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Purpose and Function of Thermistors

Thermistors are special parts that change with temperature. They are made of metallic oxides molded into different shapes. These parts are very important because they help measure temperatures very accurately and don’t cost much.

There are mainly two kinds: NTC, which measures temperature, and PTC, used for keeping circuits safe. The Steinhart-Hart equation helps turn thermistor resistance into exact temperatures.

This makes them great for many uses like checking if there’s an arc flash, protecting motors, watching voltages, and managing batteries in cars and household devices. Thermistors can be sealed tightly, bolted on, or stuck on surfaces to fit various needs.

Their ability to repeat measurements reliably and work well within certain temperature ranges is crucial for controlled systems where precise temperature control is needed. You’ll find thermistors in things like digital thermometers, cars, and labs where they do much more than just measure heat; they also control heating or cooling based on the temperatures they detect.

Let’s explore how these tiny but mighty parts make a big difference.

Defining Thermistors

Moving from the introduction, we now turn our attention to thermistors. Thermistors are special types of resistors whose resistance changes with temperature. They are semiconducting devices made from metallic oxides.

These components show a significant change in resistance for every small change in temperature.

Thermistors provide accurate temperature readings due to their high sensitivity. Their resistance decreases as the temperature increases if they are Negative Temperature Coefficient (NTC) thermistors or increases with temperature in the case of Positive Temperature Coefficient (PTC) thermistors.

This unique property makes them invaluable for measuring and controlling temperatures in various applications.

Composition of Thermistors

Thermistors are made of metallic oxides, binders, and stabilisers. Manufacturers press these materials into wafers using precise ratios. This method ensures that each thermistor has specific electrical traits.

The choice of metallic oxide impacts the thermistor’s resistance and temperature sensitivity.

Besides their composition, thermists come in various shapes like beads, disks, or cylinders. After shaping, a protective layer of epoxy or glass encloses them. This protection is crucial for preventing damage and corrosion when used in different environments.

Next up: Types of Thermistors explain the diversity within this technology.

Types of Thermistors

Thermistors come in two main varieties, each with unique responses to temperature changes. Explore more to understand how they function differently.

NTC (Negative Temperature Coefficient)

NTC thermistors work in a unique way. Their resistance decreases as the temperature goes up. This is because they are made from metallic oxides mixed with binders and stabilisers. These ingredients are pressed into discs or wafers.

The exact mix of materials affects how the resistance changes with temperature.

These components offer high precision in measuring temperatures due to their significant temperature coefficient of resistance. They’re essential for devices needing accurate temperature control and monitoring, making them a top choice for various applications where precise data is crucial.

PTC (Positive Temperature Coefficient)

Shifting from NTC thermistors with their unique ability to decrease resistance as temperature increases, we now dive into the world of PTC thermistors. These components possess a positive temperature coefficient, which means their resistance rises when the temperature goes up.

This characteristic makes them ideal for overcurrent protection in various electrical circuits.

PTC thermistors are crafted using metallic oxides mixed with binders and stabilisers. The combination is then pressed into thin wafers. By adjusting the ratios of these composite materials accurately, manufacturers can control the resistance/temperature curve of the thermistor precisely.

This process ensures that PTC devices offer reliable performance in safety-critical applications such as preventing thermal runaway and acting as resettable fuses in electronic gadgets.

Key Functions and Roles of Thermistors

Thermistors play a crucial role in various safety and monitoring systems, including arc flash detection and motor protection. These devices offer precise temperature measurement, which is essential for the effective functioning of electrical equipment.

Their accuracy and cost-effectiveness stem from their high resistance change with temperature variations, making them ideal for applications demanding high resolution and repeatability.

Manufacturers design thermistors to cater to different thermal ranges, providing options suitable for both low and high-temperature usage. This versatility allows for their integration into diverse settings such as voltage monitoring relays and pump protection systems.

With a range of base resistances and temperature response curves available, thermistors serve as adaptable components in creating efficient thermal management solutions across multiple industries.

Understanding the Operation of Thermistors

Moving from their key roles, we explore how thermistors work. These components change resistance with temperature changes. This property allows them to control or measure heat. NTC thermistors decrease resistance as temperature increases.

PTCs do the opposite, increasing resistance with rising temperatures.

This operation makes thermistors invaluable in circuits and devices needing temperature regulation or measurement. They react quickly to temperature shifts, ensuring systems function safely and efficiently.

Whether limiting inrush current in electric heaters or monitoring battery conditions, thermistors adjust their resistance to maintain optimal performance across various applications.

Thermistors in Controlled Systems

Thermistors play a critical role in controlled systems by adjusting resistance based on temperature changes. Their ability to provide accurate feedback helps maintain the desired operation levels in various devices.

Resistance and Bias Current

Resistance in thermistors changes with temperature. This property makes them ideal for temperature sensors. In controlled systems, a bias current flows through the thermistor. This current helps measure the change in resistance accurately.

Bias current’s value is chosen carefully. It must be enough to detect resistance changes but not so high that it causes heating in the thermistor. Heating can alter readings and make them less accurate.

Voltage Limits

After discussing resistance and bias current, it’s crucial to consider the voltage limits of thermistors in controlled systems. Class A thermistors can handle up to 30V while Class B types are limited to 15V.

