Thermistors are specialised resistors that can detect and respond to changes in temperature. As the temperature changes, so does the resistance of the thermistor, allowing it to accurately measure temperature in a wide variety of applications.

The first thermistors were invented in the early 20th century by Samuel Ruben and Sidney O. Greenberg at Bell Labs, who discovered that certain metal oxide materials exhibited a significant change in electrical resistance when subjected to temperature changes. Initially used in radio and radar equipment for temperature compensation and stabilisation, thermistors quickly found their way into industrial applications such as thermostats and temperature control systems in the 1940s. Over time, improvements in materials science and microfabrication have led to significant advances in thermistor technology, making them increasingly popular in applications such as self-regulating heaters and electronic circuits. Today, thermistors are widely used across a range of industries, from healthcare and consumer electronics to automotive and aerospace, due to their accuracy, sensitivity, and versatility.

This guide covers the topic as below:

  • Types of thermistors
  • Operating principles of thermistors
  • Steinhart-Hart equation
  • Advantages and disadvantages of thermistors
  • Characteristics of thermistor
  • Applications of thermistor

Types of thermistors

Thermistors can be classified into two types based on the direction of the resistance change with temperature: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC) thermistors.

  • Negative temperature coefficient (NTC): Negative Temperature Coefficient (NTC) Thermistors are designed to have a negative temperature coefficient, meaning that their electrical resistance decreases as temperature increases. NTC thermistors are made of semiconducting materials, such as metal oxides, which are highly sensitive to changes in temperature. These thermistors are widely used in temperature sensing and control applications, such as in thermostats, refrigerators, and ovens. Due to their high sensitivity and fast response time, they are particularly useful for applications that require monitoring rapid temperature changes. Furthermore, NTC thermistors can serve as self-heating components in temperature compensation circuits.
  • Positive temperature coefficient (PTC) thermistors: PTC thermistors are made of ceramic materials with a high Curie temperature, which means their electrical conductivity changes as the temperature changes. These thermistors have a positive temperature coefficient, which means their resistance increases as the temperature increases. PTC thermistors are commonly used in applications where overcurrent protection is required, such as in circuits that drive motors or other high-power devices. When the current through the PTC thermistor exceeds a certain level, its resistance increases dramatically, limiting the current flow and preventing damage to the circuit. PTC thermistors can also be used in self-regulating heaters, where the resistance of the thermistor increases as the temperature rises, reducing the amount of power consumed by the heater. However, PTC thermistors have lower sensitivity to temperature changes compared to NTC thermistors, so they are not as commonly used in temperature sensing applications.
NTC and PTC thermistor
Figure 1: NTC and PTC thermistor

To complement the two main types of thermistors, there are also hybrid thermistors that combine the properties of NTC and PTC thermistors. These hybrid thermistors have a dual response to temperature, with a negative temperature coefficient at lower temperatures and a positive temperature coefficient at higher temperatures. They are particularly useful in applications where the temperature range is large, and precise control of temperature is essential. For instance, they can be used in HVAC systems, electronic circuits, and automotive applications where temperature fluctuations are common. The hybrid thermistors offer an optimal solution by providing accurate temperature sensing and control across a broad range of temperatures.

Operating principles of thermistors

NTC thermistors are composed of semiconducting materials, particularly metal oxides, which exhibit high sensitivity to temperature changes. The number of charge carriers in the material increases with temperature, resulting in a decrease in resistance. This behavior is known as negative temperature coefficient (NTC), as the resistance decreases with increasing temperature. In contrast, PTC thermistors are composed of ceramic materials, such as barium titanate, which have a high Curie temperature. At low temperatures, the material acts as an insulator, but above the Curie temperature, it becomes a conductor. As the temperature increases, the material approaches its Curie temperature, causing an increase in resistance. This relationship between temperature and resistance is known as the positive temperature coefficient (PTC).

Steinhart-Hart equation

The Steinhart-Hart equation is a useful mathematical tool for accurately measuring temperature using a thermistor. Since the relationship between the resistance of the thermistor and the temperature of the environment is non-linear, a linear relationship cannot be used to calculate the temperature. However, the Steinhart-Hart equation can accurately relate the resistance of the thermistor to the temperature by considering the non-linear relationship.

The Steinhart-Hart equation is represented by the equation 1/T = a + b(ln R) + c(ln R)^3, where T is the absolute temperature, R is the resistance of the thermistor, and a, b, and c are constants specific to the thermistor being used. To determine the constants, the resistance of the thermistor is measured at various known temperatures, and then the data is fitted to the Steinhart-Hart equation using curve-fitting techniques.

