This article takes an in-depth look at thermistors.
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A thermistor is a form of semiconductor that exhibits greater resistance than traditional conductors and is crafted to respond dynamically to temperature changes. The resistance of a thermistor shifts depending on the materials used in its construction. Typically, thermistors are composed of metal oxides, binders, and stabilizers, which are shaped into wafers and then cut into chips. The composition and proportions of these materials determine the thermistor's resistance attributes and how they react to temperature variations.
The word "thermistor" denotes temperature-sensitive resistors, recognized for their accuracy and efficiency in gauging temperature. There are primarily two kinds of thermistors: Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC). With NTC thermistors, resistance decreases as temperature rises, while with PTC thermistors, resistance increases with a rise in temperature.
As passive components, thermistors' resistance changes with temperature shifts within a system, offering a budget-friendly, accurate, and quick method for measuring temperature.
Thermistors are employed to monitor device temperatures, directly influencing equipment performance and used for temperature detection and overload protection. They are found in numerous circuits, components, and devices, providing a cost-effective temperature monitoring solution.
Thermistors are available in several designs, including widely used options like hermetically sealed, flexible (HSTH series), bolt-on, washer types, and self-adhesive surface-mount versions. HSTH thermistors are encased in a plastic polymer jacket that fully protects the sensing elements from moisture and corrosion, ensuring longevity and reliability.
Accurate temperature monitoring is vital in many manufacturing processes and industrial automation systems. The effectiveness of temperature control directly influences product quality, safety, and efficiency in diverse applications—from food processing plants to electronics manufacturing. Thermistors, a specific type of temperature sensor, play a key role by providing a consistent, predictable, and highly precise change in electrical resistance in response to temperature variations. This makes them a reliable component for continuous temperature measurement, essential for process control, equipment protection, and energy efficiency.
Thermistors are employed wherever precise temperature measurement, thermal management, and regulation are required—from heavy-duty industrial environments to everyday home appliances and consumer electronics. Their versatility and superior sensitivity make them a popular choice for engineers and designers choosing between various types of temperature sensors such as thermocouples, RTDs, and infrared sensors. A typical application of thermistors is in advanced HVAC systems, where they play a crucial role in managing climate control, air flow, and ambient temperature, ensuring optimal comfort and energy consumption.
Thermistor sensors operate effectively within a temperature range of 32°F to 212°F (0°C to 100°C), ideal for residential, commercial, and light industrial use. Class A thermistors provide the highest accuracy for demanding applications like laboratory equipment and medical devices, whereas Class B thermistors are suitable for general-purpose uses where ultra-precise measurements are not critical. Their inherent long-term stability ensures that thermistors maintain high accuracy over extended periods without significant drift, reducing maintenance costs and calibration frequency for businesses and manufacturers.
Key advantages of thermistors over other types of temperature sensors include:
As a result, thermistors remain a fundamental component not only in traditional temperature sensing but also in modern IoT (Internet of Things) and smart home technologies, where real-time monitoring and remote diagnostics are essential.
The central part of a modern thermostat is a highly sensitive NTC or PTC thermistor, providing fast and accurate temperature feedback. The temperature control circuit in an HVAC system consists of electronic components like an operational amplifier, thermistor, and a relay, with the thermistor being the primary temperature sensor in the circuit. Thermistors enable smart thermostats to learn user preferences, optimize heating and cooling, and integrate with home automation systems, making them an integral part of energy management in residential and commercial buildings.
In automobiles, thermistors are commonly used to monitor the temperatures of oil and coolant, ensuring reliable engine performance. They play a critical role in advancing automotive safety—alerting the driver if the vehicle is overheating and preventing costly engine damage. Thanks to durable and robust sensor design, thermistors are also integrated into transmission systems, air intake sensors, climate control units, and battery management systems in electric vehicles (EVs). Thermistors are directly linked to dashboard instruments and electronic control units (ECUs), providing real-time, accurate temperature data essential for overall vehicle performance and fuel efficiency.
Modern microwaves are equipped with thermistors to monitor and regulate their internal temperature precisely, enabling advanced thermal protection during cooking cycles. These thermistors support safety protocols by sensing potential overheating, reducing fire risk, and prolonging the appliance's lifespan. The integration of thermistors allows manufacturers to add energy efficiency and advanced fault detection features, meeting regulatory safety standards and user expectations for smart kitchen appliances.
