K Type Thermocouple Daughter Board Schematic?

  • I was searching, but couldn't find it. Can anyone direct me in the location of the schematic and files for the new V1.1 Thermocouple daughter board?

    I need to make a few custom PCBs for my printers and wanted to include the thermocouple connections on a custom designed board. I know the design specs for the MAX31856 are pretty straight forward, but I just like to double check so that I don't make a fundamental mistake somewhere.

    Here is the board I am talking about: https://duet3d.dozuki.com/Wiki/Thermocouple_daughter_board


  • administrators

  • Thank you!

  • @dc42

    Quick question - I've created a similar board, but copied the Adafruit reference design. There are really two differences:

    1. Pull-ups on the MOSI, SCK, and CS lines
    2. 100 ohm resister between the negative thermocouple to the bias input.

    The circuit performs fine when connected to an Arduino, but I can't get it to function with the Duet. I'm thinking it has to do with the pull-up lines. I'm able to connect it, and the M305 command shows comes back without any errors - but then the temperature reading stays at 2000C. On the Arduino, using the same cables, it reads correctly.

    Any thoughts?

  • administrators

    Are you using the correct X parameter in the M305 command, depending on which CS line you are using?

    The pullup resistors shouldn't matter unless their values are too low.

  • So after debugging this all day, I think I've got a lead.

    Based on the Max31856 spec, it could draw about 0.2A each at 3.3V. The ethernet cable I was using happened to be a 32 AWG cable - the actual voltage at the card was closer to ~2.3V by the time it ran through the cable. Spec min is 3.0V.

    I happened to notice that the Arduino would read fine, but when I connected my smart effector as well, the reported temperatures dropped by 2/3rds. Then it started having trouble during boot - thinking it was type J instead of K and all sorts of weirdness.

    I'm going to increase the gauge of the ethernet wire (to 24+ AWG) to see if that resolves the issue.

    On a side note - what's the maximum current that the 3.3V line can handle? I currently have 4 TC boards (~0.8A max) + a microcontroller (~0.4A).

  • administrators

    @tlas said in K Type Thermocouple Daughter Board Schematic?:

    Based on the Max31856 spec, it could draw about 0.2A each at 3.3V.

    The MAX31856 datasheet that I have says on page 2 that the maximum current draw is 10mA. I suspect you have a bad or incorrect connection.

    The maximum load on 3.3V that can be tolerated will probably be limited by heating of the 3.3V regulator. The regulator has a rated output current of 800mA or 1A depending on the revision of the Duet.

  • I was looking at the maximum power on page two for current estimates.

    In any case, I just made a duplicate board and connected it to my test printer with a 24 AWG, 3ft ethernet cable and it worked just fine. Literally just plugged it in and it worked. I have a 7ft cable for my main printer coming in on Friday - I expect it to solve the problem.

    All ethernet cables are NOT created equally...


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