At what CPU temperature and active cooling ?


  • Moderator

    First you should calibrate the CPU temp display if you haven't already. It may be hotter or cooler than you think.

    https://duet3d.dozuki.com/Wiki/Calibrating_the_CPU_temperature

    https://duet3d.dozuki.com/Wiki/Mounting_and_cooling_the_board

    The documentation on mounting and cooling recommends that the board be mounted vertically to aid in convective cooling. If the case is relatively open to the air and ambient temps aren't too high and your motor current isn't at the max you probably don't need forced cooling. The drivers will be a greater source of heat and require cooling before the CPU will. The CPU itself can probably handle up to 70c without much issue. It would be the driver temps that I'd be more worried about. But again, if your motor currents aren't near the max then it may not be an issue either. And again, how the board is mounted and whether it's enclosed will make a big difference.

    The Maestro drivers need more cooling than the wifi drivers.

    I have three options - use no fan, use a fan that is also hooked to the thermostatically controlled hot end fan or turn the fan on based on a temperature measurement of some kind.

    The article also includes a gcode command for thermostatically controlling a fan based on the CPU temp and driver temp warning flags.



  • Thank you for the links, I will study them tomorrow!



  • It comes down to how long you need it to work; elevated temperature means derating the mean time before failure. Google MTBF and temperature and you'll get an idea, staying below 70 probably means the board will outlive the rest of the machine, below 50 and it'll likely outlive most of us.


  • administrators

    First, have you calibrated the CPU temperature reading? https://duet3d.dozuki.com/Wiki/Calibrating_the_CPU_temperature.

    50C CPU temperature is nothing to worry about, but as others have said it may indicate that the stepper drivers are generating a lot of heat. What currents do you have them set to?



  • Thank you everybody for your input.
    I have calibrated the cpu temperature now and it runs about ten degrees lower than what was previously indicated. During a long print run, cpu temperature is 40C which is cool as a cucumber.
    The Duet and Duex were already mounted vertical and with a good spacing on the back to allow for heat to be carried away.
    Currently the boards are in open air so no issues. Once a cover goes over the electronics, I will look at temperatures again and determine if thermostatic fan control is required or not but I have all the info I need now.



  • @dc42 Hi
    in the docs ,it wasn’t clear to me “ M912 P0 S# where #” Is it a formula or format should be “M912 P0 S-12 “ after calculating value???

    Thanks



  • @ViralTinker It's just a number. So yes, if you think the displayed value is 12 degrees too high, use M912 P0 S-12. Although 12 degrees does look a little extreme. It's best to leave the printer overnight or at least for a good few hours for all the temperatures to settle. Then make a note of the ambient, then apply power and note the CPU reading as soon as possible after applying power.



  • Thank you for confirming I was scratching my head for 5 minutes at first it look liked a formula 🙂


  • administrators

    The specified tolerances on MCU temperature reading at/around room temperature are:

    Duet 06/085: +/- 46C
    Duet WiFi, Ethernet, Maestro: +/- 13C
    Duet 3 main board: +/- 34C

    In practice the tolerances are a little higher because the above figures don't take account of ADC offset and gain errors, or the tolerance of the 3.3V voltage reference.



  • CPU temperature is it something todo with as reference for stepper drivers?..


  • administrators

    @ViralTinker said in At what CPU temperature and active cooling ?:

    CPU temperature is it something todo with as reference for stepper drivers?..

    Not directly, however when the stepper drivers generate heat, they will heat up the CPU. The stepper drivers generate temperature warning separately, but only when the driver chip temperatures exceed about 100C.

    The CPU generates some heat of its own, so don't be surprised to see it rise somewhat even when no motors are energised.


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