All About NTC Thermistor Temperature Sensors

Temperature sensing can be found everywhere around us. Your mobile phone, your car, the refrigerator in your kitchen, and even the machinery at the medical center next door all employ some method of temperature sensing. Although there are many different ways of measuring temperature, one particular kind has always stood out due to its high sensitivity, small size, and adaptability, the NTC thermistor. Though they may not receive as much publicity as some of the more sophisticated devices, the accuracy and reliability of NTC thermistors have rendered them essential in countless industrial sectors.

This article highlights the NTC thermistor, its operating principles, physical manifestations, comparative aspects with other types of sensors, and its applications.

What is an NTC Thermistor?

The word “thermistor” is a combination of two words, which are thermal and resistor. “Thermal” refers to anything related to heat while resistor refers to a passive electronic device used to resist electrical currents in circuits. In a literal sense, a thermistor is any resistor whose resistance is greatly affected by temperature. The NTC term means Negative Temperature Coefficient. This implies that the resistance of an NTC thermistor decreases when temperatures increase.

The NTC thermistors are manufactured using ceramic semiconductors, usually consisting of metal oxide compounds like manganese, nickel, cobalt, copper, or iron oxides. They are then compacted to form small sizes such as beads, discs, chips, and rods. Their semiconductive nature makes them respond highly sensitively to changes in temperatures.

An NTC thermistor can even detect temperature changes of 0.05 degrees Centigrade when required. For this reason, they are ideal for situations.

How Does an NTC Thermistor Actually Work?

A brief understanding of semiconductor physics will enable one to comprehend how an NTC thermistor operates. In any given semiconductor material, there is always a certain minimum amount of energy that electrons must possess before they can escape the bonds that hold them in place in the material in order to become charge carriers. The energy needed for this purpose is referred to as the bandgap energy.

As the temperature lowers, very few electrons have the required amount of energy to move around. In such cases, the resistance will be high. With a rise in temperature, more electrons acquire the required amount of energy, hence increasing conductivity and decreasing resistance. Unlike metals like platinum, whose resistivity increases with an increase in temperature, semiconductor resistivity decreases with temperature.

The relationship between temperature and resistance is nonlinear in an NTC thermistor and is expressed mathematically using the well-known Steinhart-Hart equation. The simplified version of this is the B-parameter equation, which looks as follows:

R(T)=R₀exp(B(1/T-1/T₀))

Hence, R(T) signifies the resistance at temperature T measured in Kelvin, whereas R₀ denotes the resistance at the reference temperature T₀, which is generally 25°C. The parameter B varies depending on the oxide employed and falls within the range of 2000K to 5000K.

What Criteria Are Important During Selection?

The following criteria describe the performance of the chosen NTC thermistor:

Resistance at 25°C (R25): This criterion indicates the resistance of a particular material at the reference temperature of 1kΩ, 10kΩ, and 100kΩ, where 10kΩ is the most common designation for consumer and medical equipment applications.
B value (beta value): The material constant that defines the steepness of the resistance-temperature dependence. The greater the B value is, the more sensitive and nonlinear the curve will be.
Tolerance: It shows how accurately the thermistor resistance corresponds to its rated one. There are various tolerances, ±1%, ±2% and ±5%
Operating temperature range: The thermistor operating temperature range is -55°C to 150°C
Response time: It shows how quickly the thermistor can stabilize at the measurement temperature; it depends on the thermistor size, weight, and the medium in which it works.
Dissipation constant: Power in mW required to increase the temperature of the thermistor itself by 1°C.

One should pay attention to the issue of self-heating. In order to measure the thermistor resistance, it is necessary to pass current through it. However, it leads to heating, hence, a small but systematic measurement error.

What Physical Forms Do NTC Thermistors Come In?

NTC thermistors come in different forms based on the application in which they will be used. In most cases, the decision to pick one form over another may be influenced by the operational environment as well as the electrical characteristics.

Bead thermistors

Bead thermistors are among the smallest and fastest types available. This type features a minute semiconductor bead, normally smaller than a millimeter across. The lead wires are built right into the bead to enhance performance. Bead thermistors can quickly respond to variations in temperature owing to their low thermal mass.

Disc or chip thermistors

Disc or chip thermistors come in a disc form. They are fabricated using ceramic powders and then sintered to create a solid object. The electrodes are added to the surface of the discs. They represent the typical surface mount thermistors that are installed onto a printed circuit board.

Rod thermistors

Rod thermistors are made in a rod or cylindrical form. They can measure liquids and gases within enclosed tubes and feature higher resistance ratings.

