FFF 3D printers can use different types of temperature sensors, the most common being NTC thermistors, thermocouples and PT100 probes..
NTC thermistors
Thermistors are the most common, cheapest and simplest sensor to implement, as they are connected directly to the printer board. It is an element that varies its electrical resistance depending on the temperature, so the printer must have the RT (resistance vs. temperature) table of the specific model being used preconfigured in its firmware. If you want to replace a thermistor with a different model, it is essential to modify the firmware of the printer including the specific RT table of the new model, otherwise, the temperature measurements will be wrong. If it is not possible to modify the printer firmware, the thermistor must always be replaced by an identical one. Among their main disadvantages are that they do not provide a linear response and are generally not suitable for high temperatures (above 300 ºC).
Image 1: NTC thermistor. Source: Filament2print
There are two main causes of thermistor problems:
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A bad configuration of parameters in the firmware: As previously mentioned, it is essential that the printer firmware has the specific RT values configured for a specific thermistor model in order to be able to accurately convert the resistance values measured into real temperature values. All thermistor manufacturers provide the RT data for each model, and firmware such as Marlin or RepRap FW include RT tables for the most common models.
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Poor condition of cables or connections: A cable in poor condition, a bad connection or an excessive length of cable, can increase the resistance measured by the board, resulting in erroneous temperature readings. It is essential to periodically check the condition of the thermistor cables and connections. The thermistor must be connected directly to the board, avoiding the use of connection plugs or connectors and using the minimum length necessary. If quick connectors are used, they must 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 connector on the board with a multimeter and see if it is the same as specified 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 ºC - 1400 ºC). They are very economical and interchangeable, however they have two important limitations:
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They have a very low accuracy (better than ºC).
- They require the installation of amplifier plates in order to be used.
Until recently they were the most common solution for high-temperature 3D printers, but they have been displaced by other technologies such as high-temperature thermistors or PT100 probes.
Image 2: Thermocouple type K. Source: RS Componentes
The main causes of problems are:
- Poor condition of wiring or connections: Like thermistors, temperature is determined by measuring the resistance of the thermocouple, so defects in the wiring or connectors cause erroneous temperature measurements.
- Electrical noise: Thermocouples are sensitive to electrical noise, so the appearance of this in the circuit alters the measurements.
RTD probes
Similar to NTC thermistors, they are composed of a metal that varies in electrical resistance with temperature. Unlike NTC thermistors where the resistance decreases with temperature, RTD probes increase with temperature. This means that they can accurately measure much higher temperatures than thermistors, up to 600 °C. Although they are very accurate over a wide temperature range, they have the disadvantage that they are more expensive and, like thermocouples, require additional electronics, which further increases their cost and complicates their installation. The most common type of RTD probe is the well-known PT100 probe.
Image 3: PT100 probe with amplifier plate. Source: E3D.
In general, they present fewer problems than NTC thermistors and thermocouples, however, as in the previous cases, it is important to check the condition of the cables and connectors, since their operation is also based on the electrical resistance reading.
Temperature-related problems
In many cases, temperature problems are not related to the sensor itself, but to the temperature control model and safety settings of the printer. To control the 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 a higher or lower heating rate, so the correct setting of these parameters is essential for accurate temperature control. This is why most 3D printers incorporate a function called PID calibration, which determines these parameters automatically. It is advisable to carry out this calibration periodically.
In addition, it is common for 3D printers to implement safety algorithms that deactivate heating when the heating speeds or temperatures reached do not correspond to those of the model. In these cases, temperature errors are frequent. When they appear, the following should be checked:
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The status of the temperature sensors
- The thermal efficiency of the hotend
- The layer fan is not facing the heater block and cooling it.
- That the heater block is not in contact with the heatsink of the hotend.
- Perform a PID calibration.
Note: This guide discusses concepts in a general way and does not focus on a particular brand or model, although they may be mentioned at some point. There may be important differences in calibration or adjustment procedures between different brands and models, so it is recommended that the manufacturer's manual be consulted before reading this guide.
