Types Of Thermistors And Its Applications

One of the key challenges in electronic circuits is temperature management. Each component has a safe operating limit, and if a failure to monitor and regulate these temperatures occurs, it can lead to malfunction and reduced performance or even damage.

Here is where a thermistor helps, by providing an efficient and affordable solution. These compact sensors offer precise temperature measurements by detecting changes in resistance, making them ideal for maintaining optimal conditions.

Now that we’ve introduced thermistors, let’s take a closer look at what they are and how they work. In the following sections, we’ll explore the different types of thermistors and the advantages they bring to various applications.

So What Exactly Is A Thermistor?

A thermistor is a type of resistor whose resistance varies significantly with temperature. Derived from the words “thermal” and “resistor,” a thermistor provides a predictable change in resistance as its temperature changes, making it highly valuable in temperature sensing and control applications.

Thermistors come in two main types: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). NTC thermistors exhibit a decrease in resistance as temperature rises, while PTC thermistors increase resistance as temperature goes up. By leveraging these properties, thermistors provide an efficient, cost-effective solution for measuring temperature changes across various industries, from consumer electronics to automotive systems.

Working Principle

The thermistor functions are based on the principle of temperature-dependent resistance. When it comes to NTC thermistors, an increase in temperature creates more electrons allowing them to move freely, and reduces resistance. This allows the NTC thermistor to be sensitive and accurate in detecting minor changes.

In contrast, a PTC thermistor experiences a rise in resistance as temperature climbs. This occurs because, beyond a specific temperature, the material in a PTC thermistor undergoes structural changes, restricting the movement of charge carriers. Both types of thermistors exhibit precise, predictable resistance-temperature curves, making them reliable tools for temperature monitoring and control.

Application And Uses Of Thermorisitors

The versatility of thermistors allows their use across numerous fields. In industrial settings, thermistors are critical in temperature measurement and control systems, where accuracy and fast response times are essential. For household appliances, such as refrigerators and air conditioners, thermistors help regulate temperature and optimize energ

Automotive systems also benefit from thermistors, especially in engine management, where monitoring engine temperature ensures optimal performance and safety. Medical devices rely on thermistors to maintain patient safety, particularly in devices such as incubators and dialysis machines, where constant temperature control is critical. In consumer electronics, thermistors protect against overheating, safeguarding sensitive components from damage.

Types Of Thermistors

Based On Temperature

Negative Temperature Coefficient (NTC) Thermistor

NTC thermistors experience a decrease in resistance as temperature increases, offering a highly sensitive solution for temperature measurement. The resistance-temperature relationship for an NTC thermistor is non-linear, with resistance decreasing exponentially as heat rises. This steep curve makes NTC thermistors ideal for applications requiring precise temperature tracking. The temperature coefficient of resistance typically falls between -3% and -6% per degree Celsius. Various factors, including material composition, can influence the exact behavior of the thermistor, but the general trend remains the same—more heat, and less resistance.

Advantages Of NTC Thermistor

• High sensitivity detects small temperature changes due to the steep resistance-temperature curve.
• The fast response time reacts quickly to temperature fluctuations, making it useful for real-time monitoring.
• Has a wide operating temperature, and can function from -55°C to +300°C.

Disadvantages Of NTC Thermistor

• The non-linear resistance-temperature relationship requires compensation circuitry for accurate readings over a broad range.
• It degrades in high temperatures, and prolonged exposure to high heat can reduce performance and longevity.

NTC Applications

NTC thermistors are commonly used in a variety of industries, such as automotive temperature sensing (engine monitoring), HVAC systems, and household appliances like ovens and refrigerators. They are also found in battery management systems for monitoring battery temperature in devices like smartphones, electric vehicles, and medical equipment. Their sensitivity to even small temperature changes makes them ideal for applications requiring real-time temperature control and precision.

Positive Temprature Coefficient (PTC) Thermistor

Unlike NTC thermistors, PTC thermistors see their resistance rise sharply once a critical temperature, known as the Curie point, is reached. Before this threshold, the resistance increases gradually with temperature. After the Curie point, which can vary depending on the thermistor type, resistance can increase rapidly, sometimes by several orders of magnitude. This characteristic makes PTC thermistors ideal for specific protective applications.

Advantages Of PTC Thermistor

• Is self-regulating and can act as both a sensor and protective device, limiting current flow at a critical temperature.
• Protects from overconsumption of current by increasing resistance when overcurrent situations.
• No additional components are required and it has a simple integration into circuits for temperature protection.

Disadvantages Of PTC Thermistor

• Reacts more slowly to temperature changes compared to NTC thermistors
• Less precise temperature sensing, as the sharp increase in resistance, makes detailed temperature feedback challenging.
• A limited and low-temperature performance.

PTC Applications

PTC thermistors are widely used for overcurrent protection in electrical circuits, acting as resettable fuses. They also find applications in self-regulating heating elements, where the thermistor heats up until its resistance increases enough to limit further current flow, effectively regulating the heat output. Other common uses include motor protection in industrial equipment and household appliances like washing machines, dishwashers, and refrigerators.

Based On Material

Bead thermistor

Known for their small size, these thermistors stand out with a size of 0.15mm to 1mm, making them perfect to use in applications where space is concerned. Typically made from a mixture of metal oxides, such thermistors exhibit excellent sensitivity to temperature and are often encased in a glass or epoxy resin coating, which provides protection and durability in various environmental conditions.

This type of thermistor is commonly used in medical devices to monitor body temperature because of its ability to perform precise readings. A compact size also allows for a faster response time ranging from a few milliseconds to a few seconds. With a simple yet robust construction, bead thermistors are found in research equipment, HVAC systems, and other electronic devices.

