Need help with bed resistor values



  • Hey all, I'm testing out duet2 wifi and must say I'm impressed (except the whole ending up with a v2 config out of the configurator, fixing it bit by bit and then realising that I should have just asked for a v3...)

    Anyhow, this printer that was on ramps (very modified anet a8) is all running nicely except the bed temp is WAY off (I have a non-contact thermometer and it's off by about 20 degrees.) I seem to be one of the "lucky" folks who ended up with something that kinda fell within the vishay (marlin no 8 ) thermistor. At least, Marlin/RAMPS was working close enough with that, unless the firmware on disk doesn't match what I had but I think it did.

    https://github.com/MarlinFirmware/Marlin/blob/2.0.x/Marlin/src/module/thermistor/thermistor_8.h is the table from marlin but I don't quite know how to convert that to Beta with a C value, or at least if I use the comment in that file (beta25 = 3950 K) with a C of 7.060000e-8 or no C or C0, it doesn't match nicely at all.

    I also don't seem to find a nice walkthrough for using a thermometer and getting the resistance values. Is there a G-code for getting the raw thermistor value I can plug into the calculator then?


  • Moderator

    @ranyardm It might be because that Marlin thermistor table specifies a 10k pullup, rather than 4k7 used by most printer electronics including the Duet 2. But then it's quite possible that the actual table has been fixed to supply the right numbers. Unfortunately, I can't see a way to show the raw thermistor value (it's not part of the Object Model), and I'm not sure that would help anyway.

    If you have thermometer and a multimeter, you could set a temperature, wait a few minutes for the temperature to stabilise (with the thermometer touching the bed), then quickly disconnect the thermistor from the Duet and measure the resistance. If you do that at three different temperatures (including checking that it reads 100k ohm at 25C) you could put the numbers into the first box here: https://www.thinksrs.com/downloads/programs/therm calc/ntccalibrator/ntccalculator.html and generate a beta or Steinhart-hart model.

    Or swap the thermistor for a more 'standard' one!

    Ian


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