Thermistor problem…



  • Just got my DuetWifi and now trying to make it work with my Kossel XL.

    Installed a brand new 12V E3D extruder and thermistor.
    Both bed and extruder thermistor change between negative values around -20 degrees and "error" with a few seconds in between.
    That's with standard settings for "chinese type" bed thermistor and original E3D thermistor.
    Any ideas what the problem might be? Thanks in advance.


  • administrators

    Have a look at this page:
    https://duet3d.com/wiki/Connecting_thermistors

    and check that the config.g is setup correctly to use your thermistors, feel free to put comment with those lines here. The B value is important to get an accurate reading but if you have it wrong it should not give as large an error as you are showing so if you don't know it for the bed thermistor then you can use the typical chinese bed thermistor value shown on that wiki page just to get in the right ballpark.



  • Any chance you've popped the VSSA fuse?

    https://www.duet3d.com/forum/thread.php?id=222#p2195



  • @T3P3Tony: Yes, configured the thermistors as you proposed (one as original E3D thermistor, the other as generic).
    @elmoret: The VSSA fuse was indeed popped.
    I looked at this thread: https://www.duet3d.com/forum/thread.php?id=222#p2195
    Then, as @AndreS had done, I replaced the SMT fuse by putting an 5x20 fuse holder with one connector at the E0 endstop GND pin and the other at thermistor E1 VSSA pin. Alas, still the same issue (and I tried two other thermistors that did work on my RAMPS board). The new fuse (12V, 2A by the way) didn't pop.
    The temperature readings still change between "error" and about -14 to -19 degrees Celsius each intervall lasting a few seconds . Other ideas?



  • I think I have also poped my VSSA fuse while unscrewing the heat block.
    I have switched to thermocouple for the hot end and it's working correctly. Is it still necessary for me to do anything with the poped fuse? Is there any adverse effect?


  • administrators

    @Radioist5:

    The new fuse (12V, 2A by the way) didn't pop.

    2A is way too high. The one we fit is 0.25A. It's chosen so that in the event of a short from +12V or+24V to VSSA, the current in the PCB traces and vias will be limited by the resistance of the fuse so that the PCB traces and vias will survive the brief current surge, The original fuse almost certainly blew because of a short, and if you get that short again then with as 2A fuse it's possible that a trace or a via on the PCB will pop before the fuse does. Perhaps that has already happened.


  • administrators

    @Jackal:

    I think I have also poped my VSSA fuse while unscrewing the heat block.
    I have switched to thermocouple for the hot end and it's working correctly. Is it still necessary for me to do anything with the poped fuse? Is there any adverse effect?

    What about your heated bed - doesn't that use a thermistor?



  • @Jackal:
    If you dont connect anything to the thermistor headers, there will be no side effect.

    @Radioist5:
    are you sure, you got the right pins on E0 and E1 headers? They are both the pins on the headers facing away from connection side of the board and to the side of the big VIN terminal.
    And please, replace the 2A fuse with a F 250mA fuse. If there is another fault, there will be 2A through the microprocessor and I think that will kill him.
    250mA is the original value.
    My printer is working with this work-around good again.



  • I wonder if the next revision could use socketed pico fuses similar to the Rambo controllers?



  • @dc42:

    @Jackal:

    I think I have also poped my VSSA fuse while unscrewing the heat block.
    I have switched to thermocouple for the hot end and it's working correctly. Is it still necessary for me to do anything with the poped fuse? Is there any adverse effect?

    What about your heated bed - doesn't that use a thermistor?

    I use a silicon heat pad (stuck onto an aluminium plate) which comes with thermocouple wiring



  • Andre and dc42: Thanks for the comments - I'll try the right specs on the fuse… Anyway, processor still working - the X,Y,Z motors are still rolling smoothly (much better than on my old RAMP), and no problems with the web interface 🙂



  • Connecting an external 250 mA fuse between E1 thermistor VSSA pin and E0 stop GND pin as a substitute for F1 fuse solved my problem 🙂 Temperature readings now ok. Thanks again, AndreS and dc42!


  • administrators

    I'm glad you got it working again. I suggest you check your hot end and bed wiring very carefully, because you must have had a short between a thermistor wire and a heater, fan or stepper motor wire.



  • hi im having the same problem and ive just switched to the e3d volcano head and the heater block wires are exposed on by the heater block so i think i may have popped the same fuse
    but i can not locate the fuse to cheek it im no engineer and my soldering skills are medium crap at best

    can anyone help locate the fuse
    and how hard is it to replace it


  • administrators

    The fuse is behind the ESP8266 WiFi adapter in a little rectangle on the silk screen labelled F1. It doesn't have metal end caps, so the easiest way to check it is to use a multimeter to measure the resistance between one of the endstop ground pins and the VSSA pin on one of the thermistor connectors. It should be about 2 ohms on production boards and about 8 ohms on pre-production boards.

    To replace the fuse you would need a hot air soldering station. That is why some users bypass it with an external fuse instead.



  • hi thanks found it it has popped and i solidered onto the (E0 vssa) pin and the paneldue ground pin



    i left a long wire soldered onto a slow blow 0.25a fuse which ill replace with a fuse holder and small glass fuse 0.25a next week which is why i left a long wire on for now
    before i plug this in and connect is this correct also will this over load the negative trace if i plug a paneldue in ?



  • here the responsible party for blowing the fuse

    not usre if its too clear but the heater wires have become exposed and must of made contact with main body of the extruder at some point



  • Just a rant but I really hate how these Chinese thermistors come with their leads sleeved in paper-thin PTFE. One reason…tighten that screw more than a little and you've just pinched right though. I imagine in some scenarios this could even lead to a potential fire.

    Another reason is PTFE isn't safe to use at temperatures as high as some hot-ends are capable of operating. I've seen several "all metal" hot ends come equipped this way. And look for type-K thermocouples on Amazon, they're all sleeved in PTFE. It's like making a kitchen pan out of plastic.....why even bother?

    @Cap: If I were you I'd get some high-temperature fiberglass sleeving and use that to insulate the thermistor leads. Also, looking at that fuse fix makes me cringe. There's a strong potential for those long fuse leads to pick up noise and interfere with your temperature readings. I also doubt that big glass fuse is even remotely capable acting fast enough to protect the processor in the event of another short. If you're in the US, I stock replacement VSSA fuses and could properly repair your board for a reasonable cost and turn it around the next day.



  • im in the uk and i now have fiberglass sleeving and new thermistors heading to me from e3d

    as for the wires they go straight down throw the case printed in abs and dont run along the board it self so hopefull they should be fine but yes tomorrow ill go to me local hardware store and change the fuse to as small glass 0.25a fuse inside a fuse holder



  • rant warning
    but i am vexed about a blown fuse i could not software reset are pull out and change where was the logic in having a fuse you need specialized equipment to change and selling the board to hobbyists…....grr vents steam
    do not get me wrong i have and will still recommend this board to others over other boards i have tried and tested Menzies rumba and ramps
    but if it needs a fuse please make an easy reset fuse are easy changeable fuse are whats the point
    i choose this board over beagle bone boards and smoothie boards but if this board dies because of a fuse i counld not easily replace then ill try a different board completely


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