"sensor short to other wiring" with Thermocouple daughterboard



  • Hello, using a Duet 2 with RRF3 on an E3d Toolchanger.

    One Hemera tool has a Maxiwatt hot end with J type thermocouple. Everything works correctly, until the extruder operates. As soon as that drive moves I get the fault in the title. It only happens when the extruder runs.

    I am using Dupont style connectors with thermocouple shielded wire. I have also completely moved the wiring away from any extruder motors or motor cables but the fault persists.

    Any advice?


  • administrators

    Is it a direct drive or a Bowden extruder? If it's direct drive, try grounding the hot end metalwork or the stepper motor body.



  • @dc42 Direct drive. Do you have any advice on how to properly ground the hot end? I've been searching based on your advice from an older post but am not really sure how to go about doing this.


  • administrators

    You have a few options:

    1. If you are using shielded thermocouple wire, and you have connected the shield to ground at the Duet end, you could use the shield to ground the hot end metalwork.
    2. Alternatively you can run a separate wire from one of the stepper motor screws alongside the extruder wires, and ground it near the Duet.
    3. You can use 4-core shielded cable to connect the stepper motor, and use the shield to ground the body of the motor.


  • @dc42 said in "sensor short to other wiring" with Thermocouple daughterboard:

    You have a few options:

    Thank you for the guidance, grounding the motor fixed the issue.

    Now I'm running into another issue -- the temperature reading is 245, but is actually much lower. I did a PID tune but don't know what else I can do but compensate with a higher nominal temperature.


  • Moderator

    @plasticfactory said in "sensor short to other wiring" with Thermocouple daughterboard:

    the temperature reading is 245, but is actually much lower.

    How are you verifying that it's actually much lower?


  • administrators

    Are you sure that you have specified the correct thermocouple type in the M305 or M308 command? What colours are the thermocouple wires? Have you run the correct type of thermocouple wire all the way from the hot end to the daughter board?



  • @Phaedrux said in "sensor short to other wiring" with Thermocouple daughterboard:

    @plasticfactory said in "sensor short to other wiring" with Thermocouple daughterboard:

    the temperature reading is 245, but is actually much lower.

    How are you verifying that it's actually much lower?

    Thermal imaging camera and obvious signs of cold material.

    @dc42 said in "sensor short to other wiring" with Thermocouple daughterboard:

    Are you sure that you have specified the correct thermocouple type in the M305 or M308 command? What colours are the thermocouple wires? Have you run the correct type of thermocouple wire all the way from the hot end to the daughter board?

    I have a feeling I'm about to learn a thing or two about wiring. I do believe it is configured correctly. Here is the relevant portion:

    M308 S3 P"duex.cs5" Y"thermocouple-max31856" T"J" A"Tool 2"
    M950 H3 C"duex.e2heat" T3
    M307 H3 A973.9 C181.7 D4.7 S1.00 V21.6 B0
    M143 H3 S400

    Thermocouple wires from the hot end are black and white, the extension cable used is white/blue and white/red. The extension cable was pulled from a type K thermocouple.

    To correct my first post, these are not shielded cables.


  • Moderator

    @plasticfactory said in "sensor short to other wiring" with Thermocouple daughterboard:

    Thermocouple wires from the hot end are black and white, the extension cable used is white/blue and white/red. The extension cable was pulled from a type K thermocouple.

    I'm not an expert on thermocouples but I do know they are pretty particular about wiring extensions. I'm not sure using a Type K wire on a Type J thermocouple will work as expected.



  • @Phaedrux said
    I'm not sure using a Type K wire on a Type J thermocouple will work as expected.

    It won't.

    For anyone not familiar with thermocouples:
    Thermocouples work by having two different metals in contact that generate a voltage when heated. Different types (K, J etc) use different alloys. And they work because one end of the wires (at the chip) is 'cold', the other end is 'hot'. The chip has a thermistor to measure its local temp, from that it can work out the temp at the hot junction. If you extend the wires with anything other than the specific type of extension wire, you at best move the cold junction away from the chip and make the setup less accurate. Using a different Type wire is going to do odd things, as now your sensor has 4 junctions, not 2.

    PS.
    This is also why thermocouples have a limited lifespan, the two different alloys corrode at the junction...



  • Thanks for all the information everyone. Next step is to learn about thermocouple wire/connectors and order some. Though I'm sure this should be fairly straightforward, any advice on manufacturers, different styles, etc. would be welcome.



  • @plasticfactory as an FYI, the heater PID tuning is tuning how the heater responds to a given power (e.g. how quickly it will heat up/cool down, time lag from power being sent to the temperature actually changing etc). This helps the Duet control the heater to achieve your demanded temperature. It is based on the measured temperature. It is not a way of tuning how accurate your temperature sensor is/the temperature measurements you get.


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