What is a 3D Printer Thermistor?
While thermistors might sound like something out of a science fiction comic, they are the most widely used temperature-sensing device in 3D printers today. Having an accurate measure of a printer’s nozzle and heated bed temperatures is essential for achieving successful prints; this is where thermistors stand out from other types of sensors like thermocouples or RTDs. This is because they are relatively inexpensive and easy to connect to controller boards, allowing them to be integrated into different types of 3D printing systems.
Where to locate Thermistors on a 3D printer
On an FDM printer, thermistors can be discovered in a few different locations. On the printhead, or hot end, and is embedded within a block of metal, often insulated by a silicone cover over it. This allows the temperature of the block to be measured accurately while two pairs of wires are connected – one powering up the heating element within it and another connecting with the thermistor itself. This intricate connection layout allows FDM printers to finely adjust the required temperatures needed to melt filaments and achieve desired prints.
The thermistor on a heated bed can be a tricky thing to track down. Its placement is so discreet that it’s easy to overlook. Taking up residence between the heated element and the print surface, it’s almost hidden away. More recently, some 3D printing setups have ventured even further with an extra thermistor monitoring the printing space as well; this adds an extra safety feature as temperatures can widely vary during a print job.
In order to measure the temperature the two wires running between the thermistor and controller board detect changes in electrical resistance. As the thermistor heats up, this change is detected and evaluated by the printer’s firmware which checks against calibration data to understand how much heat has been generated.
Although the use of thermistors is incredibly widespread, they often get overlooked in favour of more familiar electronic parts and components. In this article, we’ll explore the essential facts about thermistors – what they are and what they can be used for – as well as typical issues and problems you may encounter with them. We also have some useful tips to consider if you plan on replacing, modifying, or upgrading a thermistor in your project.
Types of thermistors
Thermistors can be divided into two categories. The type found in 3D printers is called negative temperature coefficient (NTC) devices. This means that their resistance to electrical current decreases as the temperature rises. On the other hand, you can find positive temperature coefficient (PTC) thermistors which have an increase in resistance when getting hotter and so they become an ideal choice for resettable electric fuses of machines or electrical applications.
Classic 3D printer thermistors are a special type of thermistor that is characterized by an NTC 3950 100k value. This is due to the fact that the shape of the temperature/resistance curve for these thermistors is determined by three coefficients, originally established by Steinhart and Hart in the 1960s. These coefficients (labelled as a, b and c) are more commonly simplified to B or β which will typically be within the range of 3,500 and 4,500.
NTC thermistors are highly-specialized temperature sensors that have a wide range of sensing capabilities, spanning from -50 to 250 °C. Surprisingly, the resistance change between these temperatures is not linear. Instead, it exhibits a steep drop as the temperature shifts towards lower numbers, then flattens out considerably when the value increases.
Every printer contains a unique set of calibration data pre-programmed into its firmware that matches the particular thermistor installed. These programmed settings generally work fine without alteration; but, there are some cases in which changes must be made. So, it is beneficial to have a grasp of how the calibration is performed.
The resistance at room temperature serves as the starting point, which is equal to 100 kΩ (kiloohms). Having this information is essential as it gives users real-time updates on the temperature of their device, allowing them to make necessary adjustments if need be. Fortunately, there are manufacturers that provide tables of data related to how resistance varies with respect to changing temperatures across its full operating range. This data makes it easier for 3D printers and their firmware to monitor the temperature in detail and display the readings accordingly.
Also, of particular importance is the connection between the thermistor for the extruder and the circuit board. Generally, a 4.7 kΩ pull-up resistor is utilized; this figure is then used during calibrations.
In addition to this, we need to think about the thermistor’s packaging and how it is physically attached. They range from just a millimetre wide to larger designs encased in glass beads or plastic discs. Additionally, some thermistors are built into E3D-compatible cartridges or with screw mounts, making them easier to install on specific types of 3D printers.
Fixing Thermistor Issues
While one might assume that thermistor measurements are just as reliable as any other electronic data, with 1% accuracy being standard, this is not always the case. Especially in 3D printing forums, it’s no surprise to find reports of drastic measurement errors; those as much as 15 °C higher than expected standards. The risk of greater inaccuracies exists too; some cases may even lead to errors exceeding 15 °C.
The following are several factors that may lead to such errors:
Here are a few situations that can cause these sorts of errors: Drift over time: All thermistors will change over time, especially with prolonged exposure to high temperatures. This causes an increase in resistance, which results in the recorded or displayed temperature being lower than it actually is. For the thermistor itself, manufacturers claim a change of less than 0.2 °C a year. Many anecdotal stories, however, point to a higher drift than manufacturers’ claims, although this may be due to other components.
Here are a few situations that can cause these sorts of errors:
- Drift over time: The accuracy of thermistors will deteriorate over time, especially if it is exposed to temperatures beyond their rated limits. This will cause increases in resistance, which end up resulting in a lower-than-actual temperature being recorded or displayed. Manufacturers of thermisters typically claim a maximum change of up to 0.2 degrees Celsius per year, however, this has been found to be inaccurate in many cases through anecdotal information, though it could be possible that their claim is accurate another component and its degradation could be the cause of the inaccuracy.
- Tolerance: Thermistors are a highly reliable form of temperature gauging but this accuracy is not immune to degrading over time and higher temperatures. This has the potential to be a major issue in industries such as 3D printing where thermistors are used in hot ends. With material such as ABS, tolerances of +/-3% experienced by thermistors equate to as much as +/-5°C, resulting in outcomes that may be drastically different than what was anticipated by the initial impression.
