3D PRINTER THERMISTOR: THE BASICS Considerations Thermistors up close and personal (Source: e-radionica) Types There are two types of thermistors. Those found in 3D printers are known as negative temperature coefficient (NTC) devices, meaning that their resistance decreases as they heat up. Contrastingly, positive temperature coefficient (PTC) thermistors increase resistance with rising temperatures and are used often as resettable electric fuses. A classic 3D printer thermistor is a type “NTC 3950 100k”. The number following “NTC” relates to the coefficients used to describe the shape of a thermistor’s temperature/resistance curve. The characteristic shape was established by the researchers Steinhart and Hart in the 1960s, and the three coefficients (a, b, and c) still bear their name. These are often simplified to a single value B or β, with a value typically between 3,500 and 4,500. Calibration The relationship between temperature and resistance isn't linear (Source: Physics Online via YouTube) A typical NTC thermistor operates from -50 to 250 °C; however, the change in resistance across that range isn’t linear. It’s more pronounced at lower temperatures and flattens out as it gets hotter. Each type of thermistor, therefore, comes with calibration data – pre-configured in the firmware of new printers – to help work out the right temperature. Normally, the calibration data can be ignored. Yet, there are cases where changes need to be made, so it’s worth understanding how calibration works. The starting point is the thermistor’s resistance at room temperature. 3D printers are typically designed to use thermistors with a resistance equal to 100 kΩ (kiloohms). The manufacturer also provides a table of data to show how this resistance varies as temperature changes across its full operating range. This data can be stored and used directly by a printer’s firmware. A final point on calibration relates to how thermistors are connected to the printer’s circuit board. This is nearly always by making use of a 4.7 kΩ pull-up resistor, and this value is used in calibration calculations. Packaging Thermistors come in various packaging types (Source: PintarElektro) We also need to consider how the thermistor is packaged and physically installed. The actual sensor is less than a millimeter wide, so it’s usually encased in a tiny glass bead or plastic disc. They can also be built into E3D-compatible sensor cartridges or with screw mounts, depending on the type of 3D printer fitting.
3D PRINTER THERMISTOR: THE BASICS Identifying & Fixing Issues Replacing a heated bed thermistor (Source: Creality via YouTube) With all of the data and calculations related to thermistors, we would expect that our thermistor measures within 1% accuracy. This is, however, not always the case. On 3D printing forums, it’s not uncommon to find accounts of measurement errors as high as 15 °C, and in some cases, there may be problems that would cause errors that well exceed 15 °C. The following are several factors that may lead to such 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 component degradation.
Tolerance: Thermistors tend to be reasonably consistent in performance, but their tolerances degrade with increasing temperature. The thermistors used in some 3D printer hot ends may be accurate +/-3% at higher temperatures, which equates to +/-5 °C when printing material like ABS.
Incorrect calibration data: It can happen that the applied calibration data is incorrect for the installed thermistor. This has the potential to cause the largest discrepancies, sometimes so large that they’re immediately apparent!
Connecting circuitry: Out-of-spec components on the printer’s motherboard or abnormally high resistance in connecting cables can also have a bearing. These too can change and drift over time.
Poor PID tuning: This can cause a printer’s hot end or heated bed temperatures to oscillate and, in some cases, be out of step with what’s reported. On most printers, executing a quick PID auto-tune will help. Check out our comprehensive article on PID tuning.
Complete or partial thermistor failure: A massive underreporting of actual temperatures can be the result of a problematic thermistor. On older printers (or those without thermal protection enabled), this could result in thermal runaway and potentially even a fire. The sensor itself can fail, but most likely, the issue is a broken or loose wire. Thermistor wires are relatively fragile, and care must be taken to avoid damage when, for example, changing a nozzle.
