Adding second (backup)thermistor to buildplate

[luka1952]

Hi,
I want to add second thermistor to my bed(as a backup). So it can work in case of failur of the main one and shut down the heating.
This is what i have in the config.g:
; Heaters
M308 S0 P"bedtemp" Y"thermistor" T100000 B4138 ; configure sensor 0 as thermistor on pin bedtemp
M950 H0 C"bedheat" T0 ; create bed heater output on bedheat and map it to sensor 0
M307 H0 B0 S1.00 ; disable bang-bang mode for the bed heater and set PWM limit
M140 H0 ; map heated bed to heater 0
M143 H0 S120 ; set temperature limit for heater 0 to 120C
M308 S1 P"spi.cs1" Y"rtd-max31865" ; configure sensor 1 as PT100 on pin spi.cs1
M950 H1 C"e0heat" T1 ; create nozzle heater output on e0heat and map it to sensor 1
M307 H1 B0 S1.00 ; disable bang-bang mode for heater and set PWM limit
M143 H1 S350 ; set temperature limit for heater 1 to 350C
M308 S2 P"e1temp" Y"thermistor" T100000 B4138 ; configure sensor 2 as thermistor on pin e1temp
M950 H2 C"e1heat" T2 ; create chamber heater output on e1heat and map it to sensor 2
M307 H2 B1 S1.00 ; enable bang-bang mode for the chamber heater and set PWM limit
M141 H2 ; map chamber to heater 2
M143 H2 S70 ; set temperature limit for heater 2 to 70C
M308 S3 P"e0temp" Y"thermistor" T100000 B4138 A"Backup Bed Thermistor" ;configure sensor 3 as thermistor on pin e0temp

How can I configure it to shut off the heater?

Thanks

[dc42]

@luka1952 use M308 to configure the second thermistor as an additional temperature sensor. Then use M143 to configure an additional monitor on that heater, using that sensor.

[luka1952]

@dc42
I have used M308 to configure the second thermistor(as you can see in my config.g)
But when I use M143 got errors after restart.
Do you have an example how does that command should look like?
Thanks

[dc42]

@luka1952 post your config.g file so that we can see how you have used M308 and M143 for the second sensor.

[mikeabuilder]

If your goal is to add a thermal safety to your buildplate heater, you might also consider putting a "thermal control switch" (amazon search term) in the power line to your bed heater. These switches are commonly used to hold the temperature of coffee or tea pots just below boiling. They are inexpensive and robust.

You get one which is normally closed and that opens when the control temperature is reached, disconnecting the power from your bed heater. They operate at a specified temperature, and you select one that opens at a temperature 10 or 15 degrees above the max you think you'll run you bed. The good thing about this is that even if the duet board goes bonkers, or you accidentally set the bed temperature to 1000 degrees, the switch will do the job. On one printer I'm working on, we plan two switches in series, one at 70C for low temperature filaments and one at 115C for high temp filament. We have a toggle switch in parallel with the lower temperature thermal switch to take it out of the circuit during high temperature operations.

[luka1952]

@mikeabuilder
This is really great idea. But it would required the switch to be somehow attached to build plate to sense the temperature, right?
Unfortunately I can’t placed it there because I’ve just insulated everything under the bed. And don’t really feel like undoing it. But maybe for next printer or if I’ll do any changes in this one I can still use it. Thanks

[luka1952]

@dc42
I’ve posted my config.g in the original post.
I’ve removed the M143 from it because it causes errors (Temperature reading error on sensor 3) and I need my printer to do some prints at the moment.

But the missing command (last line) was looking like that:
M143 H0 T3 P1 S130 A2