K-Thermistor vs Ender 3 Stock Thermistor: Problem with bad reads

Question

I have changed my hotend on my Ender 3 v2 with Dragonfly BMS.

In effect, because I wanted to reach higher temperatures, I also changed the thermistor with one that is 3mm wide and does not have the bead shape.

This is what it looks like

The connector is the same as the Creality Ender 3 v2 one (JST).

It was listed as "HT-NTC 100K 3950 Thermistor for Reprap Hot-End" and "Working temperature: -50 °C ~ +350 °C, can replace the original K-thermistor. Suitable for printing high temperature filaments."

I replaced the stock one and did a PID callibration.

However, when I attempted to print parts, first I observed bubbles and degradation of the fillament (PETG at 240 and PLA at 200). I tried lowering the temperature and it started printing well (PETG) at 200 C, which obviously is way too low. Same goes for PLA, which I can't print over 170.

Here's the product on amazon

Does anyone have experience with this kind of problem? I have access to modifying Marlin firmware. I am not sure if this is a different type of temperature sensor.

I'm using OCTOPRINT to see the temperature.

When the sensors are off the hot end reports 22.1 degrees while the bed reports 22.

Answer 1

Even if it works properly (which it seems not to do), this type of thermistor will only have a resistance of around 100 Ohms at 350°C, which is too low to be useful in your printer. It is rather suspicious that the seller does not specify an accuracy for the thermistor, so it is probably of a low accuracy, which will make things even worse. I am not surprised that you are getting poor results with it.

Maybe use a Platinum RTD, instead of a thermistor. A PT1000 RTD will have a temperature of 1000 Ohms at 0°C, and so should be usable in your printer without needing an amplifier board. E3D-Online sell one that is rated up to 500°C. You will need to change the sensor type in your printer's firmware, which will probably mean recompiling and re-flashing the firmware.

I used this thermistor calculator to calculate the expected resistance at 350°C.