Thermistors not recognized, open circuit temperature at 2000°C



  • Hello,

    I have been converting my Da Vinci AIO to run on a Duet Maestro.

    All is going well except I am having problems with the Thermistors. I believe these are NTC Thermistors and the readings I took confirm this. When plugged in only by USB the bed temperature reads around 4°C and when I turn on the main power it reads at 2000°C. The extruder Thermistor does exactly the same.

    Using other Forum posts I have used other Da Vinci owners resistance at 25°C and β coefficient values, this did not solve my problem.

    I then heated the bed up myself and took these resistance readings of the thermistors.

    • 25°C = 336000Ω
    • 50°C = 143000Ω
    • 90°C = 94000Ω

    The resistance across the thermistors is changing and appear to be working.

    Putting all three into the RRF Configuration tool I get the resulting coefficients:
    R25: NaN Ω
    β: 189 K
    C : -1.094496e-5

    Putting the 25°C and 90°C into the RRF Configuration tool I get the resulting coefficients:
    R25: 362000 Ω
    β: 2246 K
    C : 0

    Putting the 25°C and 50°C into the RRF Configuration tool I get the resulting coefficients:
    R25: 362000 Ω
    β: 3579 K
    C : 0

    And then putting the 50°C and 90°C into the RRF Configuration tool I get the resulting coefficients:
    R25: 196808 Ω
    β: 1231 K
    C : 0

    After trying these values and not getting a change, I decided to go into the config file via the duet web Interface and edit the M305. I had a thermocouple on the bed measuring the temperature at 22°C and changed the β coefficient to 26000, which whilst the Maestro was powered only by my USB measured 22°C. Despite this when turning the Power on the temperature immediately reverts to 2000°C again.

    Is the board actually recognising my themistors as it seems to always read open? So I powered up the board without the thermistor connected (simulating an open circuit) and got the same result (2000°C).

    I can confirm that the bed thermistor is plugged into the Bed thermistor socket on the board.

    On a separate but possibly related note I have not wired the 5V power in as the wiring diagram shows this as optional. When is this optional 5v input required and would connecting a 5v supply to the board solve my incorrect thermistor issues? I mention this because when connecting via the USB only I must be getting a 5volt supply but when I switch on the mains 12 volt supply it reverts back to the above fault.

    Any help will be greatly appreciated as I believe I am nearly there and ironed out all other previous problems. I have all the XYZ motors working and the endstops configured correctly and am itching to get printing.



  • @scally123 2000 deg C is what you normally get if the wiring is open circuit. If you are sure that the wiring is correct, post your config.g file and also config-overide.g if you use it. AFAIK the 5V rail is only used for fans so won't have any effect on the thermistor readings (but USB only power might).



  • I'd start by finding out why there is a difference when supplying 12v and when connecting usb which aslo supplies 5v.

    The board should make its own 5v from the 12v supply and as such there shouldn't be any difference.

    These might help
    https://duet3d.dozuki.com/Wiki/Power_Wiring
    https://duet3d.dozuki.com/Wiki/Duet_2_Maestro_Wiring_Diagram


  • administrators

    @scally123 said in Thermistors not recognized, open circuit temperature at 2000°C:

    I then heated the bed up myself and took these resistance readings of the thermistors.

    25°C = 336000Ω
    50°C = 143000Ω
    90°C = 94000Ω

    The resistance across the thermistors is changing and appear to be working.

    That set of readings doesn't make sense, which you can see by plugging those values into the calculator at https://www.thinksrs.com/downloads/programs/therm calc/ntccalibrator/ntccalculator.html. I think at least one of them must be highly inaccurate.

    High resistance thermistors such as that one are difficult to read using the standard series resistors used on Duets and other standard 3D printer electronics. Here are a couple of solutions:

    • If possible, replace the thermistors by either standard thermistors with a resistance of 100K at 25C, or by PT1000 sensors.
    • If you are certain that the thermistor is connected properly, you can avoid the 2000C reading by connecting a high-value resistor in parallel with the thermistor. I suggest you try 470K, 330K or 220K. But you will lose the thermistor-disconnected detection.


