What is a Thermistor? The Basics of How it Works

Thermistors, a contraction of the words "thermal" and "resistor," are specialized resistors whose resistance varies significantly with temperature. They are essential components in many thermal sensing and control applications due to their high sensitivity to temperature changes. Unlike standard resistors, thermistors are designed to have a predictable change in resistance when exposed to different temperatures.

This property makes them ideal for measuring, controlling, and compensating temperature in a wide range of applications, from household appliances to industrial machinery.

Published: December 14, 2023.

The fundamental principle behind a thermistor is the relationship between temperature and resistance. As the temperature changes, the resistance of the thermistor adjusts in a predictable manner, which can be measured and converted into a temperature reading.

This sensitivity to temperature changes allows for precise control and monitoring in various applications. Their small size, low cost, and reliability make thermistors a popular choice for temperature sensing.

Types of Thermistors: NTC Thermistors vs. PTC Thermistors

Thermistors are broadly categorized into two types based on their temperature coefficient:

• Negative Temperature Coefficient (NTC),
• Positive Temperature Coefficient (PTC) thermistors.

NTC thermistors decrease in resistance as temperature increases, which is useful for a wide range of temperature-sensing applications. Their high sensitivity in the common temperature range makes them ideal for environmental, medical, and automotive applications.

On the other hand, PTC thermistors behave oppositely, increasing their resistance with an increase in temperature. This characteristic makes them suitable for over-current protection and self-regulating heating elements, as they can act as current limiters or heating elements that stabilize at a certain temperature.

The distinct properties of NTC and PTC thermistors determine their suitability for different applications, making them versatile components in temperature-related fields.

NTC Thermistor and PTC Thermistor Symbols

 Symbols for NTC and PTC thermistors vary depending on different standards.

Generally, in electronic schematics, a thermistor is represented by a resistor symbol with a diagonal line through it, indicating its temperature sensitivity.

The NTC thermistor symbol has "-t°" added to emphasize that it is a Negative Temperature Coefficient thermistor.

The PTC thermistor symbol has "+t°" added to emphasize that it is a Positive Temperature Coefficient thermistor.

These symbols can differ slightly based on the standard used, such as IEEE, ANSI, JIS, or IEC.

Steinhart-Hart Equation Explained

The Steinhart-Hart Equation is a mathematical model used to calculate the resistance of a thermistor based on temperature.

It's a more accurate method compared to simpler models (for example, first-order approximation or Beta formula), as it considers the non-linear relationship between temperature and resistance.

1/T = A + B(ln R) + C(ln R)² + D(ln R)³ + E(ln R)⁴...

• T - temperature in Kelvins (Celsius + 273.15)
• R - resistance at T(°K), given in Ohms ()
• A, B, C, D, E - Steinhart-Hart coefficients for each thermistor, provided by the manufacturer, often abbreviated to A, B, and C, which shortens Steinhart-Hart equation to:

1/T = A + B(ln R) + C(ln R)²

The Steinhart-Hart equation generally offers an accuracy of approximately ±0.15 °C in the temperature range of -50°C to +150 °C, which suffices for most uses.

For applications requiring greater precision, narrowing the temperature range can enhance accuracy. For instance, achieving an accuracy of better than ±0.01 °C is possible within a 0°C to +100 °C range.

Working Principles of Thermistors: Temperature and Resistance

The working principle of thermistors revolves around the change in electrical resistance with varying temperatures.

In NTC thermistors, the resistance decreases as the temperature rises, which is attributed to an increase in charge carriers facilitating easier current flow.

PTC thermistors, conversely, show an increase in resistance with rising temperature, primarily due to the lattice structure of the material that impedes the flow of electrons. This predictable resistance change is key to their functionality and allows for the precise measurement and control of temperature in various devices.

Applications of Thermistors: From Industry to Everyday Use

Thermistors are ubiquitous in both industrial and everyday applications due to their precision and reliability.

In industrial settings, they are vital for temperature monitoring and control in processes like chemical manufacturing and metal refining.

In consumer electronics, they are found in devices like refrigerators, ovens, and air conditioners for temperature regulation.

NTC thermistors are commonly used in medical devices, such as electronic thermometers, due to their accuracy over a wide temperature range.

In the automotive industry, they help in monitoring engine and cabin temperatures, enhancing vehicle performance and safety.

Materials and Manufacturing: Inside Thermistor Technology

Thermistor technology is deeply rooted in the materials used for their construction.

NTC thermistors are typically made from semiconducting ceramic materials like manganese, nickel, cobalt oxides, which are sintered at high temperatures to create a compact, sensitive element.

PTC thermistors, on the other hand, are often made from polycrystalline ceramics containing barium titanate. These materials are key to the thermistor's characteristics, as they dictate the temperature-resistance relationship.

Manufacturing processes for thermistors are precise and sophisticated, ensuring the reliability and accuracy of these components in various temperature ranges.

Thermistor Configurations

Thermistors can be found in various shapes and configurations, depending on their intended use, material, and type.

• Ceramic Switching PTC Thermistors: These are used for over-current and over-temperature protection, characterized by a sharp increase in resistance at a specific temperature.
• Polymeric (PPTC) Thermistors: Made from a conductive polymer, these thermistors reset themselves after a fault condition, which is useful for circuit protection.
• Bolt-On or Washer Thermistors: Designed for surface temperature sensing, they are ideal for mounting on heat sinks or other surfaces.
• Hermetically Sealed Flexible Thermistors (HSTH): Encased to protect against moisture and contaminants, these are flexible for use in diverse environments.
• Surface-Mounted Thermistors: Compact and suitable for PCBs, these are used in electronic devices for temperature sensing.
• Glass Encapsulated Thermistors: Encased in glass for durability and moisture protection, suitable for harsh environments.

Choosing the Right Thermistor: Factors to Consider

Selecting the appropriate thermistor for a specific application involves considering several key factors.

The temperature range of the application is crucial; NTC thermistors are preferred for broader temperature ranges, while PTCs are suited for specific temperature points.

The desired sensitivity and accuracy are also important, as some applications require more precise temperature control than others.

Additionally, the physical size and response time are important considerations, especially in compact or fast-response systems. Considering these factors ensures the thermistor not only fits physically but also performs optimally in the intended application.

Thermistor vs. Thermocouple

Thermistors and thermocouples are both temperature-sensing devices, but they operate on different principles and are suited for different applications.

• Thermistors, known for their high sensitivity and accuracy within a limited temperature range, work on the principle of resistance change with temperature. They are ideal for precision temperature measurement in consumer electronics and medical devices.
• Thermocouples, on the other hand, generate a voltage in response to temperature differences and can measure a much wider temperature range. They are robust and suitable for high-temperature industrial environments.

The choice between a thermistor and a thermocouple depends on the required temperature range, accuracy, and application environment.

How to Test a Thermistor?

Testing a thermistor typically involves measuring its resistance and comparing it to a temperature-resistance chart specific to that thermistor.

You can use a multimeter to measure the resistance. First, disconnect the thermistor from any power source or circuit. Then, set the multimeter to the resistance measurement mode and connect its leads to the thermistor's terminals.

The resistance reading should correspond to the ambient temperature as per the thermistor's specifications. Additionally, for a more thorough test, you can vary the thermistor's temperature (like placing it in ice water or near a heat source) and observe the resistance change.

Consistent resistance changes in line with the specified values indicate a functioning thermistor.