3D Printer Thermistor. What It Does in a 3D Printer?

by Mike Brooks | Last Updated: March 6, 2022

What Is a 3D Printer Thermistor?

A thermistor is a resistor whose resistance varies rapidly and in a predictive manner with changes in temperature.

Thermistors are made from semiconductors, mostly germanium and silicon, and their resistance value depends on the magnitude in their temperature range.

What Does a Thermistor Do in a 3D Printer?

Thermistors are used in 3D printers as temperature sensors. They are fixed in temperature-sensitive points such as the heat bed and the hot end. In these points, they monitor the temperature and send the data to the microcontroller.

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To control the printer temperature, the printer’s microcontroller uses the related data to regulate the temperature of the printer.

Thermistors are made of a pair of wires joining each thermistor. The pair, which has a short wire length, is usually led back to the controller board.

When they heat up, their resistance change is detected and mapped against calibration data by the printer’s firmware to calculate the temperature changes.

Types of Thermistors

There are two types of thermistors:

a) Negative Temperature Coefficient (NTC)

This is the most commonly used type of thermistor in 3D printers. The NTC thermistor offers variable resistance depending on temperature. When the temperature rises, the resistance goes from high to low and allows the current to go through.

At power-on, NTC offers an additional series of resistance when used to mitigate for inrush current. Resistance drops to a negligible amount during normal temperatures when the thermistor self heats with a current, allowing normal current flow.

b) Positive Temperature Coefficient (PTC)

The PTC thermistor also offers variable resistance depending on temperature. As temperature increases, resistance rises from low to high and prevents the overcurrent.

PTC thermistors are used in certain scenarios rather than NTC thermistors. These are: in extreme temperature conditions, in equipment with almost zero-reset time such as digital thermometers (check at Amazon), or in a system that experiences frequent shots.

Why Are Thermistors Good Sensors?

Thermistors are advantageous for use in many applications as sensors because of their high durability and ease of use since their behavior is quite predictable. Their use in applications for temperature measurement and control is outstanding.

Evidently, a change in electrical resistance of a thermistor due to a corresponding temperature change is notable irrespective of whether the thermistor’s body temperature is altered due to radiation or conduction from the surrounding environment or due to “self-heating” caused by power dissipation within the device.

The thermistor’s body temperature will be dependent on the thermal conductivity of its environment and also its temperature when a thermistor is used in a circuit where the power dissipated within the device is enough to cause “self-heating”.

Thermistors are “self-heated” for use in applications such as airflow detection, liquid level detection, or thermal conductivity measurements

How Do I Change Thermistor on My 3D Printer Heated Bed?

The following steps may be used to replace the thermistor and the heating tube.

You will require the following tools:

• A thermistor
• Heating tube
• A pair of scissors
• Tweezers
• Dual-purpose screwdriver
• 1.5 mm hex key wrench
• Cable tie (check at Amazon)

Step 1: Dislodging the fan next to the nozzle

Firstly, ensure that the nozzle is cooled to room temperature, to avoid injuries. Detach the two side covers by using a screwdriver, then loosen the two screws next to the fan.

Unplug the connector of the fan using tweezers to hold down the port, then dislodge the fan.

Step 2: Removing the thermistor and the old heating tube 

Now, using the tweezers, unplug the connectors of the old heating tube and thermistor, then cut the old heating tube and thermistor.

After cutting, use the 1.5mm hex key wrench to loosen the screw in the heat block, and extract the old heating tube and thermistor.

Step 3: Installing the thermistor and the new heating tube

Bind the new heating tube and thermistor up with a cable tie, then insert the new heating tube and thermistor into the heat block. Part of the heating tube should extend from the other side of the heat block.

Ensure both ends of the heating tube to the heated block are of equal length.

Ensure the thermistor is within the heat block, then tighten the screw and put the new heating tube and thermistor through the module. Plug the connectors into the ports.

The heating tube and thermistor can not reach the hot end.

Step 4: Installing the fan

Plug the fan connector into the interface then attach it to the module.

Step 5: Assembling the parts

Install the two side covers.

Step 6: Testing the thermistor

Connect the module to the controller, then switch on the power. Load the printer filament, calibrate the heated bed and start a short print to test.

Check if there could be issues as it prints.

How Do I Test the Resistance of a Temperature Sensor?

There are various ways of checking for resistance. For this discussion, we will consider checking resistance using a multimeter.

Resistance is not a value that you can measure directly. To find the thermistor’s resistance, you’ll have to infer current flow in the thermistor, then measure the resulting resistance to it.

The reading will vary across temperatures since it is a thermistor. It Is preferable to take your reading to room temperature resistance (25℃)

Steps on How to Check the Resistance:

To check temperature resistance we shall need; multimeter and multimeter probes

Step 1: Remove the fiberglass insulation on the pair of wires connecting the thermistor.