After determining the constants, the Steinhart-Hart equation can be used to calculate the temperature of the environment based on the resistance of the thermistor. This is typically done using a microcontroller or other electronic device that is programmed to perform the necessary calculations.

The Steinhart-Hart equation is particularly useful in applications where high accuracy and precision are required, such as in industrial, scientific, and medical settings. By accurately measuring temperature over a wide range, it can help ensure the safety and effectiveness of various processes and experiments.

Advantages and disadvantages of thermistor

Thermistors are widely used in various industries for their high sensitivity, fast response time, small size, wide temperature range, and low cost. Their high sensitivity to temperature changes makes them ideal for accurate temperature measurement, whilst their fast response time is particularly useful in applications that require rapid temperature monitoring. Their small size makes them suitable for applications where space is limited, and they can measure temperature across a wide range of temperatures, making them versatile and useful in many different applications. Moreover, thermistors are relatively inexpensive compared to other types of temperature sensors, making them a cost-effective option.

However, thermistors do have some disadvantages. Their non-linear response to temperature changes can make calibration and measurement more challenging than other types of temperature sensors, such as RTDs and thermocouples. Thermistors also have limited accuracy compared to these types of sensors. Additionally, some types of thermistors have a limited temperature range, which may restrict their use in certain applications.

Despite these limitations, thermistors remain a popular choice for many temperatures measurement and control applications due to their versatility, reliability, and affordability. The advantages of thermistors outweigh their limitations for many applications, making them a valuable tool in industries such as automotive, aerospace, medical, and consumer electronics.

Characteristics of thermistors

Thermistors are temperature-sensitive resistors that exhibit specific characteristics that make them useful in a wide range of applications. Understanding the characteristics of thermistors is important in order to select the appropriate thermistor for a specific application and to ensure accurate and reliable operation.

Temperature coefficient of resistance (TCR)

The temperature coefficient of resistance (TCR) measures how the resistance of a thermistor changes in response to temperature changes. TCR is typically expressed as the percentage change in resistance per degree Celsius (°C).

NTC thermistors have a negative TCR, meaning that their resistance decreases as the temperature increases. The TCR of NTC thermistors usually ranges from -2% to -6% per °C. PTC thermistors have a positive TCR, meaning that their resistance increases as the temperature increases. The TCR of PTC thermistors typically ranges from 0.3% to 0.6% per °C.

It's important to note that the TCR of a thermistor isn't constant over its operating range. Instead, it varies with temperature and is highest at lower temperatures. This means that the rate of change in resistance with temperature is faster at lower temperatures and slower at higher temperatures.

To calculate the temperature of the environment in which the thermistor is placed, the TCR can be used. By measuring the resistance of the thermistor at two different temperatures, the TCR can be calculated using the equation:

TCR = (ln(R2/R1))/(T2 - T1)

where R1 and R2 are the resistances of the thermistor at temperatures T1 and T2, respectively. Once the TCR is known, the temperature of the environment can be calculated based on the resistance of the thermistor using the equation:

T = (R - R0)/(R0 x TCR)

where R is the resistance of the thermistor at the temperature of interest, R0 is the resistance of the thermistor at a known reference temperature, and TCR is the temperature coefficient of resistance.

Sensitivity

Thermistor sensitivity refers to the extent of its resistance changes in response to a given temperature change, and it is usually expressed as a change in resistance per degree Celsius (°C). The sensitivity is closely related to the temperature coefficient of resistance (TCR) of the thermistor, which varies with its material composition, size, and shape. NTC (Negative Temperature Coefficient) thermistors are generally more sensitive than PTC (Positive Temperature Coefficient) thermistors because they have a higher TCR. In addition, smaller thermistors are more sensitive than larger ones because they have less mass and respond faster to temperature changes. Thermistors made from materials with a higher TCR, such as semiconductors, are also more sensitive than those made from materials with a lower TCR, such as ceramics. A calibration curve can be used to characterise the sensitivity of a thermistor, relating its resistance to temperature over a range of values. This curve can be created by measuring the thermistor's resistance at various known temperatures and plotting the results. The sensitivity of the thermistor can then be calculated by determining the slope of the curve, which represents the change in resistance per degree Celsius of temperature change.