  • @deckingman Thanks for your very quick reply. I have attached the latest config file below.

    0_1559994604914_config (2).g

    I am very new to all of this though I am sure I have followed the wiring diagram correctly, thus making me believe it maybe something I have done wrong when using the RRF Configurator. If there is something wrong with the wiring, is there any common things that cause this fault that I may of overlooked?

    The thing that really confuses me is why it changes when switching from USB power to Mains power.

    On the attached Config file I changed the B value to 26000 which reads right when powered only via 5 Volts.

    Many Thanks



  • @dc42 Thanks for your prompt reply.

    I was thinking of buying new thermistors that have a proven track record rather than using stock Da Vinci ones. I was just apprehensive as the thermistors were changing and I couldn't definitively say it was whether it was what I put into the RRFC or something else.

    Many Thanks


  • administrators

    @scally123 said in Thermistors not recognized, open circuit temperature at 2000°C:

    The thing that really confuses me is why it changes when switching from USB power to Mains power.
    On the attached Config file I changed the B value to 26000 which reads right when powered only via 5 Volts.

    That suggests that there is a short circuit or leakage between one of the thermistor wires and something else, such as the hot end heater. Disconnect the thermistor from the Duet, then use your multimeter on its highest resistance range to check whether you can measure any leakage resistance between the thermistor and the hot end metalwork or the heater.



  • @bearer Thanks for your quick reply I couldn't tell what caused the difference, I assumed it should be the same like you said.

    Many Thanks



  • Thank you everybody for the help so far. I really appreciate it.

    I have tried putting a spare 330K resistor in parallel across the Bed Thermistor. Whilst plugged into the USB it read 48°C. Then I turned the power on and it stayed at 48°C. Though it obviously wasn’t 48°C this was the first time I got the temperature to stay the same when going from 5V to the mains power.

    I believe it would be best to replace the thermistors, I don’t want to go down the route of having to buy the daughterboard and PT1000 sensors just yet but are there any 100k resistors at 25°C that are commonly used within 3D printing that are reliable and have known values. I saw that there were some listed in the RRF configurators’ calculator but I was wondering which one would be the most recommended.



  • Semitec 104GT are the most used and best known thermistors for the duet.

    you can get them from
    https://e3d-online.com/thermistor-cartridge
    or if you can wait
    https://www.aliexpress.com/item/32843785247.html?spm=2114.search0104.3.9.38d551fdWS6i4n

    also the maestro can run pt1000 well and you dont need a daughter board. (dont confuse them with pt100)
    https://www.precisionpiezo.co.uk/product-page/pt1000-sensor
    or
    https://www.aliexpress.com/item/32859917151.html



  • @scally123 said in Thermistors not recognized, open circuit temperature at 2000°C:

    Though it obviously wasn’t 48°C this was the first time I got the temperature to stay the same when going from 5V to the mains power.

    Did you try disconnecting both the hotend heater and bed heater from the Maestro and seeing if you still are getting different readings as suggested by dc42? (at least indirectly)



  • Hi as I have mentioned I have been updating my Da Vinci with a new Duet2 Maestro board.

    After having trouble with the stock thermistors I have bought two new PT1000.

    I have connected them to the board and re-configured the firmware with the RRF cofigurator {using the standard settings for PT1000s to the bed a and extruder).

    I am getting readings of 147C for the extruder and 208C for the Bed at an ambient temperature of around 21C. If I swap the two thermistors on the board I still get the same readings 147C for the extruder and 208C for the Bed. I have removed the PT1000s from the Bed and the extruder to eliminate any false readings through leakage. with these hanging free in fresh air at around 21C I still get 147C for the extruder and 208C for the Bed.

    I am at a loss on what to do next any welcome.

    Apologies for bringing the PT1000s were only delivered last weekend.

    Many Thanks


  • administrators

    Please post your config.g file, and tell or show us how you have connected the PT1000 sensors to the Duet.



