Type K Thermocouple Daughterboard connection problem



  • Hi there,

    On the custom printer I am building, I need to keep the thermocouple daughterboard on the extruder mount in order to be as close to the heating blocks as possible due to the massive size of the printer (otherwise I would have to run 20 + feet of Type K thermocouple wire which I was told would not work-please correct me if this information is wrong). I built a shielded 10 pin connector/cable to run from the control box to where the thermocouple board is mounted but cannot get a stable temperature reading. It will read 2000C on both but will sporadically pick up a temperature for a second or two then return to the 2000C reading. I double checked the continuity and correct placement of all the wires and have measured 3.3 v signal coming to the end of the cable but not on the corresponding top pins of the thermocouple board (which does work perfectly when plugged directly into the duet).

    Any suggestions will be greatly appreciated!

    Ari


  • administrators

    @aplanetrider the issue is the SPI bus that connects the thermocouple board to the Duet is definitely not designed for long distances! In fact it will be even less likely to work than long thermocouple wires.



  • Thanks for such a quick reply and your help! I was afraid of that, I know the thermocouple board uses 3.3v but would it be possible to use 5v ?

    The bed it self is 4'x8', making it a pretty far stretch from the extruder to the control board, we were trying to avoid using thermocouple wires that long (doubtful it would even work).


  • administrators

    @aplanetrider its not just the voltage - its also the capacitance on the SPI bus wires- SPI is not designed to work at any distance at all.

    Thermocouples can be extended if shielded and routed properly - I believe you can even get specific "extension wires" for type K etc. I have not tested long ones with this one, the datasheet:
    https://datasheets.maximintegrated.com/en/ds/MAX31855.pdf
    does not mention any maximum length - the lower resistance the extension you use then longer length will be possible. I does recommend using a twisted pair extension for "long" wires.


  • administrators

    As Tony says, you can extend the thermocouple wires. Use type K twisted pair thermocouple wire for the extension. Some thermocouple manufacturers will make thermocouples with your choice of lead length.

    If you do use an extension cable, keep the join between the two cables away from sources of interference, especially stepper motor cables.

    Don't forget that the thermocouple tip must be insulated from its casing, in other words choose a mineral-insulated thermocouple.


 

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