How Do NTC Thermistors Compare to Other Temperature Sensors?

Placing the NTC thermistors in comparison with other temperature sensors will enable one to comprehend where exactly these thermistors can operate efficiently.

While compared with thermocouples, NTC thermistors display much higher sensitivity and accuracy of measurements than their rivals. For example, using a thermocouple for measuring inside the furnace where temperatures reach 1200°C is the only option to make such a measurement. Nevertheless, for measuring human body temperature, which is 37°C, NTC thermistor is more efficient than its alternative. The thermocouple needs reference junction compensation, while the NTC thermistor does not need it, besides being immune to electrical interference.

RTD’s advantages are that it has high linearity, a broader operating temperature range, and good stability compared to NTC thermistors. On the contrary, NTC thermistors are cheaper and more sensitive than RTD thermometers. Therefore, for pharmaceutical production or precise calibration instruments, the preferred instrument is RTD. As for consumer electronics or clinical use, NTC thermistors are obviously better for these purposes.

Compared to the semiconductor IC sensors, NTC thermistors need additional conditioning for their signal since their output is non-linear; however, they react quickly, are smaller in size, and have high sensitivity. IC sensors are ready-made and easy to use with digital devices; nonetheless, they draw more current and lack precision at the endpoints of their measuring range. Studies have revealed that within the temperature range of 35°C to 42°C in the human body, a properly specified NTC thermistor would beat IC sensors by five times or more in accuracy.

Where Are NTC Thermistors Actually Used?

The scope of uses for NTC thermistors is even broader than one could imagine. The sensitivity, compactness, quick response, and low price of NTC thermistors enable their application in almost any industry that requires temperature monitoring.

Health Care & Medical Industry

NTC thermistors are utilized in digital thermometers, patient monitors, incubators for babies, dialysis machines, and breathing apparatuses. Due to the rapid response and high accuracy, especially in the 35°C to 42°C range, NTC thermistors serve as the best fit. Disposable health care items also feature NTC thermistors to prevent contamination while taking body temperature measurements during operations.

Battery Management

Portable electronic devices like mobile phones, computers, power banks, and others employ NTC thermistors to detect the temperature of each cell within a battery pack. Since lithium-ion batteries need to be monitored constantly due to the fact that any working conditions above 45°C may decrease the lifetime of a battery by 20%, NTC thermistors are essential. Operating at a temperature of over 60°C poses risks since thermal runaway may easily occur.

Automotive Industry

Contemporary automobiles utilize NTC thermistors to detect the coolant temperature, the air intake temperature, the cabin air temperature, and the battery temperature in electric cars. The coolant temperature matters a lot since it is necessary in fuel injection timing and ignition timing advancement. Automotive NTC thermistors have operating ranges starting from -40 °C up to 130 °C.

Air Conditioners and Other Domestic Appliances

In addition to the above uses, the NTC thermistor is used in domestic applications such as air conditioners, heat pumps, refrigerators, washing machines, dish washers, and many other devices. For instance, when using a refrigeration system, the NTC thermistor is placed inside the refrigeration machine and in the evaporator part of the machine.

Applications in Industrial Process Monitoring

While RTD sensors are common in almost all industrial process monitoring applications, there is some level of involvement of NTC thermistors in lower-temperature industrial applications, food processing systems, laboratory instruments, among others, due to their cost-effectiveness.

3D Printing Application

Thermistors are commonly used as temperature sensors within FDM 3D printer hotends. The sensors are located near the printing nozzle to provide temperature information to the printer controller responsible for controlling the heating elements depending on the type of material being used for the print job.

What Are the Limitations Worth Knowing About?

Output nonlinearity requires mathematical correction or lookup tables for accurate readings across a wide range
Limited operating temperature range compared to thermocouples and platinum RTDs
Self-heating error if excitation current is not carefully controlled in circuit design
Long-term stability is lower than platinum RTDs in applications requiring precision over many years
Interchangeability can be inconsistent with lower-tolerance or lower-grade components

Conclusion

The NTC thermistors have finally found their place in the realm of measuring temperature. Batteries of this kind would be perfect for different settings, like normal gadgets, life-preserving medical devices, monitoring the condition of patients constantly, electric cars, and household items.

It is vital to know everything about their operation, what can be developed with their help, and in what conditions they will do good.

And once applied correctly, an NTC thermistor will be a small, affordable, and highly reliable solution to one of the core problems of engineering: accurately measuring temperature.

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