These limits ensure the thermistor functions correctly without damage or inaccurate results. If a system exceeds these voltages, it could permanently harm the thermistor.

Matching the voltage requirements with the thermistor’s capacity is essential for safety and accuracy. Using voltage monitoring relays helps protect thermistors by keeping within specified limits.

Ignoring these guidelines risks losing functionality and compromising system integrity.

Thermistors Compared with Other Sensors

Thermistors offer unique properties when compared with other types of temperature sensors. The table below highlights key differences to provide clarity on the functionality and applicability of thermistors in comparison to other sensors.

Sensor TypeResistance Change/Temperature ChangeAccuracyStabilityTemperature RangeCost
ThermistorsHighHighHigh within limited rangeLimited rangeLow to Moderate
RTDs (Resistance Temperature Detectors)ModerateHighVery HighWideHigh
ThermocouplesLowLow to ModerateLow to ModerateVery WideLow

This comparison underlines the fact that thermistors have a higher resistance change per degree of temperature, offering greater resolution, repeatability, and stability within their operational temperature range. They are highly accurate and stable, making them ideal for applications where precise temperature measurement, control, or compensation is required. However, their utility is best within a limited temperature range, unlike RTDs and thermocouples which can operate over wider temperature ranges. The cost of thermistors is generally low to moderate, making them a cost-effective choice for many applications.

Moving on, let’s delve into the significance of the Steinhart-Hart equation in thermistor functionality.

The Steinhart-Hart Equation in Thermistor Functionality

The Steinhart-Hart equation plays a key role in making thermistors highly accurate for measuring temperatures. This mathematical formula converts the resistance of a thermistor into temperature readings.

Engineers and scientists value this tool because it provides precise data essential for controlling heating elements, managing thermal conductivity, and ensuring circuit protection across various applications.

Using the Steinhart-Hart coefficient, devices like refrigerators, freezers, and automotive sensors achieve optimal performance by closely monitoring temperatures. This equation is crucial not only for achieving accuracy but also for enabling technology to operate safely and efficiently in environments where temperature control is vital.

Practical Applications of Thermistors

Thermistors play a crucial role in various industries by monitoring and controlling temperatures. They ensure devices operate safely and efficiently in environments from cars to home appliances.

Automotive Sensors

Littelfuse produces a broad spectrum of sensor products for cars. These sensors keep an eye on passenger safety, comfort, and various car functions like powertrain, chassis, and emissions.

They are crucial for making sure the vehicle operates smoothly and efficiently.

These automotive sensors use changes in electrical resistance to monitor different conditions within the car. For example, temperature sensors detect heat levels in engines and cabins.

This helps prevent overheating or ensures that the interior is comfortable for passengers.

Battery Management

Battery management plays a crucial role in ensuring devices operate efficiently and safely. Thermistors, especially NTC and PTC types, are key components in this process. They monitor temperatures within batteries to prevent overheating and protect circuits from excessive current flow.

By accurately detecting temperature changes, thermistors help preserve battery life and safeguard against over-discharge. This is essential for maintaining the performance of many electronic devices.

Manufacturers like Littelfuse provide a broad selection of battery management solutions that make use of thermistors. These components not only contribute to more reliable battery operation but also aid in the conservation of battery power by facilitating efficient recharging protocols.

Their application extends across various sectors including automotive sensors, enhancing safety and durability in these technologies.

Temperature Sensors

Thermistors serve a vital role as temperature sensors in various devices, from digital thermometers to household appliances. Their ability to measure temperature accurately and cost-effectively makes them indispensable.

Unlike other sensors that might struggle with precision, thermistors excel due to their high resistance change per degree of temperature change. This feature provides greater resolution along with repeatable and stable readings.

Manufacturers prefer NTC thermistors for measuring temperatures because they are reliable and efficient. These thermistors help keep vehicles running smoothly and ensure household devices operate within safe temperatures.

By closely monitoring temperatures, they prevent overheating in sensitive electronics, making them crucial for battery management systems too. Moving forward, the discussion will shift towards how these components fit into controlled systems for optimal performance.


These devices serve as crucial components in a range of applications, from automotive to industrial settings. They offer precise temperature measurements and protect circuits, showcasing their versatility.

The understanding and use of thermistors highlight advancements in technology and engineering. Their role in improving safety, efficiency, and the reliability of various systems cannot be overstated.

This makes them invaluable tools in our continuous quest for innovation and improvement across multiple sectors.

For more information on how thermistors are utilised in complex systems, explore our detailed guide on air conditioning printed circuit boards.


1. What is a thermistor?

A thermistor is a type of resistor whose resistance changes with temperature. It’s used for sensing temperatures in various devices.

2. How do thermistors work?

Thermistors work by changing their resistance when the temperature around them changes. This allows them to measure heat or cold accurately.

3. Are there different types of thermistors?

Yes, there are two main types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors decrease in resistance as temperature rises, while PTC ones increase in resistance.

4. Can thermistors help control electric currents?

Indeed, they can! Thermistors act as inrush current limiters to protect circuits from sudden surges of electricity when devices turn on.

5. Where might I find a thermistor being used?

Thermistors are found in many places like immersion heaters, electric ovens, and even medical devices to monitor temperatures closely.

6. Why are some thermistors hermetically sealed?

They’re sealed tightly to prevent moisture from getting inside which could affect how well they sense temperature or even damage them.

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