Type K Thermocouple Daughter Board and external SD card problem



  • I have a problem with an external sd card connected to a duet wifi board. When I run gcode from this card the thermocouple readings start to crazy and show peaks around -1900 degrees. When i run gcode from sd card on the board this problem does not occur. When I switch to thermistor the problem does not occur. The board is new and has the latest software. Did anyone have the same problem



  • Hi,
    The Thermocouple Type K Daughterboard is known to suffer from temperature spikes that goes to ~2000[deg].
    (So I am not sure if your problem is related to the SD card at all)
    Thermocouples are based on "Seebeck effect", which state that a two different metal junction create a voltage difference proportional to the junction temperature.
    The voltage that is created is only micro-volts, so any electrical interference near the thermocouple wires can interfere the thermocouple "signal" and causes an incorrect temperature reading.
    Electrical interference can be caused by any wires that carry a changing current especially if its high currents, for example stepper motor wires, or heater wires.
    So keep away the thermocouple wires from these kind of wires, and use shielded thermocouple wires.

    According to DC42 there is a modification that can be done to the thermocouple daughterboard that could solve the temperature spikes problem. You can read more about the mod in the following link:
    https://www.duet3d.com/forum/thread.php?pid=14255#p14255
    For this mod, it is recommended to use 0805 size SMD capacitors, 0.01[uF].

    Switching from thermocouple to thermistor is also a good solution. Thermistor relay on resistance change proportional to the temperature, so they are not affected by external electrical interference.
    The downside is that thermistor max service temperature (~300[deg °C]) is much lower (and maybe less accurate in rage of 200-300[deg].) compare to thermocouple type K max temperature (~1300[deg °C])
    For polymers 3d printing application, thermistors are good enough.

    Regrading the negative temperature reading, try to reverse the polarity of the thermocouple connection to the daughterboard, the reading should be a positive temperature.


  • administrators

    The thermocouple and the external SD card use the same SPI bus. They should run independently, but perhaps they are interfering. I'll test it when I am back in the office.



  • Perhaps the SD card extension is putting noise on the SPI bus?


  • administrators

    @elmoret:

    Perhaps the SD card extension is putting noise on the SPI bus?

    Yes, or the length of cable plus the SD card is loading the bus too much for the thermocouple chip. In which case it won't make any difference where the Gcode comes from, but whether the external SD card is plugged in or not may make a difference.



  • I checked signals on SPI bus and MISO signal is faulty if the thermocouple and the external sd card are working simultaneously. The second picture shows the correct signal when the thermocouple is working and the print goes from the internal card. It looks like the thermocouple and the sd card are transmitting at the same time.
    https://drive.google.com/open?id=0BxTGGx-AlVmKdEhBdEJoVldnY3c
    https://drive.google.com/open?id=0BxTGGx-AlVmKYldEdUdHbTJqdEU


  • administrators

    Can you use your oscilloscope to check whether the CS signals overlap?



  • Problem solved. It turned out that SD card loaded the miso rail so I separated the miso from SD card with a 1kohm resistor. Signal from thermocouple is now correct and everything works. Thanks for any suggestions.


  • administrators

    Hmm… I don't think that should have been necessary, assuming that the firmware is not overlapping the CS signals and the SD card sets its DO pin to high impedance when its CS pin is not active. I'll test it here tomorrow.


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