One thing that always made me confuse when using my 3D printer (A Rigidbot) was that, despite being able to print nice things on PLA, I could never print anything using ABS. Thare was simply no extrusion with this kind of filament.

After some research, if found some other users having issues with their printer's thermistor (The temperature sensor). They were getting wrong temperature readings. So I got an external thermometer and found out that my printer was indicating a temperature 40 degress below the actual extrusor temperature, and that seems to be a common issue. It means that the maximum temperature I was able to print was actually 210 degress, wich is too cold for ABS.

So, I had two options to fix that:

  1. Change the firmware code, so it would calculate the correct temperature for my printer
  2. Insert a resistor in series with the thermistor, to increase its reading

Option (1) has several disadvantages. The most important for me is that I would end up with a non official firmware. In order to be able to easily get firmware updates in the future, I should go for option (2)

Going for option (2) is not complicated: The thermistor sensor is a voltage-divider, so I just had to increate the resistance in order to increate the raw date to the correct temperature value.

So, I cut one of the thermistor wires and inserted a potentiometer on it, as depicted here:

After that, I set the extruder temperature to 220 (mid-point between PLA and ABS) and waited until the printer PID set the temperature.

Then I started increasing the potentiometer resistance, while checking the temperature both on the printer and on the external thermometer. When they matched, I removed the potentiometer and checked its resistance: Around 70 Ohms.

That seems a little odd, since the thermistor nominal impedance goes up to 100k. But I checked and saw that other users found values around 300 Ohms, so I went for it. I turned the printer off, swapped the potentiometer for a 68 Ohms and turned it again (You cannot change this with the pronter powered on, because it will raise a "MINTEMP ERROR"):

By checking the temperature again, I could see that both the printer LCD and the external thermometer agreed on the temperature: 220 degress.

The final check was by printing a nice ABS part. After months, ABS was finally flowing through my printer nozzle!