PT100 converted analogue into duet.



  • I would like to convert all wiring on the x carriage to extra flex to remove the possibility of cable breakage. I want to use a pt100 temp sensor on the hotend. The pt100 output is tiny and subject to noise so to reduce the pt100 cable length I can easily mount a small signal conditioning chip to the x carriage and connect to the duet using extra flex. The output is an analogue voltage proportional to temperature. My problem is that a direct analogue input is not supported in the list of temperature sensors.

    Is this possible ?


  • administrators

    @thecrofter While it does not completely answer your question, using one of our PT100 daughterboards:
    https://duet3d.dozuki.com/Wiki/Connecting_PT100_temperature_sensors

    It allows for use of 4 wire connections to the PT100 sensors that removes the effect of the cable resistance from the sensor. The MAX31865 used also has filters in it to help reject 50/60Hz noise from AC sources and common mode noise.


  • administrators

    It's technically possible, but not supported by the firmware. As soon as you introduce additional analog circuitry, you introduce additional errors that will likely result in the loss of the high accuracy that is the main advantage of using PT100 sensors. That's why we chose to use a daughter board based on a one chip that was specifically designed for this purpose and one precision resistor. A solution involving an analog output and subsequent A to D conversion as you describe introduces multiple sources of error.

    Noise in the PT100 circuit is not normally a problem unless you run unshielded PT100 wires parallel to stepper motor wires for an appreciable distance. If you do need to run PT100 wires close to motor cables, make sure the motor cables are twisted and use 2- or 4-core shielded cable for the PT100.



  • Thanks for the replies, I will stop worrying. I see the daughter board is using a MAX31865 so I presume you use the spi connection to read temp. Several sources sell a MAX31865 based interface card so, just out of interest, could you connect using the expansion header ?.


  • administrators

    @thecrofter it's best to connect it using the dedicated SPI daughter board header because that has the Power, SPI and CS pins. If you are using a different daughter board from ours then you will almost certainly need an interface cable. Try and keep the leads as short as possible. Alternatively just use one of our daughter boards 😉



  • Thank you.


 

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