currently I only have one thermistor in the middle of the bed and one near the edge that's just used for monitoring. However, yesterday I experienced and issue where one of the SSRs controlling one of the zones died (SSRs typically short closed) so one zone got too hot and I was unaware. I'd like to add a thermistor for each zone and perhaps have an external trigger, trigger a heater fault if one zone is for instance 5C different than another. Is this possible? I know I can have a limit set with a M143, but that doesn't verify that the three thermistors are in agreement.

Thanks.

[p8blr]

@KenW Thanks, I might pursue this avenue. However, I'd need something rated a bit higher as I'm hoping to print PEI eventually.

[jens55]

@KenW, I believe my thermal fuses are 165C. They are readily available via Aliexpress.

[p8blr]

@dc42 Does the latest firmware still support virtual heaters? I'm not sure how to make the M950 command since heater 103 is out of range. If I were to setup a virtual heater and tie my extra thermistors to it, I could set a max temp limit so that heater-fault.g would disable a contactor powering my bed SSRs.

[dc42]

@p8blr there is no need for virtual heaters any more, because RRF3 treats temperature sensors and heaters as separate entities.

You can configure the heater to use one sensor to control it, and use the other sensor to enforce a temperature limit using the M143 command.

[p8blr]

@dc42 I'm using an array of heaters and SSRs on a large print bed that each have their own thermistor for safety. I've created a script that runs in daemon.g that disengages a contactor that provides power to the SSRs if one temperature reading drifts too far from the rest in the array (A SSR failing closed could cause one set of heaters to get too hot or cold). It works, but I think it would be useful if a comparison of multiple thermistors (more than three) was an option in M143. Thanks.