During battery recharging—whether for consumer electronics, power tools, or electric vehicles—excess heat must be carefully managed. Battery chargers and battery management systems (BMS) often incorporate a low-resistance NTC thermistor to actively monitor cell temperature. If the battery temperature rises excessively, the thermistor rapidly triggers a cutoff, halting the charging process to prevent overheating, fire hazards, and potential battery damage. Battery thermistors are essential for the safety, efficiency, and longevity of lithium-ion, nickel-metal hydride, and lead-acid batteries across multiple industries, including renewable energy, robotics, and portable medical devices.
As smartphones and mobile devices become increasingly compact and powerful, the risk of component overheating grows. Thermistors are used to continuously monitor internal temperature, transmitting real-time data to the phone’s integrated circuit (IC). By detecting heat, thermistors help maintain optimal operating temperatures and ensure the phone’s microprocessors and batteries function safely and efficiently. Advanced mobile devices rely on thermistors for thermal throttling, battery protection, and system diagnostics, directly influencing user experience, device longevity, and overall product reliability.
In modern washing machines, a thermistor’s role is to monitor and confirm the optimal temperature for effective washing and rinsing cycles. This sensor helps control water heating elements, ensuring that garments are cleaned at the correct temperature for specific fabric types. If a heating error or irregularity appears on the machine’s display, it may signal an issue with the thermistor or its electrical connections. By ensuring proper water temperature, thermistors not only improve cleaning performance but also protect energy efficiency and prevent overheating or damage to sensitive machine components. Similar technology is integrated into dryers for accurate temperature consistency during drying cycles.
Surge protectors are essential for preventing equipment damage caused by electrical overloads, transient voltage spikes, or power surges—which generate hazardous heat. PTC thermistors (Positive Temperature Coefficient) are integrated into surge protectors to sense and manage energy surges. When overheating is detected due to an overload, the thermistor increases its resistance and interrupts current flow to safeguard connected devices. PTC thermistors in surge protectors are critical for protecting sensitive electronics, computers, home theater systems, industrial controllers, and networking equipment from electrical faults, minimizing downtime and protecting valuable investments.
In a refrigerator, multiple thermistors are used to gather temperature data from key components—such as the freezer compartment, evaporator coils, and main refrigerator space. These sensors transmit critical temperature readings to the electronic control board, regulating compressor cycles, defrosting, and overall cooling efficiency. For example, a thermistor is typically mounted on the top of the evaporator coil to monitor ice buildup and initiate automatic defrost when needed. High-precision thermistors enable smart fridges to maintain ideal food storage conditions, reduce energy consumption, and provide advanced error diagnostics codes. Premium models may use as many as nine thermistors to oversee and optimize various aspects of operation.
Like all resistors, a thermistor resists electrical current. However, unlike a standard resistor, a thermistor's resistance varies with temperature changes. The resistance of a thermistor adjusts in response to temperature fluctuations, following a consistent principle across all thermistors.
NTC thermistors are the most widely used type compared to PTC thermistors.
NTC thermistors exhibit a decrease in resistance as temperature rises. They are composed of semiconductor materials that have conductivity levels between that of conductors and insulators. When the temperature of the component increases, electrons are freed from their lattice positions, allowing for easier flow of electricity. As the temperature goes up, the thermistor conducts electricity more rapidly and efficiently.
The performance of an NTC thermistor depends on its specific materials. Manufacturers adjust the ratios of oxides and doping metals to achieve the required characteristics. Additionally, factors such as the oxygen content during firing and variations in the cooling rate during production can influence the thermistor’s behavior.
NTC thermistors are made in discs, rods, plates, beads, or chips using a sintered metal oxide. Metallic oxide NTC thermistors are made from a fine power that is compressed and sintered. The materials include manganese, nickel, copper, iron, and titanium, as well as silicon or germanium crystals.
The conduction method in NTC thermistors varies based on the materials used in their production. The selection of these materials is influenced by the specific temperature range that the thermistor needs to measure.
Germanium � Operates within the range of 1 Kelvin (K) to 100 K, or from -457.6°F to -279.4°F (-272°C to -173°C).
Silicon � Effective up to 250 Kelvin (K) or -9.4°F (-23°C).
