Duet 0.85 thermocouple and max31855 breakout



  • Hi,

    I connected a thermocouple to my printhead and got always a temperature reading of 2000 degrees.
    I have a duet 0.85, an Adafruit MAX31855 breakout and a standard k-type thermocouple also from Adafruit.
    The breakoutboard is connected as follows with duet 0.85
    Vin to pin 25
    Gnd to pin 21
    Do to pin 26
    CS to pin 20
    CLK to pin 24

    In the config file i put M305 P1 X100

    I replaced the thermocouple already with a normal cable to check. the readings stays at 2000 degrees. i checked already with another thermocouple the reading stays at 2000 degrees. i connected a 100uF capacitor between the two leads of the thermocouple to no avail

    I checked all the documents and manuals i could find concerning thermocouples combined with a duet 0.85
    Any idea what i am missing?
    I am out of ideas


  • administrators

    You have used the pin connections for the expansion connector on a DueX4 expansion board. If you don't have a DueX4 in your system, you need to use the ones in the previous column of the table at https://duet3d.com/wiki/Connecting_thermocouples#Duet_0.6_and_Duet_0.8.5.

    A good way to check things out is to disconnect the thermocouple and connect a short link wire between the two thermocouple terminals in the MAX31855 board. You should get a reading of room temperature. Is that what you did when you say you connected a "normal cable"?



  • i have a duet 0.85 which uses as far as i know the DueX4 expansion board, so i supposed the pins for the Duex4 connector could be used?

    With a "normal" cable i meant indeed a short link wire 🙂


  • administrators

    The DueX4 expansion connector referred to is a 26-way expansion connector on the DueX4 itself, not the main one that connects to the Duet. I'll clarify that in the table.



  • OK, i replaced the wires.
    I took 27 as CS

    config stays M305 P1 X100

    The 2000 is gone and replaced by 0 degrees.
    I replaced the thermocouple with a short link wire to check. still 0 degrees.

    i disconnected the capacitor, still 0 degrees (is a 10V, en 100 uF no ceramic)

    other suggestions?



  • could it be possible that i fried the breakout max31855 by connecting it as i did? I would expect that those pins weren't used, and didn't fry the board.

    Is there a way to check if it is fried?

    Is it possible to connect the older AD8495 breakout thermocouple board with the duet? i have still one.



  • Hi, I did buy one original MAX31855-based daughter board for my Duex5 expansion but i also do not get a correct reading from it. I tried the M305 P103 X100 S"test" command (changing numbers from 100-103 and 150-153) with a typ K thermocouple as well as a shortend contact between the two terminals of each terminal block and i do get the test sensor displayed in the "extras" folder of the web gui 🙂 but the reading always says 2000°C. Can you tell me what i might possibly be doing wrong here?
    Before using a "virtual sensor" like P103, I tried M305 P1 X100 as you suggest but that didnt work either in any combination, however it scrambled my pt100 settings (pt100 daughter board on DuetWifi) somehow, could there be a firmware based interference between the two different daugther boards (just updated to 1.192)? Also I dont understand what this M141 command is really good for, I dont find it well documented on reprap wiki but that is possibly just me 😉

    PS: Autolevelling of my four axis bed now works thanks to your updates and advice!


  • administrators

    Stellator, did you plug the daughter board into the Duet, or into the DueX5? If you plugged it into the DueX5 then the channel number are 104 to 107. Test the board and configuration with a link wire if you are getting 2000c readings with a thermocouple connected.

    Lazy_mosquito, all the pins on the expansion connector are used, so it's quite possible that you damaged the thermocouple interface board.



  • Dear dc42, thanks for the quick reply, i never saw these numbers but it worked! One also can see immediatly that the unshielded straight wired TC is much more prone to noise than the 3wire pt100 but it has such a small footprint compared to any pt sensor i know that some shielding will be worth the effort.

    thx!


  • administrators

    @stellator:

    Dear dc42, thanks for the quick reply, i never saw these numbers but it worked! One also can see immediatly that the unshielded straight wired TC is much more prone to noise than the 3wire pt100 but it has such a small footprint compared to any pt sensor i know that some shielding will be worth the effort.

    thx!

    If you have one of the MAX31855 daughter boards and it doesn't already have the extra capacitors added as described at https://www.duet3d.com/forum/thread.php?pid=14255#p14255, then adding those capacitors will make it much more resistant to noise. The later MAX31855 boards we shipped have those capacitors fitted already.



  • probably fried it :-(.
    I ordered a pt100 with a max 31865 board and will try that instead. It will also give me more correct readings.
    Thnx for the help.


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