Step 2: Set the Multimeter range to the rated resistance of the thermistor e.g. 100k.

Step 3: Apply the multimeter probes to the two wires. The multimeter should display the temperature resistance.

How Do I Know if a Thermistor Is Bad?

Most 3D printing thermistors have a temperature resistance of 100k at room temperature. Various symptoms of a bad thermistor in a 3D printer include:

• When Print Temperatures are Higher than Usual

Printer materials normally have recommended print temperatures. If the printer requires a higher temperature to extrude the materials than the rated temperature, the thermistor can be bad.

• Thermal Runaway

A thermal runaway is a situation in which a 3D printer heats up to extremely high temperatures and can not stop. When this happens, the printer may catch fire!

Thermal runaway may be due to a variety of reasons. However, the most probable cause is when the thermistor is not properly aligned.

Due to the poor alignment, the firmware will continually increase the heat so that it reaches the target.

The increase is due to its inability to properly sense the current bed temperature or current nozzle.

To prevent thermal runaway, you need to install thermal runaway protection in the printer’s firmware. The firmware alone does not prevent the thermal runaway. It only tries to halt the printer’s overheating in its tracks.

• Failure to print by the printer, or many print errors due to temperature issues
• Abnormal temperature variations readouts on the printer monitor

How to Calibrate Your 3d Thermistor

There are different ways of calibrating a 3D thermistor. We are going to learn how to calibrate using a multimeter.

1. Testing the Thermocouple

Test whether the multimeter’s thermocouple is accurate. Boil a small amount of water, then dip the thermocouple into the water. A reading of 100℃ indicates that the thermocouple is accurate

2. Identifying the Hot End

Now open the printer’s firmware. Check to identify the hot end. There is usually a file in the printer’s program file controlling the hot end. The owner’s manual may indicate the location of the file for your printer.

3. Connecting Hot End to the Multimeter

Join the multimeter’s thermocouple to the hot end. An appropriate way is to find a space between the hot end and the nozzle and stick it in.

4. Copying Table

Open the temperature table, in the firmware. The table contains the values of thermistor resistance versus temperature. To determine the temperature from the resistance measured, the printer uses this file. Now copy the table, then delete the temperature column in the new table.

5. Filling Out the Table

In the old table, set the hot end to the temperature value, then measure the accurate temperature reading on the multimeter. Copy the readings to the resistance value on the new table matching the value on the old table, then repeat the steps for all resistance values.

6. Replace the Table

Delete the old thermistor table and replace it with the new one after you have found the desired temperature for the resistance values.

Factors That May Lead to an Error in Temperature Range

Despite all data computation and calculations relating to thermistors, thermistor measures still display abnormally high margins of errors of over the acceptable 1% margin in 3D printers. These errors can be attributed to:

The following are several factors that will cause such errors:

1. Apparent Thermistor Failure

Thermistor wires are quite fragile. They can easily be spoiled when changing the nozzle. The damaged thermistor could lead to an extremely low-temperature reading. This could in turn be disastrous to the printer since the reported readings are not the actual temperature. In some rare cases, it can lead to fire breakout.

2. Drift Over Time

Thermistors wear out over time as a result of continued exposure to high temperatures. The exposure to high temperatures leads to a rise in resistance, which may translate to abnormally lower temperature readings that really is.

This exposure is an additional factor to the normal drift in the time expected in any machine or component of a machine due to degradation.

3. Inaccurate Calibration Data

Incorrect calibration data may cause errors in thermistors. Therefore, in addition to errors caused by tolerance, the margin of error here will be much higher than expected.

4. Tolerance

Thermistors are relatively consistent in performance, however, their tolerances decrease with a rise in temperature.

The deviation for thermistors used in some 3D printer hot ends may not necessarily be accurate at 1%. It may be higher like 2%, and even higher than this during printing.

5. Motherboard Circuit

Some components on the printer’s motherboard or abnormally high resistance in connecting cables can cause thermistor temperature reading errors. These incidents can change and drift over time.

6. Poor PID Tuning

Poor PID tuning can make a printer’s heated bed or hotbed temperatures swing back and, in some cases, be wrongly reported.

Buying a Thermistor

The main reason for one should buy a thermistor is if the one they have has become faulty.

Secondly, one could buy a new one if they wish to modify their printer to print at a higher temperature than the one usually designed for.

The most important factor in buying a new one is to ensure the compatibility of the firmware.

You should also consider buying from a recognized dealer so that whatever you buy is genuine.

Conclusion

A thermistor is a small component in a 3D printer, yet plays a pivotal role in printing. The status of the thermistor should not be overlooked since it could bring everything to a grinding halt.

Michael Brooks is the founder of M3DZone.com. He sees a very bright future for 3D printing that's why his mission is to try and make this easy for everyone. Discover your hidden talent and creativity. You can follow here: Facebook, Twitter & Pinterest.