Thermistors and thermocouples. Types and common problems

FFF 3D printers can use different types of temperature sensors, with NTC thermistors, thermocouples, and PT100 probes being the most common.

NTC Thermistors

Thermistors are the most common, economical, and easy-to-implement sensor, as they connect directly to the printer's board. They are an element that varies its electrical resistance depending on the temperature, so the printer must have the specific RT (resistance versus temperature) table preconfigured in its firmware for the particular model being used. If replacing one thermistor with a different model, it is essential to modify the printer's firmware to include the specific RT table of the new model; otherwise, temperature readings will be incorrect. If unable to modify the printer's firmware, it is essential to always replace the thermistor with an identical one. Among its main disadvantages are not providing a linear response and generally not being suitable for high temperatures (above 300°C).

Image 1: NTC Thermistor. Source: Filament2print

The main causes of problems with thermistors are two:

• Incorrect parameter configuration in the firmware: As mentioned earlier, it is essential for the printer's firmware to have the specific RT values of a particular thermistor model configured to accurately convert measured resistance values into real temperature values. All thermistor manufacturers provide their own RT data for each model, and firmware such as Marlin or RepRap FW includes RT tables for the most common models.

• Poor condition of cables or connections: A damaged cable, poor connection, or excessive cable length can increase the resistance measured by the board, resulting in inaccurate temperature readings. It is essential to periodically check the condition of the thermistor cables and connections. The thermistor should be directly connected to the board, avoiding the use of junctions or connectors and using the minimum necessary length. In the case of using quick connectors, they should be of the highest possible quality and crimped correctly. To determine if a thermistor is installed correctly, the best way is to measure the resistance at the board connector with a multimeter and see if it matches the specified value in the RT table at 25°C.

Thermocouples

They are composed of a bimetallic junction that varies its conductivity depending on the temperature. There are several types, with type K being the most common in 3D printing due to the wide temperature range they cover (-200°C to 1400°C). They are very economical and interchangeable; however, they have two important limitations:

• They have very low accuracy (greater than 1°C).

• They require amplifier boards to be used.

Until recently, they were the most common solution in high-temperature 3D printers; however, they have been displaced by other technologies such as high-temperature thermistors or PT100 probes.

Image 2: Type K Thermocouple. Source: RS Components

The main causes of problems are:

• Poor condition of cables or connections: Like thermistors, temperature is determined by measuring the resistance of the thermocouple, so defects in wiring or connectors result in erroneous temperature measurements.

• Electrical noise: Thermocouples are sensitive to electrical noise, so the appearance of noise in the circuit alters the measurements.

RTD Probes

Similar to NTC thermistors, they are composed of a metal that varies its electrical resistance with temperature. Unlike NTC thermistors where resistance decreases with temperature, in RTD probes, it increases. This allows them to accurately measure temperatures much higher than thermistors, up to 600°C. Although they present very good accuracy over a wide range of temperatures, they have the disadvantage of being more expensive and, like thermocouples, requiring additional electronics, which further increases their cost and complicates their installation. The most common type of RTD probe is the known PT100 probe.

Image 3: PT100 Probe with Amplifier Board. Source: E3D

In general, they present fewer problems than NTC thermistors and thermocouples; however, as with the previous cases, it is important to check the condition of the cables and connectors, as their operation also relies on reading electrical resistance.

Temperature-related Issues

Many times temperature problems are not related to the sensor itself but to the temperature control model and printer safety settings. To control temperature, FFF 3D printers use a pulse frequency-based model known as PID. The coefficients of this model determine the pulse frequency required to achieve higher or lower heating rates, so correct configuration of these parameters is essential for precise temperature control. This is why most 3D printers incorporate a function called PID calibration, which automatically determines these parameters. It is recommended to periodically perform this calibration.

Additionally, it is common for 3D printers to implement safety algorithms that deactivate heating when heating speeds or temperatures reached do not match those of the model. In these cases, temperature errors are frequent. When they occur, the following should be checked:

• The state of the temperature sensors

• The thermal performance of the hotend

• That the layer fan is not directed at the heating block and cooling it.

• That the heating block is not in contact with the hotend heatsink.

• Perform a PID calibration.

Note:  This guide discusses concepts in a general manner without focusing on a specific brand or model, although they may be mentioned at some point. There may be significant differences in calibration or adjustment procedures between different brands and models, so it is recommended to consult the manufacturer's manual before reading this guide.