What Is a Thermistor? How Does It Work?

I. Introduction

In electronic circuits, resistors are among the most common components. The resistance of an ordinary resistor is essentially fixed and does not change noticeably with environmental conditions. However, there is a special class of resistors—thermistors—whose resistance changes significantly with temperature. Thanks to this characteristic, thermistors are widely used in temperature measurement, temperature compensation, overcurrent protection, and many other applications, making them indispensable sensitive components in modern electronic devices. This article introduces the basic concept, classification, operating principles, and typical applications of thermistors.

II. What Is a Thermistor?

A thermistor is a temperature-sensitive resistor whose resistance changes significantly with temperature. The English name "Thermistor" is short for "Thermal Resistor."

Thermistors are typically made from semiconductor ceramic materials, with common materials including metal oxides such as manganese, cobalt, nickel, and iron. Compared with ordinary metal resistors (such as copper and platinum), thermistors have a much larger temperature coefficient, making them extremely sensitive to temperature changes—even a tiny temperature fluctuation can cause a noticeable change in resistance.

Common thermistor forms include bead, chip, disk, and rod shapes, with a variety of packaging options to meet the installation and measurement needs of different scenarios.

III. Classification of Thermistors

Depending on the direction in which resistance changes with temperature, thermistors are mainly divided into two types:

3.1 Negative Temperature Coefficient (NTC) Thermistors

The resistance of an NTC (Negative Temperature Coefficient) thermistor decreases as temperature rises. This is the most widely used type of thermistor, commonly found in temperature measurement and temperature compensation circuits.

3.2 Positive Temperature Coefficient (PTC) Thermistors

The resistance of a PTC (Positive Temperature Coefficient) thermistor increases as temperature rises. Depending on the mechanism, PTC thermistors can be further divided into ceramic PTC (such as barium titanate) and silicon PTC, commonly used in overcurrent protection and self-resetting fuses.

IV. Operating Principle

4.1 Operating Principle of NTC Thermistors

NTC thermistors are made from semiconductor ceramic materials. In a semiconductor, the concentration of charge carriers (electrons and holes) increases with temperature. As temperature rises, more electrons gain enough energy to jump to the conduction band, the number of carriers participating in conduction increases, the material's conductivity improves, and therefore the resistance decreases.

In simple terms, the higher the temperature, the more "free electrons" are available to carry current inside the material, so the resistance becomes smaller. Conversely, as temperature drops, the carriers become fewer and the resistance rises. This is the fundamental reason why NTC thermistors exhibit a negative temperature coefficient. The resistance-temperature relationship is nonlinear—it changes slowly at low temperatures and more rapidly at high temperatures—so high-precision measurement applications usually require linearization or lookup-table compensation.

4.2 Operating Principle of PTC Thermistors

The resistance of a PTC thermistor increases as temperature rises, and the mechanism varies by material:

• Ceramic PTC (such as barium titanate): Near the Curie temperature, the material undergoes a phase transition, and the grain-boundary barrier rises sharply, causing the resistance to surge by several orders of magnitude within a narrow temperature range. This characteristic makes it highly suitable for overcurrent protection and self-resetting fuses.

• Silicon PTC: Using semiconductor doping characteristics, the carrier concentration decreases as temperature rises, thereby achieving a positive temperature coefficient.

4.3 Self-Heating Effect

It should be noted that when a thermistor operates, the current flowing through it generates heat (Joule heating), causing its own temperature to be higher than the ambient temperature—a phenomenon known as the self-heating effect. In temperature measurement applications, the measurement current usually needs to be limited to reduce self-heating error; in overcurrent protection applications, however, the self-heating effect is precisely exploited to raise the component's temperature and change its resistance to achieve protection.

V. Typical Applications

• Temperature measurement: NTC thermistors are widely used in electronic thermometers, appliance temperature control, automotive temperature sensors, and more.

• Temperature compensation: Compensates for parameter drift in other circuit components caused by temperature changes, improving circuit stability.

• Overcurrent protection: PTC thermistors serve as self-resetting fuses—when current is excessive and heat builds up, resistance rises sharply, limiting the current, and automatically recovers after the fault is cleared.

• Inrush current suppression: Power-type NTC thermistors are connected in series at the power supply input; their resistance is relatively high when cold, suppressing the inrush current at startup, and drops after heating up during operation, reducing power consumption under normal conditions.

• Temperature control and heating: PTC heating elements leverage their constant-temperature characteristics and are widely used in fan heaters, electric water heaters, and other products.

VI. Selection and Usage Considerations

1. Clarify the purpose: Whether for measurement, compensation, or protection, different applications have different requirements for accuracy, response speed, and power rating.

2. Pay attention to key parameters: Nominal resistance (at 25°C), B value, accuracy, operating temperature range, maximum power, etc.

3. Reduce self-heating error: When measuring temperature, control the current flowing through the thermistor to avoid self-heating affecting measurement accuracy.

4. Note the nonlinearity: The resistance-temperature relationship of a thermistor is nonlinear; for high-precision measurement, linearization or lookup-table compensation is required.

VII. Conclusion

Thanks to their sensitive response to temperature, thermistors play an important role in measurement, compensation, and protection. NTC and PTC thermistors have different mechanisms and different uses: the former excels at temperature measurement and compensation, while the latter is well suited to overcurrent protection and constant-temperature heating. Understanding their operating principles and characteristics helps in proper selection and correct use during circuit design, thereby improving system reliability and accuracy. With the rapid development of fields such as the Internet of Things, smart appliances, and new energy vehicles, thermistors will have even broader application prospects.