Termistor headache



  • So bought a hermera hotend the trermistor was defective out if the box "broken wire" the termister comes in a 3mm tub with the hi temp wires allready attached to a plug in case you need to replace it
    I pickud up three p1000 sensors today because the 3 replacements sent from my fav shop were the glass cr10 style and of no use unless i macgyver a bunch of ideas together 🙄 not my style..
    All three of the t1000 sensors read open circuit with my vom
    ???? Only one read 1089 ohms twice then reads open consistantly?????
    Are they sensitive to voltage????
    How do you check them if not with an ohm
    meter ... does it need to be read maybe on a high resistive scale to prevent the sensor failer?? I just dont know
    I think the likelyhoid that all three are bad are small
    So most likely im not seeing somthing or doing somthing wrong.. any input would be appreciated



  • Given the purpose for the trermister and temp sensors are a critical part of the safety precaution that the duet provides gives me concern about reliability ?? Thoughts there would also be appreciated. thanks



  • My first though would be to change supplier if only one out of three pt1000 is fine. I use genuine E3D pt1000 and had never single issue with them.

    What concerns safety I belive firmware will not let You to use heater when thermistor fault is detected



  • So my assuption that a v.o.m. any digital ohm meter is fine to use to test ?? And im not blowing up the sensor (p1000) and using the same method??? I should see a resistance value for both the high temp p1000 sensor and the termistors ???
    sorry to be so blatant with my questions i just dont know untill i ask,
    I haven't ever worked with thiese two devices
    I am also assuming the diferance between the p1000 and p100 is the temp curve or limits????
    P1000 up to 500c. (Sensor)
    And p100 up to 300c (termistor) ??????



  • @ziggymanpopo You can use the ohm meter for sure.

    Difference between pt100 and pt1000 is basically in a resistance (due to thickness of Platinum layer). Both of them can be made for high temperatures.

    It is just a resistor with layer of platinum, which changes it's resistance with temperature. And If layer is longer/thinner You get 1000Ohm at 0C or 100Ohm at 0C



  • Thanks. 🙂


  • Moderator



  • So no pt 100 without a daughter board is what i got out of all this the tesistance would be to low and read shorted hence froth a fault

    But the pt 1000 can be directly attached
    Is the inline resistor fixed on the duet wifi2



  • The links helps to understand
    Thanks ian



  • @ziggymanpopo Just to make sure... what meter You use and on which measurement range ?



  • What the limit of the device was i asumed the two were of diferant uses the tp100 being toped out at
    About 300c for example. thats not the case if i read the posted info correctly
    The data stated that the temp range is independant on "manufacturing specifications" and not type
    Also that the pt100 cannot be used without a daughter board thats because of the onboard resistor used doesn't match with the pt100.
    Did i get it right ?



  • @ziggymanpopo

    What the limit of the device was i asumed the two were of diferant uses the tp100 being toped out at
    About 300c for example. thats not the case if i read the posted info correctly

    Pt100 and Pt1000 can be used to about 800-1000*C however particular sensor might be limited due to other materials used in it.

    Also that the pt100 cannot be used without a daughter board thats because of the onboard resistor used doesn't match with the pt100.

    Basically Yes.



  • Thanks ...again 🙂 i like the extra bits you guys put in.


  • Moderator

    @ziggymanpopo

    Also that the pt100 cannot be used without a daughter board thats because of the onboard resistor used doesn't match with the pt100.

    Basically Yes.

    It's a bit more complicated than just changing the resistor in the on-board voltage divider circuit to support PT100. Even if the resistor was changed for an appropriate one, I don't think it would be accurate or sensitive enough to provide good temperature feedback. That's why the daugtherboard uses the MAX31865 for precise feedback.

    Ian



  • @droftarts In general Yes. I just simplified the answer. It could work with changing pullup but pt100 would draw significant current due to low resistance and would also heat up itself and this would lead to inaccuracy (and possibly could destroy platinum strip).

    Best would be to use current source and amplifier, but MAX31865 for sure solves a lot of issues with reading pt100 (and also pt1000 with proper setup).

    I could bet that it is more acurate to use PT100 with MAX31865 than pt1000 with simple pullup.

    Measuring low resistances is a large topic itself, I just did not want to go into it too much.

    SO pt1000 and some other thermistors have resistance that allow to match pullup resistor, that will not cause too much current flowing to heatup sensor significantly, and yet voltage drop on such sensor is still large enough that can be measured by ADC without fancy amplifiers etc.


  • Moderator

    @BoA said in Termistor headache:

    I could bet that it is more acurate to use PT100 with MAX31865 than pt1000 with simple pullup.

    https://duet3d.dozuki.com/Wiki/Connecting_thermistors_or_PT1000_temperature_sensors#Section_PT1000_sensors

    The accuracy of PT1000 sensors should be very good on the Duet 3 and Duet 2 Maestro and generally good on the Duet 2 Wifi and Ethernet. However, it may be poor on the Duet 0.6, Duet 0.85 and other SAM3X8E-based electronics. For these boards, you can calibrate the Duet's on-board analogue-to-digital converter to improve this

    Then see https://duet3d.dozuki.com/Wiki/Connecting_thermistors_or_PT1000_temperature_sensors#Section_Temperature_calibration_and_ADC_tuning further down the page.

    Yes, PT100 + daughterboard should generally be even more accurate than PT1000, and because it's a daughterboard, should be the same across all Duets.

    Ian



  • Thsnks much better inderstanding now and thanks for the links 😁


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