Stability

The stability of a thermistor is crucial for its reliability in temperature sensing and feedback control systems. Factors that affect stability include material composition, manufacturing process, and operating environment. High-quality thermistors made from precise techniques tend to be more stable than those made from lower-quality materials or less precise methods. Environmental factors, such as temperature cycling, mechanical stress, and moisture exposure, can also impact thermistor stability, leading to measurement errors and reduced accuracy over time.

To maintain thermistor stability, it's essential to choose a high-quality thermistor designed for the specific application and operating conditions. Proper installation and protection from environmental contaminants and mechanical stress are also necessary. Regular calibration of the thermistor is essential to ensure accurate temperature measurements. Calibration involves comparing the thermistor's resistance-temperature relationship to a known reference standard and making any necessary adjustments.

In some applications, temperature compensation may also be necessary to account for any drift or variation in the thermistor's resistance-temperature relationship over time. This can be achieved by using an external reference sensor or by implementing software-based compensation algorithms. In summary, stability is a vital characteristic to consider when selecting and using thermistors in temperature sensing and feedback control systems, and proper maintenance and calibration are essential for accurate and reliable performance over time.

Non-Linearity

Thermistors exhibit a non-linear relationship between resistance and temperature. This means that their resistance changes at different rates depending upon the temperature, which can cause measurement errors and reduced accuracy in applications where precise temperature sensing is required. The non-linearity of a thermistor is determined by its material properties and the physics of how it works. The Steinhart-Hart equation describes the non-linear relationship between resistance and temperature, which can be characterised by a "beta" value that changes with temperature. In practical applications, the non-linearity of a thermistor can be a significant source of error. To compensate for this, it's important to calibrate the thermistor using a known reference standard and apply appropriate compensation methods. One common compensation method is to create a lookup table that relates the thermistor's resistance to its corresponding temperature. This table can be created by measuring the thermistor's resistance at various known temperatures and plotting the results. Another method is to use software algorithms that model the thermistor's non-linear resistance-temperature relationship and apply correction factors to improve the accuracy of temperature measurements. By using these compensation methods, the effects of non-linearity can be mitigated, resulting in more accurate and reliable temperature measurements.

Response Time

The response time of a thermistor is an important characteristic that refers to the duration taken by the device to detect a change in temperature. Rapid temperature changes or precise temperature control applications require a thermistor with a fast response time. Several factors influence the response time of a thermistor, including its size, shape, thermal mass, and operating environment. Generally, smaller thermistors with lower thermal mass respond more quickly to temperature changes than larger devices with higher thermal mass. Also, thermistors in direct contact with the material being measured tend to have faster response times than those located further away. The response time of a thermistor can also be influenced by its material composition. Different materials have varying thermal conductivities and heat capacities that can affect their response time. Ceramic thermistors, for instance, may have slower response times than those made from metals or other materials. To improve the response time of a thermistor, it is essential to minimise its thermal mass and ensure that it is in direct contact with the material being measured. This can be accomplished by using small, compact thermistors and mounting them directly onto the surface of the material being measured.

Applications of thermistor

Thermistors have a wide range of applications across a variety of industries, due to their sensitivity, accuracy, and ease of use. Some common applications of thermistors include:

  • Temperature measurement: Thermistors are commonly used for temperature measurement in a variety of applications, including environmental monitoring, HVAC systems, medical devices, and industrial processes. They can provide accurate and reliable temperature readings over a wide range of temperatures, making them ideal for many different types of applications.
  • Temperature control: In addition to measuring temperature, thermistors can also be used for temperature control. By monitoring temperature and sending signals to a control system, thermistors can help regulate temperature in a variety of systems, including refrigeration units, ovens, and HVAC systems.
  • Overheat protection: Thermistors can be used as overheat protection devices in a variety of applications, including electronic devices, motors, and power supplies. By monitoring temperature and triggering a shutdown or alarm when temperatures exceed a certain threshold, thermistors can help prevent damage or accidents due to overheating.
  • Liquid level detection: Thermistors can also be used to detect liquid levels in tanks and other containers. By measuring the temperature difference between the liquid and the surrounding air, thermistors can determine the level of liquid in the container.
  • Battery management: Thermistors are often used in battery management systems to monitor battery temperature and prevent overcharging or overheating. By monitoring temperature and providing feedback to a control system, thermistors can help extend the life of batteries and improve their overall performance.
  • Automotive applications: Thermistors are commonly used in automotive applications, including engine temperature monitoring, cabin temperature control, and battery management. They are also used in airbag systems, tire pressure monitoring systems, and other safety systems.

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