Disc And Chip Style Thermistors

Disc thermistors use temperature sensing and control that ranges in diameter from 3mm to 25mm they come in a flat and disc shape which is typically constructed with metal oxides. Their design allows for stable performance across broad temperatures between -50℃ to +150℃. And are commonly found in appliances like fridges, where temperature monitoring is essential.

On the other hand, chip thermistors are more compact and often find their place in circuit boards and electronic devices. You can expect an uncomprimized performance despite its size offering an accurate temperature reading with a tolerance as low as ±1%.  Both disc and chip thermistors are known for their long-term stability, making them ideal for industries that require constant temperature measurements.

Cylindrical Thermistor

Characterized by a tubular design, the cylindrical thermistor ranges from 1 to 5mm in diameter. It offers a robust solution where mechanical stress is common and the cylindrical shape helps in easy mounting in various configurations. They’re often found in automotive systems where temperature monitoring is critical for safety and performance.

The construction that involves wrapping a resistive material around a cylindrical core, results in excellent thermal conductivity. Ensuring their ability to handle high temperatures which can extend from -40℃ to +250℃ and their vibration resistance makes them preferred for industries. Cylindrical thermistors are also employed in aerospace systems and heavy machinery, where reliability and durability are paramount.

Metalized Surface Thermistor

Metalized surface thermistors are distinct due to their specialized metal coating that enhances conductivity and strength. This surface allows for improved heat transfer with temperatures like other thermistors, this too is used in environments where accurate and rapid temperature sensing is required and the metal coating also provides a level of protection against environmental factors such as moisture and dust.

Often found in laboratory equipment where thermal management is essential. In addition to their accuracy, the metalized thermosisters are also valued for their stability across wide temperatures ranging from -55°C to +200°C. Their ability to perform consistently even in challenging conditions, makes them an attractive option for various industrial processes.

Glass Encapsulated Thermistors

Designed for harsh environments, the glass-encapsulated thermistors protect against moisture chemicals, and physical damage. Encased in a glass sheath, they offer superior protection compared to other types making them ideal to use in temperatures as high as 300°C. One of the key benefits of glass encapsulations is their resistance to thermal shock, allowing the thermistors to maintain accurate readings even with temperature fluctuations.

This durability makes glass-encapsulated thermistors perfect for use in oil and gas industries, where consistent performance is required under extreme conditions. In addition to their ruggedness, these thermistors also offer high precision and stability, ensuring reliable temperature measurements over time.

PAN Thermistor

Positive Anisotropic Network thermistors, use a specialized construction that involves an anisotropic network of resistive materials. The anisotropic structure allows for temperature sensing in specific directions, providing more targeted and efficient thermal measurements. PAN thermistors are commonly used in directional temperature monitoring, such as in aerospace and engineering systems.  The design enhances sensitivity while reducing the overall size.

Additionally, their unique ability to focus on temperature changes in particular areas makes them stand out from other thermistor types. In addition to their precision, they have a high-speed response of 1 to 5 seconds. Ensuring real-time temperature monitoring in fast-paced environments.

Precision Interchangeable Thermistors

These interchangeable components allow you to swap one thermistor out with another without needing to recalibrate the entire system and streamline the entire process. These thermistors are often used in medical instruments, laboratory equipment, and high-precision electronics, where exact temperature readings are critical.

The production of precision interchangeable thermistors involves strict manufacturing controls to ensure that each unit performs within a narrow tolerance range which goes as low as ±0.1°C, and the ability to maintain accuracy over a range of temperatures from -80°C to +150°C, further enhancing their dependability. By eliminating the need for recalibration, precision interchangeable thermistors are affordable and have increased efficiency in complex temperature monitoring systems.

Polymeric Thermistors

Polymeric thermistors are designed using conductive polymers, which makes them both flexible and durable. Unlike standard thermistors made from ceramic or metal, polymer variants excel in applications that require bending or stretching. This adaptability makes them highly suitable for wearable technology and flexible electronics. These thermistors respond to temperature changes by adjusting their resistance, which provides accurate, real-time, temperature readings.

Common uses include smart fabrics, health monitoring devices, and other flexible systems. Operating between -20°C to +85°C, polymeric thermistors typically have fast response times of 0.5 to 2 seconds. Their lightweight nature, combined with corrosion resistance, further enhances their reliability in diverse environments.

Ceramic Thermistors

Ceramic thermistors are well regarded for their ability to operate in high-temperature environments with impressive stability. Made with ceramic materials like meral oxic=des. These thermistors provide excellent thermal conductivity, which ensures accurate temperature sensing even in extreme conditions. They are used in kilns, power plants, and automotive systems, where both high and low temperatures need to be monitored.

Ceramic thermistors can maintain consistent temperature over ranges, typically from -100°C to +300°C. They’re suitable for precision tasks that require accurate readings in harsh conditions, combined with a quick response time of 2 to 10 seconds, ensuring they are both, efficient and resistant, also their material makes them resistant to physical wear.

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Conclusion

After reading our article, you may have gained insight into the huge impact that thermistors have on both industrial and electronic applications, thanks to their ability to respond to temperature changes. Whether you need an NTC thermistor for precision or a PTC thermistor for safety, understanding what each type offers is the key to choosing the right one and better managing the electronic circuit in temperature management.

By factoring in features like temperature range, accuracy, and environmental conditions, you can ensure the chosen thermistor meets the needs of your application and provides accurate temperature data making them a valuable component of modern technology.