About bed heater failsafe using multiple thermistors

Schild0r

Hello there,

if this exact question has been answered before I could not find it (if you do please link me to it).

I have a 6mm aluminium plate with a keenovo silicone heater with built in thermistor as my heated bed.
Screwed into the aluminium plate is also one thermistor with an M3 thread. That one is for the actual PID control so it controlls the temp of the aluminium plate and not of the silicone heater.
The glue of the silicone heater is stable up to about 150°C (long term) obviously the heater should not exceed that temperature when heating the aluminium plate due to the delay that comes from the thermal mass of the aluminium
Screwed onto the aluminium plate is a 125°C thermal fuse (no insulation around it so the bed plate will probably be at little over 150°C once the fuse actually fails which is okay for the glue to still not be damaged)
Maximum working temperature for the buildplate should be 120°C

So I use

M308 S0 P"temp0" Y"thermistor" A"heater_temp" T100000 B3950 ; define bed temperature sensor
M308 S1 P"temp1" Y"thermistor" A"plate_temp" T100000 B3950  ; define bed temperature sensor
M950 H0 C"out0" T1                                          ; create bed heater using T1 for control
M143 P0 H0 S125 T1 A0                                       ; set the maximum bed plate temperature to maximum working temperature + 5°C overshoot
M143 P1 H0 S140 T0 A2                                       ; switch of the heater temporarily if it exceeds 140°C on the silicone heater during heating due to thermal delay

That makes sure that the heater can work properly and doesn't damage the glue when heating up.

Now lets consider the failure modes of a bed heater:

Mosfet fails as a short

heater fault will be triggered without an effect but
thermal fuse will take care of that

Silicone heater somehow detaches from the aluminium plate

T1 will read lower than expected for extended period of time and the controller will trigger a heater fault (according to the wiki) correct?
During that time the heater will stay below 140°C due to Heater Monitor 1

Thermistor breaks

If T1 fails and reads -273°C I will get a heater fault immedeately
But what happens if T0 fails???

How do I set up a heater monitor to trigger a heater fault when T0 fails
Is it as simple as

M143 P2 H0 S-10 T0 C1 A0

? or doesn't the C parameter require an S parameter (because the S parameter is explicitly described as maximum permitted temperature and not as trigger temperature)?

And on that note should the thermal fuse be installed on the silicone heater with high temp silicone or should it be screwed into the buildplate? I am not sure whether it is more likely that the silicone heater removes itself from the aluminium plate due to overheating before the fuse trips or whether the glued on fuse falls off the silicone heater due to overheating before it trips...

Phaedrux

I would suggest that you should use a citrus oil based glue remover to detach the heater from the plate and replace the 3M adhesive with a high temp RTV silicone instead.

https://www.amazon.ca/Permatex-81878-Copper-Silicone-Gasket/dp/B01LZ7WZAT/ref=sr_1_39?keywords=rtv+silicone&qid=1637961534&sr=8-39

Or have you already used RTV silicone? As you do mention it for attaching the sensor.

I would attach the fuse to the heater pad itself.