Sudden Open Circuit in thermistors



  • Hey guys so i have an odd problem, I was working on the bed of my corexy printer, after turning it on both my bed and hotend thermistors are reading open circuit. I can understand on the bed if i might have knocked something loose but the extruder as well seems very unlikely. I've since isolated the thermistor wiring, for both the hotend and bed, as well as replaced the bed thermistor as it looked like the fiberglass sleeving could have frayed and caused a short, this was not the case.

    In the open circuit state the hotend fan run constantly as I have it thermostaticaly controlled, and occasionally you can hear it wind down, it just completely quit spinning for half a second, as if it momentarily finds ground and loses its open circuit state. Is there a test point I can check could it perhaps be a cold solder joint somewhere expanding and contracting.

    I'm at a loss and this is really frustrating…

    Thanks for your time

    https://www.youtube.com/watch?v=DiCKHPRGRKc

    Edit: further digging and checking(had to find my meter,)
    The disconnected thermistor circuits from the connector through the thermistor and back for the bed reads 110K Ohms, the Hotend reads 108K Ohms, which is where they should be, (checking for a bad wire)

    Here is the M122 output

    === Diagnostics ===
    Used output buffers: 1 of 32 (13 max)
    === Platform ===
    Memory usage:
    Program static ram used: 20304
    Dynamic ram used: 73872
    Recycled dynamic ram: 32
    Current stack ram used: 3288
    Maximum stack ram used: 4120
    Never used ram: 32744
    Last reset 00:00:42 ago, cause: power up
    Last software reset code 0x0003, HFSR 0x00000000, CFSR 0x00000000, ICSR 0x00400000, BFAR 0xe000ed38, SP 0xffffffff
    Spinning module during software reset: GCodes, available RAM 33048 bytes (slot 3)
    Error status: 0
    Free file entries: 10
    SD card 0 detected, interface speed: 20.0MBytes/sec
    SD card longest block write time: 0.0ms
    MCU temperature: min 14.1, current 20.3, max 23.2
    Supply voltage: min 12.2, current 12.3, max 12.4, under voltage events: 0, over voltage events: 0
    Driver 0: standstill
    Driver 1: standstill
    Driver 2: standstill
    Driver 3: standstill
    Driver 4: standstill
    Current date and time: 2017-05-12 22:51:59
    Slowest main loop (seconds): 0.011509; fastest: 0.000037
    === Move ===
    MaxReps: 0, StepErrors: 0, MaxWait: 0ms, Underruns: 0, 0
    Bed compensation in use: none
    Bed probe heights: 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
    Probe change coordinates:
    === Heat ===
    Bed heater = 0, chamber heater = -1
    Heater 1 is on, I-accum = 0.0
    === GCodes ===
    Segments left: 0
    Stack records: 2 allocated, 0 in use
    Movement lock held by null
    http is ready with "M122"
    telnet is idle
    file is idle
    serial is idle
    aux is idle
    daemon is idle
    === Network ===
    WiFiServer is running
    SPI underruns 0, overruns 0
    === Webserver ===
    HTTP sessions: 1 of 8
    
    

    Next up is reflashing firmware in case it might be a corrupted eeprom as suggested by a friend in the field

    Edit2:
    So flashing firmware to 1.18.1(from 1.18 beta3) had no change

    here's the M122 output from after the update.

    === Diagnostics ===
    Used output buffers: 1 of 32 (4 max)
    === Platform ===
    Static ram used: 20320
    Dynamic ram used: 72808
    Recycled dynamic ram: 1080
    Stack ram used: 968 current, 3816 maximum
    Never used ram: 33048
    Last reset 00:06:09 ago, cause: software
    Last software reset code 0x0003, HFSR 0x00000000, CFSR 0x00000000, ICSR 0x00400000, BFAR 0xe000ed38, SP 0xffffffff
    Spinning module during software reset: GCodes, available RAM 33032 bytes (slot 4)
    Error status: 0
    Free file entries: 10
    SD card 0 detected, interface speed: 20.0MBytes/sec
    SD card longest block write time: 0.0ms
    MCU temperature: min 22.8, current 26.3, max 28.8
    Supply voltage: min 12.2, current 12.3, max 12.4, under voltage events: 0, over voltage events: 0
    Driver 0: standstill
    Driver 1: standstill
    Driver 2: standstill
    Driver 3: standstill
    Driver 4: standstill
    Date/time: 1970-01-01 00:00:00
    Slowest main loop (seconds): 0.004193; fastest: 0.000037
    === Move ===
    MaxReps: 0, StepErrors: 0, MaxWait: 0ms, Underruns: 0, 0
    Scheduled moves: 0, completed moves: 0
    Bed compensation in use: none
    Bed probe heights: 0.000 0.000 0.000 0.000 0.000
    Probe change coordinates:
    === Heat ===
    Bed heater = 0, chamber heater = -1
    Heater 1 is on, I-accum = 0.0
    === GCodes ===
    Segments left: 0
    Stack records: 2 allocated, 0 in use
    Movement lock held by null
    http is idle in state(s) 0
    telnet is idle in state(s) 0
    file is idle in state(s) 0
    serial is idle in state(s) 0
    aux is idle in state(s) 0
    daemon is idle in state(s) 0
    queue is idle in state(s) 0
    Code queue is empty.
    === Network ===
    WiFiServer is running
    SPI underruns 0, overruns 0
    === Webserver ===
    HTTP sessions: 1 of 8
    
    

    I'm at a loss and going to bed


  • administrators

    If it is a Duet WiFi and both thermistors are reading -273 then you probably had or have a short between +12/+24V and a thermistor connection which has blown the VSSA fuse. So you need to identify and fix the fault, and either replace the VSSA fuse or bypass it. There are at least 2 other threads here with more information - try searching on 'VSSA fuse'.


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