Thermal runaway can cause such disasters and much worse (Source: Hedkin via Reddit) All of these effects are cumulative and can result in notable differences between actual and measured temperatures. The simple way around this problem is to print temperature calibration towers and adjust any future G-code to compensate. However, this approach places a dependency on that exact printer, which might not be convenient for those with more than one. Alternatively, a thermistor’s response can be calibrated in situ, resulting in a customized calibration curve that can then be used to update the printer’s firmware. See Chris Riley’s great video on this. There’s also an M305 G-code option for most firmware that allows thermistor parameters to be changed or reported during operation.
3D PRINTER THERMISTOR: THE BASICS Buying a Thermistor Although there are tons of options online, there are some things to keep in mind before you buy (Source: FastTech) It’s not uncommon to see online advertisements encouraging users to “upgrade” their 3D printer’s thermistors. Unless there’s a known issue with the type of thermistor installed, there aren’t really any upgrade options, as all of the known brands perform much the same. The primary reason for changing a thermistor is because it has become faulty. A secondary reason would be as part of a modification to print materials at a higher temperature than the printer was originally designed for. Let’s look at these cases in more detail. Like with Like Replacement If you’re replacing a faulty thermistor, the best advice is to replace like with like, bought from a reputable source, and installed exactly as the original with no significant wiring changes. As always, take care not to overtighten connecting cables or, if used, any grub screws that hold the thermistor in place. This approach is easy and doesn’t require any changes to the printer’s firmware. Replacement with a Different Type or Brand If an exact equivalent isn’t available, err towards a known brand for replacement. These include brands like Semitec, TDK Electronics (formerly EPCOS), Honeywell, and Hisense. It’s also wise to choose one explicitly suitable for 3D printing. Typically, these will include something like “compatible with Marlin firmware” (followed by the number of the existing Marlin firmware calibration data) or mention other firmware settings. If the thermistor you choose has a different β value from the original, the printer’s firmware will need to be changed so that it uses the correct calibration data. The exact process varies by firmware type and worked examples can be found for each. Most firmware packages already have preconfigured options for the most common thermistor types. It’s also usually best to stick with the same packaging type. Hot ends that use glass bead thermistors may also have threaded connectors to accept thermistors packaged that way, but there’s no consensus that this is an improvement. Modifying for High-Temperature Printing Slice Engineering produces high-quality high-temperature thermistors (Source: 3DJake) It’s not uncommon for printers to be modified to print materials at a higher temperature than the original design caters for. In these cases, many printer modifications will be required, and the thermistor is no exception. Many thermistors with an effective temperature range of over 300 °C are now available. Beware, however, that many sold online don’t necessarily use sheathing for wires (or other materials) that can handle these temperatures for long. Again, it’s wise to buy from reputable sources. Suppliers such as Slice Engineering have thermistors that operate consistently up to 450 °C. As expected, they provide full calibration data for various types of firmware. It’s also worth noting that, with certain controller boards, high-temperature thermistors don’t always work well at lower temperatures, and additional firmware changes may be required to avoid “Min Temp” errors. See this Marlin example. A final note on high-temperature thermistors is that their accuracy may be improved by using high-temperature heat-paste such as boron nitride.
3D PRINTER THERMISTOR: THE BASICS Alternatives for High Temperatures RTD temperature sensors may need additional circuitry to work with 3D printer controllers (Source: Adafruit) Often when modifying printers for higher temperatures, PT100 and PT1000 devices are mentioned in the same sentence as thermistors. These are, however, RTDs – not thermistors. They use a small strip of platinum with a resistance of 100 or 1,000 ohms at 0 °C, hence the names. RTDs are accurate, stable, and easier to calibrate. PT100s have traditionally been used with 3D printers, but they require extra circuitry to work with most 3D printer controllers: typically an interface board known as the MAX31865. Until relatively recently, PT1000s have been hard to come by in formats suitable for 3D printers, which may explain why few have heard of them. Despite the relative obscurity, they work with most controller boards and are notionally better suited to 3D printing. Check out this comparison by Omega. Regardless of which PT device you choose, changes to firmware are required. As with thermistors, these are straightforward and supported by most firmware options.