I'm a new Smoothieware user running Panacutt Azteeg X5 Mini V3 controller with a homebuilt Rostock style Delta printer.
My problem was that my shop is only heated to 50F (10C) where my printer is located. With the thermistors I'm using on my Bed and Extruder, both temperatures are reading negative temp. C values.
This causes the system to go into HALT mode when a temperature setpoint is entered for either the Extruder or Bed, so I'm dead right out of the gate.
The thermistors I'm using are from generic Chinese ebay components, and are listed as:
100K ohm NTC 3950
This means it is a Negative Temperature Coefficient thermistor with a Beta value of 3950.
I entered this Beta value, and get temperature values from the board of around -8C, preventing startup of the board.
So, I figured, just change the Beta value, or enter more accurate three point Steinhart–Hart equation values into the config file, and I should be fine.
I have access to a precision oven system with digital temperature readout and control, so I rigged up a Fluke digital multimeter and one of the 100K
thermistors, and developed an accurate temperature vs. resistance curve for plugging into the config file.
When I did so, I still got negative temperature values, so I was stumped.
Upon review of my derived R vs. T values, it appears that at low room temperatures, the R value is far greater than 100Kohms, more like 180K at 10C.
My guess was that this high resistance was causing inaccuracy in the voltage divider circuit used to derive a resistance for the R vs. T calculation.
My solution? lower the resistance at lower temperatures by adding a 100Kohm resistor in parallel to the thermistor and develop another calibration
curve with this modified setup. And it worked!
Here is the S-H three point config file solution for the NTC 3950 thermistor with a 100kohm resistor across the leads:
For the hotend:
temperature_control.hotend.rt_curve 20,55600,100,6480,220,415 #3 point temp curve derived from 100K 3950 thermistor with 100Kohm fixed resistor.
For the Bed:
temperature_control.bed.rt_curve 20,55600,100,6480,220,415 #3 point temp curve derived from 100K 3950 thermistor with 100Kohm fixed resistor.