Does this look like a faulty Thermistor ?



  • Just sorted out my ABL and Z Offset ,,, then my Keenovo clone 750W 220V AC bed heater (powered via a SSR) starts failing on my first print.

    I measured the resistance at 25C to be 108K Ohms but I had no details of the thermistor so I had to research and guess the rest of the thermistor settings.

    This setup was working on my Marlin, but I suspect I've been unlucky and have another hardware failure when moving from Marlin to RepRap.

    ; Heaters
    ; H=0 Bed
    M143 H0 S140 ; Set temperature limit for heater 0 to 150C
    ;M301 H0 S1.00 P10 I0.1 D200 T0.4 W180 B30 ; Use PID on bed heater (may require further tuning)
    ;M307 H0 B0 ; Setting for PID Tuning starting with big bang
    M307 H0 A90 C700 D10 B0 ; Set Bed PID Tuning
    M305 P0 T108000 B4066 C0 R4700 ; Set thermistor + ADC parameters for heater 0

    The cable connection pass's continuity testing.
    The heater passed the PID Calibration OK.

    I'm assuming that this has to be a faulty thermistor?

    I've ordered some replacements .... anybody disagree with my diagnosis.

    In the first diagram I stuck my heater chamber 100K Thermistor the buildplate for comparison.

    1_1525617488482_Capture2.PNG 0_1525617488482_Capture.PNG


  • administrators

    I suspect either a faulty thermistor, or an intermittent short between the thermistor and ground, or leakage between the thermistor and the heating element.



  • Thanks,
    I'll replace the thermistor next week. If you see below while its heating ... its OK and relativity accurate its when it gets to its target temperature that its starts becoming erratic
    0_1525618479679_Capture3.PNG



  • 0_1525618568566_7c869157-0728-4666-a199-a0616b9d12c7-image.png



  • Right.

    1. Re soldered new thermistor on heat bed .... no change, the problem still exists.
    2. Opened the controller box case and buzzed the wire out OK Again.
    3. Rewired the thermistor cable directly to the DUET with a new connector and cable. Tested OK.
      4 Reconnected the original thermistor and original wiring loom again and connected it to the duet,

    I then noticed that when reassembling the case the power supply that goes in last before screwing the case shut was pressing down on the wire that connected the thermistor to the Duet. This must have been causing the connector on the board to have a bad connection.

    Once I moved the wire to relieve the pressure and reconnect everything,.. the original thermistor, cable and wiring loom had no issues.

    Problem Solved. It wasn't the thermistor.


  • administrators

    Thanks for sharing, glad it's fixed!


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