Bed Thermistor M305 Setting From Configurator



  • When running the Reprapfirmware configurator using version 1.17 and selecting the E3D thermistor (4719 K for the beta value and 3.166739e-6 for the C value) for the heater results in this line in config.g:

    1. M305 P1 T100000 B-6788 C3.166739e-6 R4700 ; Set thermistor + ADC parameters for heater 1

    E3D recommends using 4719 K for beta and 7.08e-8 for C so I manually change the C in the RepRapFirmware configurator and end up with this line in config.g:

    1. M305 P1 T100000 B-6788 C7.080000e-8 R4700 ; Set thermistor + ADC parameters for heater 1

    In both of these cases the configurator correctly shows 4719 K as the beta value and 3.166739e-6 and 7.08e-8 respectively in the C value column before I output the configuration files.

    These don't work. The temperature reading starts to drop when you try to heat it up. I then manually changed the config.g B value to 4719 to have this line in config.g:

    1. M305 P1 T100000 B4719 C7.080000e-8 R4700 ; Set thermistor + ADC parameters for heater 1

    My question is: Did I do the right thing in changing the B value to 4719 (and thus is the 3rd M305 line I listed correct?)


  • administrators

    Yes. Looks like there is a bug in configtool. I'm guessing that the 3.166739e-6 value is wrong too. Btw I wrote that section of the E3D wiki with the parameters to use for RRF.



  • Thanks for the incredibly fast response!

    Setup of DuetWifi has been a breeze for the most part because of these forums and the wiki. I'm incredibly glad I decided to switch to it (and will probably end up getting a 2nd PanelDue & Duet combo for the other printer I'm working on.)


  • administrators

    I just recalculated the thermistor coefficients again from the table at http://www.atcsemitec.co.uk/gt-2-glass-thermistors.html to see which C value is correct. I now recommend B4725 C7.06e-8. Configtool will be changed to these values (and the bug fixed) and I'll update the E3D wiki.



  • Any chance of a sticky thread to keep current recommended default settings (thermistor and any others) more visible?

    Rediscovering this info was by somewhat by luck and chance.

    Also, the Configurator Tool Heaters tab still shows thermistor default β values of 3988 and 4138 for Bed and E0 respectively.


  • administrators

    following on from the earlier post this is better on the wiki than in the forum I think.



  • BTW, https://duet3d.com/wiki/Changes_to_config_gcodes_and_firmware_behaviour says:
    "The hot end thermistor B value defaults to 4388 (M305 command). This is the correct value for the Semitec 104GT-2 thermistor used in the popular E3D hot ends, to give accurate readings around 220C."

    dc42, Is that still good generic info for the Semitec 104GT-2 B term without a C term, or does it need updating to reflect your latest recommended values?


  • administrators

    Yes, 4388 is still good if you don't use a C term.



  • @dc42:

    Yes, 4388 is still good if you don't use a C term.

    Thanks dc42


Log in to reply