  • 0_1561055016382_config.g

    Hi @dc42 I have attached the config.g file as well as the images below showing that one PT1000 is hooked up to the Bed Thermmistor and the other one to the extruder is connected to E0 Thermistor 2.

    0_1561055043576_20190620_185340.jpg

    0_1561055045922_20190620_185348.jpg

    0_1561055406123_20190620_185330.jpg



  • can you measure the resistance of your thermistors.
    also post your config.g thermistor config.



  • I have measured the resistance of the thermistors and its 1092 ohms.

    I have measured across the two pins where the thermistors connect to the board and I get 4.68K ohms without the thermistors connected. I then did the same check with everything disconnected from the board and I still get 4.68k ohms.

    Now with everything connected to the board including the thermistors I measured across the thermistor pins on the board and I get 0.835 and 0.837k ohms.

    In my config.g file it says:[2_1561283639142_config.json](Uploading 0%) [1_1561283639142_config.g.bak](Uploading 0%) [0_1561283639141_config.g](Uploading 0%)

    ; Heaters
    M307 H0 B0 S1.00 ; Disable bang-bang mode for the bed heater and set PWM limit
    M305 P0 T362000 B26000 R4700 ; Set thermistor + ADC parameters for heater 0
    M143 H0 S120 ; Set temperature limit for heater 0 to 120C
    M305 P1 T110000 B3950 R4700 ; Set thermistor + ADC parameters for heater 1
    M143 H1 S280 ; Set temperature limit for heater 1 to 280C

    0_1561283667474_config.g



  • R4700 is for the duet wifi not the maesto. the maestro uses R2200.

    pt1000 are configured like this

    M305 P1 X501 R2200 ; Configure PT1000 for heater 1



  • I have changed the settings to

    ; Heaters
    M307 H0 B0 S1.00 ; Disable bang-bang mode for the bed heater and set PWM limit
    M305 P0 X500 R2200 ; Configure PT1000 for heater 0
    M143 H0 S120 ; Set temperature limit for heater 0 to 120C
    M305 P1 X501 R2200 ; Configure PT1000 for heater 1
    M143 H1 S280 ; Set temperature limit for heater 1 to 280C

    But still seem to be getting the same temperature readings.


  • administrators

    You can leave out the R parameter in the M305 commands because the firmware knows the correct value. It's only on really old boards (Duet 0.6 and Duet 0.8.5) that it is needed.

    If the PT1000 sensors read 1092 ohms at room temperature. that is about right and you should get good readings from them.

    Try sending M305 P0 with no other parameters, and check it reports that it is configured for a PT1000. Likewise M305 P1.

    If that looks OK, try disconnecting both PT1000 sensors at the Duet and connect 1K resistors between the pins of the thermistor connectors. That should give you a reading close to 0C. if that works, then you may have a short between the PT1000 wiring and the heater wiring, perhaps via the hot end metalwork.



  • also measure the resistance across R15, R19 and R53 on the board.



  • So I have edited the config.g to state;

    ; Heaters
    M307 H0 B0 S1.00 ; Disable bang-bang mode for the bed heater and set PWM limit
    M305 P0 ; Configure PT1000 for heater 0
    M143 H0 S120 ; Set temperature limit for heater 0 to 120C
    M305 P1 ; Configure PT1000 for heater 1
    M143 H1 S280 ; Set temperature limit for heater 1 to 280C

    Using the G Code Console to send M305 P0 and M305 P1 to the machine and it replied with these settings

    1:58:45 PMM305 P0
    Heater 0 uses Thermistor sensor channel 0, T:362000.0 B:3950.0 C:0.00e+0 R:4700.0
    1:58:18 PMM305 P1
    Heater 1 uses Thermistor sensor channel 1, T:110000.0 B:3950.0 C:0.00e+0 R:4700.0

    0_1561294823093_7ca286f0-827b-4056-a49b-f009cec6e7de-image.png

    As I am only testing it and trying to get it ambient temperature the PT1000's are currently free in mid air in an attempt to prevent shorts against the metal work.


 

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