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