Bedtemp open circuit at -5°C

  • Hi, I am running a separately powered peltier bed system with the duet wifi to get down to -10 to -30 C, when the bedtemp hits -5C I immediately get -273C showing open circuit.

    I have tried many different thermistor types and wiring and checked the disconnected thermistors are giving the correct resistance and not shorting at these low temps, and it always fails at -5.0C exactly, so am thinking this might be a hard minimum setting somewhere but am having no luck finding it.

    thanks for any help and merry christmas 🙂

  • administrators

    It's difficult to tell the difference between a high-resistance thermistor at low temperatures and an open-circuit thermistor. For this reason, RRF treats any temperature below -5C as an open circuit thermistor, on the basis that operating a 3D printer below -5C would be unusual.

    This does not apply to PT100 or PT1000 sensors or thermocouples. So if you need to measure lower temperatures, I suggest you use one of those sensor types.

    It would be possible to measure lower temperatures accurately using a lower resistance thermistor, for example a thermistor that has a resistance of 1K at 25C. I will change the firmware to use a lower temperature limit when using a low resistance thermistor.

  • Ah, thanks a lot. It is a research project so hopefully not too many people are doing this... 🙂
    I am amazed and super happy there is an inversion option if I wanted to power any of it off the board tbh!

  • administrators

    Firmware 3.2RC2 which has just been released should allow you to measure below -5C if you use a 1K or 10K thermistor.

  • what a Christmas pressie, thanks a lot!

    here is my work if it is of any interest.

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