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    Multiple/Combined Thermistors for a single Tool (Heatbed)

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    Duet Hardware and wiring
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    • vpundefined
      vp
      last edited by

      If we assume linear relations, why does in series connection the cooler dominate ?

      When using 2 thermistors, we want Rges* = (R1+R2)/2. When T is adjusted to T/2 this is exactly what happens when connecting them in series.

      Taking the nonlinear behavior into account, Rges would be higher and by that showing a smaller temperature than it has.

      A parallel connection will also show a sightly too small value, even with linear relations.

      e.g.
      R1 = 105
      R2 = 95

      Rges in series = (105 + 95)/2 = 100 (T value adjusted to T/2)
      Rges in parallel = 105*95/(105 + 95)2 = 99.75 (T value adjusted to T2)

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      • dc42undefined
        dc42 administrators
        last edited by

        If we assume linear relations then neither thermistor dominates in the series case. But the relationship is not linear: the resistance change per degC is smaller as the thermistors get hotter. That is why I said that the cooler thermistor will dominate. In an extreme case, if one of the thermistors fell out of the bed and was at room temperature instead, the bed could reach infinite temperature but the temperature calculated from the total resistance would only be about 40C

        Duet WiFi hardware designer and firmware engineer
        Please do not ask me for Duet support via PM or email, use the forum
        http://www.escher3d.com, https://miscsolutions.wordpress.com

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        • vpundefined
          vp
          last edited by

          now it is clear to me, that was what i meant with

          Taking the nonlinear behavior into account, Rges would be higher and by that showing a smaller temperature than it has.

          To sum it up, in case a thermistor breaks completely, connection in series is better, because it auto detects the problem, in case a thermistor is completely wrong (too cold) but still alive, connection in parallel would be better.

          So it is a lose or lose situation;)

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          • dc42undefined
            dc42 administrators
            last edited by

            Yes, that about sums it up.

            Duet WiFi hardware designer and firmware engineer
            Please do not ask me for Duet support via PM or email, use the forum
            http://www.escher3d.com, https://miscsolutions.wordpress.com

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            • Synapsisundefined
              Synapsis
              last edited by

              If I want to connect a second thermistor for a virtual heater do i connect to Pin 35 for thermistoR3 but the other wire goes to ground? I could not find a wiring diagram.

              M305 P103 T100000 R1000 B4200 STEMP2 but how do i tell it is Thermistor3

              Thanks

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              • dc42undefined
                dc42 administrators
                last edited by

                The other wire should go to AGND.

                Add parameter X3 to that M305 command to tell it to use thermistor 3 (the one labelled E2).

                Duet WiFi hardware designer and firmware engineer
                Please do not ask me for Duet support via PM or email, use the forum
                http://www.escher3d.com, https://miscsolutions.wordpress.com

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                • Synapsisundefined
                  Synapsis
                  last edited by

                  Great thanks David

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                  • Synapsisundefined
                    Synapsis
                    last edited by

                    Ok gots things hooked up and setup, my questione is wheere do i find the reading in the Web Control if you can see it that is?
                    Under Extra I have only a MCU which has always been there.

                    ok found out this:

                    M305 P103
                    Heater 103 uses Thermistor sensor channel 3, T:100000.0 B:4700.0 C:0.00e+00 R:1000.0 L:0 H:0

                    does this meah I have to create a tool Heater 103?

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                    • Synapsisundefined
                      Synapsis
                      last edited by

                      @dc42:

                      …. (the one labelled E2).

                      Sorry but I can,t find a E2 there are other two like E2_Step, E2_N amd more. (which you surly know πŸ™‚ )I thought it was the Pin 35 Labeled Thermistor3-

                      Before I burn something I would rather be sure. Thanks

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                      • dc42undefined
                        dc42 administrators
                        last edited by

                        Yes it's the pin labelled Thermistor 3 if you are connecting directly to the expansion connector.

                        Duet WiFi hardware designer and firmware engineer
                        Please do not ask me for Duet support via PM or email, use the forum
                        http://www.escher3d.com, https://miscsolutions.wordpress.com

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                        • Synapsisundefined
                          Synapsis
                          last edited by

                          Strange then, please what is the G code to read the temperature as I can not find it.
                          Also I see nothing the the Web Control.

                          Just a thought is there a specific GND to use I used Pin 42.

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                          • dc42undefined
                            dc42 administrators
                            last edited by

                            Connect it to the AGND pin (also called VSSA), not GND.

                            You will find the additional temperature in DWC if you follow the Extra link above the heaters display, provided that you give the heater a name in the M305 command.

                            Duet WiFi hardware designer and firmware engineer
                            Please do not ask me for Duet support via PM or email, use the forum
                            http://www.escher3d.com, https://miscsolutions.wordpress.com

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                            • Synapsisundefined
                              Synapsis
                              last edited by

                              Thanks David did not know VSSA was also called AGND…

                              I thought it was a typo error and you wanted to say a GND. πŸ™‚

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                              • Synapsisundefined
                                Synapsis
                                last edited by

                                Not so good I'm getting a reading of 1100,00 .
                                Now if I connect to the main bedheater connector I get the same value as the one there is now.
                                So I know its not the thermistor giving strange readings seeing when connected there it works.
                                Gave it a name etc. I tried different B and R values but nothing it may go a low as 600C.
                                Any ideas as to why itws not giveing correct readings. Connected to Pin 35 and 40 on the expansione bus.

                                This is my setup I copied the first heater 0 settings
                                ; Heaters
                                M305 P0 T100000 B3950 C0 R4700 ; Set thermistor + ADC parameters for heater 0
                                M143 H0 S120 ; Set temperature limit for heater 0 to 120C
                                M305 P1 T100000 B4725 C7.060000e-8 R4700 ; Set thermistor + ADC parameters for heater 1
                                M143 H1 S280 ; Set temperature limit for heater 1 to 280C
                                M305 P103 T100000 B3950 C0 R4700 S"TEMP2" X3

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                                • dc42undefined
                                  dc42 administrators
                                  last edited by

                                  I just checked the schematics, and you also need to connect a 4.7K 1% resistor between the thermistor input and ADVREF.

                                  Duet WiFi hardware designer and firmware engineer
                                  Please do not ask me for Duet support via PM or email, use the forum
                                  http://www.escher3d.com, https://miscsolutions.wordpress.com

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                                  • Synapsisundefined
                                    Synapsis
                                    last edited by

                                    OK think I'll buy the daughter board πŸ™‚

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