NTC thermistor bridge

(Backdate: Dec. 1, 2016)

Today I built the thermistor-based temperature measurement bridge we’ll use for extruder barrel thermal control system. I specced a kind of glass-bead thermistor last week, one of the very few on Digikey that can get up above 250 degrees Celsius (just in case my control algorithm somehow goes off the rails). In order to do the temperature management, I wired the thermistor up in a Wheatstone bridge with three other 25K resistors in order to measure precise changes in resistance.

With a little bit of math and a little bit of calibration (serial output on an Arduino and a thermometer) you can pretty easily get calibration points for the thermistor bridge using this online tool. I then put those voltage-temperature pairs into Excel and calculated a linear fit for the region I care most about, around the extrusion temperature from 180 C to 220 C. For now, I’ve implemented a thermostat-style control system with this as an input, turning on the heater at 190 C and off at 210 C.