Metallic Oxide � Suitable for temperatures ranging from 200 Kelvin (K) to 700 K, or from -99.4°F to 798.8°F (-73°C to 426°C).
For high-temperature applications, thermistors are constructed from materials such as aluminum oxide (Al₂O�), beryllium oxide (BeO), zirconium dioxide (ZrO�), yttrium oxide (Y₂O�), and dysprosium oxide (Dy₂O�).
NTC thermistors are available in numerous sizes to accommodate different applications. In the electronics industry, they are commonly used in small bead sizes. The different sizes affect the thermistor’s properties and performance characteristics.
Glass-encapsulated NTC thermistors are fully sealed to prevent reading errors. This encapsulation makes them suitable for harsh environmental conditions with minimal restrictions on their use. They operate within a temperature range of -67°F to 392°F (-55°C to 200°C). These thermistors are known for their exceptional accuracy, fast response times, and compact size, which facilitates easy installation.
PTC thermistors function differently from NTC thermistors; their resistance increases as temperature rises. There are two main types of PTC thermistors: one that exhibits a gradual, linear increase in resistance and another that displays abrupt changes in resistance. These types are referred to as switching thermistors and silistors.
Switching PTC thermistors exhibit non-linear behavior. Initially, their resistance decreases slightly as the temperature rises. However, once the temperature reaches a specific point, the resistance increases sharply, making them suitable for protective applications.
Silistor PTC thermistors are characterized by their linear behavior and are based on semiconductor materials. Typically used in various temperature-sensing devices, they are made from doped silicon, with the doping level influencing their specific properties.
Switching PTC thermistors are commonly utilized and are typically constructed from polycrystalline materials like barium carbonate, titanium oxide, silica, tantalum, and manganese. During production, these materials are ground into a powder and compressed to form the thermistor shape. Most PTC thermistors come with lead wires, but they are also available in chip form. Generally, they are manufactured by embedding the chip in tape wire and soldering it through either immersion or manual techniques.
Switching PTC thermistors are commonly used for self-heating applications and as sensors.
In self-heating mode, current flows through the thermistor, causing it to heat up. Once the thermistor reaches its critical temperature, its temperature rises dramatically. This property makes it well-suited for use as a regulator.
In sensor mode, a switching PTC thermistor has only a minimal current passing through it while monitoring the surrounding temperature. Its role is to ensure that the temperature of the environment does not impact the monitored device. When the ambient temperature reaches a critical threshold, the resistance of the thermistor rises sharply.
Thermistors are manufactured through various methods, often involving metallic oxides, binders, and pressed wafers that are cut into chips, discs, or other shapes. The composition of these materials defines the thermistor's temperature curve, which is carefully controlled to ensure optimal performance.
The word "thermistor" is a blend of "thermal" and "resistor," reflecting its function as a temperature-sensitive resistor. The materials used in thermistor production are electrically resistive, and their properties vary depending on whether the thermistor is of the NTC or PTC type.
The primary materials used in thermistor manufacturing include manganese, nickel, and cobalt, which offer resistivities ranging from 100 ohms to 450,000 ohms.
Bead-shaped thermistors are produced by coating platinum alloy lead wires with a slurry of metal oxides and a binder. The binder plays a crucial role by providing the necessary surface tension to shape the material into beads. Bead thermistors are known for their high stability, rapid response, ability to operate at elevated temperatures, and low dissipation constant. They can range in size from as small as 0.0004 inches (0.01 mm) to 0.05 inches (1.2 mm).
Disc-type thermistors are produced by compressing oxide powders into circular molds. These pressed materials are then sintered at high temperatures under pressure to create cylindrical shapes, with diameters ranging from 0.094 inches to 0.98 inches (2.5 mm to 25 mm). The broad range of sizes available for disc-type thermistors provides options suitable for various applications.
While there are various thermistor configurations, the three most prevalent types are hermetically sealed flexible (HSTH) thermistors, bolt-on or washer thermistors, and surface-mounted thermistors.
HSTH thermistor sensors feature hermetic sealing at the sensor tip to protect against corrosive environments. This seal is made from PerFluoroAlkoxy (PFA), a clear and flexible fluoropolymer that is chemically inert and suitable for chemical and solvent applications. HSTH thermistors are available in three resistance values: 2252Ω, 5000Ω, and 10000Ω.
