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    ilium007
    Dec 13, 2019
    Edited: Dec 13, 2019

    Heatbed thermistor issue solved

    in Get Started

    I have spent hours this week trying to get an accurate reading from the TinyFab heatbed thermistor (NTC 100k) via the opamp buffer circuit on the new beta board. The way the voltage divider has been laid out (to suit the stock CPU evidently) causes an inverted temp reading. I messed around with config for hours before I realised that on the underside of the TinyFab CPU is a 4 pin JST connector for the separate heatbed MOSFET driver that is connected to a voltage divider on the CPU PCB that puts the thermistor on the GND side of the series resistor pair:


    TinyFab CPU thermistor input with thermistor on the low side of the voltage divider - this now works














    Beta mainboard thermistor input circuit with thermistor on the high side of the voltage divider



    By connecting the TinyFab heatbed thermistor SEN+ and SEN- to pins 3 and 2 of the TinyFab CPU underside connector and using the config below (CPU input 0.26 and beta 3950) I was able to again get an accurate bed temp reading and use the onboard heatbed MOSFET driver and shared 24V power source without requiring the external TinyFab heatbed driver board.


    temperature_control.bed.enable                  true
    temperature_control.bed.sensor                  thermistor
    temperature_control.bed.thermistor_pin          0.26
    temperature_control.bed.heater_pin              2.5
    temperature_control.bed.set_m_code              140
    temperature_control.bed.set_and_wait_m_code     190
    temperature_control.bed.designator              B
    temperature_control.bed.pwm_frequency           2000   
    temperature_control.bed.bang_bang               false
    temperature_control.bed.hysteresis              2.0
    temperature_control.bed.p_factor                13.7
    temperature_control.bed.i_factor                0.097
    temperature_control.bed.d_factor                24
    temperature_control.bed.max_pwm                 255
    temperature_control.bed.beta                    3950
    temperature_control.bed.max_temp                150
    temperature_control.bed.min_temp                -10

    3 comments
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    3 Comments
    GM
    Dec 06, 2020

    I ended up going with your workaround, which works just fine so far.

    Like

    GM
    Dec 06, 2020

    I have the strong suspicion that my thermistor troubles are connected to this inverted temp reading as well - using the tinyfab mainboard, heatbed and cpu on cetus mk3 mechanical parts, I am getting temp readings around 233 which get lower when the bed is heated. I'd be grateful for suggestions - anything other than manually connecting it to the underside of the CPU board?

    Like

    alfredoe.rhoton
    Dec 14, 2019

    Great that you figured it out! I've been playing around with my thermistor settings but I know they are not quite right. I'll switch to using it like this instead. Thanks for sharing.

    Like
    3 comments
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