- Incorrect calibration data: Errors in the calibration data for installed thermistors can be disastrous. If improperly calibrated, a thermistor can produce wildly inaccurate temperature readings. These discrepancies may continue to grow, with the potential for huge systematic bias that is immediately obvious.
- Poor PID tuning: If you’re seeing the temperature varying from what your display is suggesting, it could be a result of improper PID tuning. It can take a bit of technical knowledge to get it done right but luckily there are various guides available to make this task easier.
- Complete or partial thermistor failure: Malfunctioning thermistors can lead to an alarming underreporting of the actual temperature, which is why it’s so important that technicians pay close attention to these components and ensure that all thermal protection settings are enabled on their printers. Electrical wires connected to the thermistor can easily become loose or broken over time, especially when changing a nozzle as these wires tend to be quite fragile. In serious cases, this could even cause thermal runaway and dangerous fires
All of these effects add up, and as a result, there may be significant discrepancies between the temperatures actual and measured temperatures. To overcome this difficulty, calibration towers can be printed and used to adjust future G-code accordingly, however, this requires the same model of 3D printer each time – an impractical commitment for those with multiple machines.
Another option is calibrating a thermistor to customize the calibration curve and then updating the printer’s firmware can be done in situ with great results. Chris Riley’s video shows just how quickly this process can be accomplished, giving users more control over the temperature readings. A helpful G-code option that M305 offers is also available to most firmware which will allow thermistor parameters to be reported or changed during operation.
Advertisements for upgrading 3D printer thermistors can be tempting and make it sound like a necessary move, yet often this isn’t the case. The truth is that all of the brands of thermistors on the market today perform essentially the same way. Unless there has been an issue with failure or other problems with a certain model, there usually isn’t a good reason to bother switching brands as not much benefit will result from it.
The need to change out a thermistor comes when it is no longer functioning properly. In most cases, this means that the existing thermistor has not been able to maintain the correct temperature in order to keep the printer at optimal performance levels. Let’s take a closer look at these specific instances.
Same Brand Replacement
Replacing a faulty thermistor can be a tricky process, but the simplest approach is to replace like with like. If a trustworthy, reliable source is chosen and the old installation directions are followed exactly, there should be no issues.
Special attention should be paid not to overtighten any cables or grub screws that may keep the thermistor in place. While it may take more effort than doing something else, this is generally the safest and most suggested route when it comes to replacing faulty thermistors. Also, this approach doesn’t require any changes made to your printer’s firmware!
Different Brand Thermistor Replacement
If an exact comparison isn’t accessible, opting for a known brand like Semitec, TDK Electronics (formerly EPCOS), Honeywell, or Hisense can ensure that you don’t run into any issues with the component fitting properly. It’s especially important to make sure any replacement folio is explicitly designed to be “compatible with Marlin firmware” and lists the specific Marlin firmware calibration data it should work with.
If you find that your new thermistor has a different β value, you will need to instruct the firmware to use a different calibration data set. Depending on the firmware installed, this process could range from extremely simple to somewhat intricate. Thankfully, most firmware packages have accounted for the common thermistor types and preconfigured options for those types are already in existence.
When procuring these components, it is typically advisable to keep with the same package style because there is no agreement that using other connection or thread types would increase accuracy. As a result, employing glass bead thermistors with threaded connectors is still a viable choice.
Setting Values for High-Temperature Printing
Modifying printers to print material at higher temperatures than intended can be a complex process and requires several modifications, including changes to the thermistor.
Thankfully, there’s now a wide range of thermistors capable of an effective temperature range of over 300 °C. It’s important to note that any wires or sheathing used with these components must also be able to handle additional heat. Unfortunately, some of the items sold online don’t come with proper wiring protection against such heat, so it’s best to shop around before making a purchase.
If you are in the market for thermistors, it is always a wise idea to purchase from reputable providers who can provide full calibration data. Slice Engineering is trusted by many as they have thermistors that operate up to a remarkable temperature of 450°C.
However, if your controller board requires high-temperature thermistors and then uses them at lower temperatures, it may require firmware changes to avoid “Min Temp” errors. For more details on how this might work with Marlin, be sure to check out their example.
Finally, High-temperature heat paste can help to improve the accuracy of thermistors when working with higher temperatures. This type of paste, such as boron nitride, is specifically designed to handle very high temperatures without introducing significant lag from thermal insulation or other effects.
On the other hand, PT100 and PT1000 resistive temperature devices (RTDs) use a small strip of platinum with a consistent resistance regardless of changes in temperature. Therefore, these RTDs (These are not thermistors) are often used in applications where maintaining a consistent reading is more important than the speed of response.
RTDs, or Resistance Temperature Detectors, have become increasingly popular as temperature sensors due to their great accuracy and stability, and relatively easy-to-complete calibration process. For many years, the PT100 has been standard among 3D printers, but its use requires more components such as a MAX31865 interface board to properly communicate with most 3D printer controls.
PT1000s, while not necessarily well known, offer better performance for 3D printing than their predecessors but are still not easy to find in formats suited to the task. Luckily for those looking for a higher-quality solution, Omega has already compiled a comprehensive comparison of the two options making it quick and easy to determine which one is right for you.
Furthermore – like thermistors – Firmware adjustments can be made easily and are supported by virtually any firmware option on the market today.
I am Ganesh Divte. I work as a Quality Assurance Engineer at Dhruvtara WireTech PVT LTD. I have experience in SLS, DMSL, FDM, and SLA additive manufacturing processes. I am very enthusiastic about additive manufacturing and its potential to change the way we manufacture products. I believe that Additive Manufacturing has the potential to revolutionize the manufacturing industry and make it more efficient and sustainable.