Adding an extra thermistor



  • So I have a second hand duetwifi board where the H0 thermistor channel doesn't work for some reason. I'm using the H1 thermistor for my hotend and the bed thermistor for the bed.

    I'd like to monitor chamber temperature (possibly activating a cooling fan if it gets too high).

    How do I (presuming its possible) monitor a thermistor from an unused input pin?



  • Simon there are several thermistor pins on the Expansion header not sure what they are but sure they are documented in the wiki somewhere?

    In fact see

    https://duet3d.com/wiki/DuetWifi_Pinout_table

    HTH

    Doug



  • Thanks Doug I'll dig into that.


  • administrators

    You will need to connect a 4K7 1% resistor between your chosen additional thermistor pin and +3.3v, and connect the thermistor between the chosen pin and the VSSA pin on the expansion bus.



  • This helps too https://github.com/T3P3/Duet/blob/master/DuetWifi/Old%20versions/DuetWifiv1.0/DuetWifi_Schematic.pdf

    There are thermistors 3-7 on the expansion header.

    Thanks David. That was my next question.

    Just so I can be clear and not balls it up, connect 3.3v to the thermistor 3 (for example) pin on the expansion connector via a 4700ohm resistor and one lead from the thermistor, and then GND it via vssa on the header?


  • administrators

    Yes.



  • Okay so trying to figure out whats going on.

    The board I have, had some issues - VSSA fuse blown (was repaired).

    Mostly everything else working normally (one of the pwm fans is always on) except when I connect a thermistor to H2 input it reads 1970.4 deg C.
    So I assumed it was damaged when the board was shorted and followed the advice above, connecting a 4.7k ohm resistor from 3.3v on the expansion connector to thermistor 3 pin, then my thermistor to this and the other side to vssa on the expansion connector.

    This also reads 1970.4 deg C.

    (heater 1 showing a fault as I removed its known good thermistor to try on the other two possible channels).

    It makes no difference whether I have a thermistor connected to heater 2 or 3 here it reads 1970.4 occasionally flickering to 1163 deg C.

    Any ideas? Its not mission critical just wanted a chamber temp monitor.


  • administrators

    What M305 parameters do you have for heaters 2 and 3 in config.g, and what thermistor are you using?



  • David:

    M305 P0 R4700 H0 L0 ; Put your own H and/or L values here to set the bed thermistor ADC correction
    M305 P1 R4740 H0 L0 ; Put your own H and/or L values here to set the first nozzle thermistor ADC correction
    M305 P2 R4700 H0 L0 ; Put your own H and/or L values here to set the second nozzle thermistor ADC correction
    M301 H1 P10 I0.10 D100 T0.50 S1.0 ; PID settings for extruder 0
    M301 H2 P10 I0.10 D100 T0.50 S1.0 ; PID settings for extruder 1

    Im using generic epcos chinese thermistor, same one Im using for the bed which is working fine. The thermistor was reading 88k ohm at 30 deg C ambient last night so I believe it to be functional. It was working before I installed the duet wifi card. I have a bunch so I can try a new one if you think it might be worthwhile.

    CalvinX - yes I have one of those, but it would be nice to have my chamber fan set to turn on if it goes overtemp, isnt that why we all bought a fancy controller board?



  • Simon,

    I've got this for the thermistors that I have stuck around the printer in various locations

    M305 P2 T100000 B3950 R4700; Set thermistor + ADC parameters for heater 2 - this is used to measure stepper chip temperature on Duet
    M305 P3 T100000 B3950 R4700; Set thermistor + ADC parameters for heater 3 - this is used to measure stepper chip temperature on Duex5
    M305 P4 T100000 B3950 R4700; Set thermistor + ADC parameters for heater 4 - this is used to measure left XY stepper temperature
    M305 P5 T100000 B3950 R4700; Set thermistor + ADC parameters for heater 5 - this is used to measure right XY stepper temperature

    So maybe you need T and B parameters in your M305s?
    Ian



  • Thanks Ian will try it, have a print running right now, good idea though. although why the bed and hotend work without them is still a mystery.



  • Print wasnt looking good so I cancelled it.

    With M305 P2 T100000 B3950 R4700 in use for H2

    It now reads 7841.8 deg C, so whilst its not fixed it has changed something, though it seems to make no difference whether the thermistor is attached or not.


  • administrators

    You said that it is a second hand Duet WiFi with a fault on the H0 thermistor channel. What I suspect is that it has experienced some trauma that has damaged the ADC multiplexer in a way that affects more than one channel. I suggest you try the remaining thermistor inputs on the expansion bus (with a 4.7K pullup to +3.3V again) to see if one of them works. Failing that, you could switch your hot end to a PT100 so that you can use the E1 thermistor channel for the chamber temperature. Or find someone who can replace the processor (user W3DRK can but he lives in the US).



  • Dj is much closer to you than the states David he is in Bristol



  • David thanks for the advice I'll try the other channels. It's no problem really, it prints beautifully and I got it for a swap for a few piezo modules so I didn't pay full price by any means. I do have a pt100 board and sensor from e3d but I suspect that connects via a thermistor input so I might invest in a duet pt100 daughter board if none of the other thermistor channels work.

    Ian just wondering why use thermistors to monitor your drivers/processor when those heater channels are built in?



  • @DjDemonD:

    Ian just wondering why use thermistors to monitor your drivers/processor when those heater channels are built in?

    I only monitor the step[per driver chips, not the processor. I did it a long time ago, before anything was implemented in the firmware. AFAIK, the driver chips only report a high temp alarm and I don't know what temperature that is set at. I want to keep things cool and turn on fans, well below any high temperature warning threshold so I use 45 deg C as the threshold above which the fans come on.


  • administrators

    The driver chips flag a temperature warning at nominally 100C and report shutdown at nominally 150C. But as with most silicon temperature sensors that are not laser trimmed, there is quite a wide tolerance on these figures.



  • Hi Ian that makes sense, I'd want to fire up the fans before I hit 100 deg C for sure. Though I find one 40mm fan blowing under the board which is vertically mounted seems to keep it pretty cool. I'm moving a little less mass around though.



  • Simon,

    I have both the duet and duex5 mounted vertically one above the other so that their Vin terminals are opposite each other and close together if you get my meaning. I have an 80mm fan blowing air on to the back of each board and each fan is controlled via a thermistor stuck to one of the stepper chips. The fans are virtually silent but not particularly high flow. Today, I've seen the highest ever chip temperature at 56 degC (measured at the surface you understand) with the ambient in my study at 35 deg C. That was on one of the XY steppers on the duet main board and pushing a print speed of 260mm/sec with my heavy gantry. (Yes you did read that right - a print speed of 260mm/sec - not pretty but doable - blog post and video coming soon).

    The duex5 only drives extruders and I've never yet seen the chip get above 45 degC to trigger the fan.
    Ian



  • Just to finish this of by saying I ordered the pt100 daughter board (hopefully nothing is amiss with that system). I tried H7 but when I enabled it as my second tool numbered 99, in tool definitions the DWC was now displaying 7 tools of which 4 (h2-h5) showed 1970 deg C and 2 (h6 and H7) error with or without a thermistor attached, the two working ones are handily bed and h1. So thanks for the advice but yes it seems as though something is amiss with the adc system though thankfully not the heaters, motors or anything else.


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