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.



  • I know this is an old thread, but thought I'd drop my experiences in here.

    I converted a Raise3D N2 and N2+ to Duet2 Wifi. I used a PT100 board for the bed heater, and the Raise printers use type K thermocouples in the print head.

    I did not know there was an issue mounting the thermocouple board remote. I used 26ga ribbon cable and 10 pin ribbon cable connectors to make an extension cord. The PT100 board is on the Duet board. I mounted the thermocouple board up on the print head.

    On the smaller N2 the ribbon cable is roughly 1.5M long. It's been running fine since June. At one point I had an issue with a thermocouple reporting 2000 degrees - I replaced the thermocouple and the error went away. The thermocouple did not appear to be bad when it was off the machine.

    The larger N2+ has almost 2M of ribbon cable to the thermocouple board. I had persistent issues on that machine with the thermocouples faulting on power up and reporting 2000 degrees. I swapped some thermocouples around and got it so E0 was reporting OK, but not E1. Poking around the forum at that point was when I learned I had done everything wrong.

    On the N2+ I moved the thermocouple board down to the control board and ran shielded, twisted, type K thermocouple wire up to the print head. That did not resolve the startup fault problems - but the machine was working and I had work to do.

    I purchased more thermocouple wire so I could rewire the N2 if it started giving trouble. I haven't had the need.

    After 6 months or so of heavy use, the N2+ started getting temperature spikes on E0 and faulting. I tried a different thermocouple, but that did not resolve the spikes. The thermocouple extension wire is foil shielded, so it may be that the foil split in the flex to the print head. Just speculation.

    Since the smaller N2 has worked great for 9 months or so, I decided to try reverting the N2+ to the ribbon cable extension, knowing that it's not supposed to work.

    When I got the ribbon cable re-routed and powered up the printer last night, the thermocouple that had been faulting reported 2000 degrees. I replaced it and that seemed to make things happy.

    I've now been printing straight for about 24 hours and haven't seen a temperature spike.

    I don't know enough about this sort of thing to make any claim about right or wrong, but it's working for me. I know I might just be lucky. (I don't usually consider myself lucky)


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