Bolt-on or washer thermistors are engineered for rapid response, durability in harsh environments, and versatility across various applications. They are cost-effective and simple to install. These thermistors are created by compressing the thermistor material under high pressure into flat, cylindrical shapes with diameters ranging from 0.12 inches to 0.98 inches (3 mm to 25 mm).
Surface-mounted thermistors have an adhesive material on the bottom of their sensor that can adhere to any type of surface. They are a type of NTC chip thermistor and are ideal for use in temperature compensation networks.
Ceramic switching PTC thermistors are constructed from polycrystalline ceramics, including barium titanate and doped with rare earth materials to impart their positive temperature coefficient resistance. They exhibit a highly non-linear resistance-temperature characteristic.
Polymeric (PPTC) thermistors are composed of non-conductive crystalline organic materials combined with carbon black particles, which make them conductive. These thermistors respond to variations in ambient temperature and automatically reset once fault conditions are resolved.
Glass-encapsulated thermistors are hermetically sealed to protect against moisture ingress. These NTC thermistors are designed to operate in harsh environmental conditions and extreme temperatures, with a temperature range extending from -67°F to 392°F (-55°C to 200°C). This wide temperature range is achieved through the use of beaded glass for sealing rather than solder. Additionally, glass-encapsulated thermistors are compact, allowing them to be integrated into a diverse array of housings and devices.
There are several differences between thermistors and resistance temperature sensors (RTDs), with RTDs and integrated circuits being the most common types of sensors.
Although thermistors are small, they are essential parts of larger circuit temperature control systems. They are an inexpensive low temperature device compared to thermocouples, which are more expensive and used as high temperature devices. Unlike thermocouples, thermistors last longer and do not suffer from thermal drift.
Thermistors are available in various styles, with radial and axial configurations being the most common. In radial thermistors, both wires extend in the same direction from the bead, whereas in axial thermistors, the wires emerge from the top and bottom of the bead, which is positioned centrally along the length of the wires.
The fundamental principle behind a thermistor is that its resistance varies with temperature. This resistance is measured using an ohmmeter, which gauges electrical resistance. It is important to note that thermistors do not provide direct readings; instead, their resistance changes in response to temperature variations. The level of resistance depends on the material used in the device. Unlike linear sensors, thermistors are non-linear, and their temperature-resistance relationship is represented by a non-linear graph.
Understanding how temperature variations influence the resistance of a thermistor allows for the calculation of temperature readings from the acquired data. This relationship is non-linear, resulting in a curve rather than a straight line.
All resistors exhibit changes in resistance with temperature, quantified by the temperature coefficient of resistance. While standard resistors show some variation in response to temperature, thermistors are designed to have a significant temperature coefficient of resistance to enable precise temperature measurements.
A thermistor is positioned within a device to monitor its temperature and is integrated into an electrical circuit. As the temperature within the device varies, the thermistor's resistance changes accordingly. This resistance variation is detected by the connected circuit and calibrated to match the corresponding temperature.
Thermistors typically feature two wires, one of which connects to an excitation source that measures the thermistor’s voltage. The primary advantage of thermistors is their ability to exhibit a substantial change in resistance with temperature variations, providing highly sensitive and precise readings.
The principles of a thermistor are grounded in the Steinhart-Hart equation, a mathematical approach for obtaining accurate temperature readings. Developed by John Steinhart and Stanley Hart in 1968, this polynomial formula is used to determine the relationship between temperature and resistance in NTC thermistors. The formula allows for the calculation of resistance when the temperature is known, and conversely, for finding the temperature when the resistance is known.
Temperature measurements are very common and something that most people monitor every day. Every home has a large number of temperature measuring devices, the majority of which include thermistors. Thermistors can be found in fire alarms, refrigerators, ovens, boilers, and microwaves. Their unique ability to change electrical resistance into temperature readings makes them a very beneficial and accurate tool.
Various types of sensors are used for temperature measurement, such as thermocouples and resistance temperature detectors (RTDs). While each of these devices delivers accurate data, many manufacturers prefer thermistors for their specific advantages.
One of the key factors driving the popularity of thermistors is their cost-effectiveness. They offer accurate and precise temperature measurements over a limited range, all at a relatively low price.
Thermistors come in a compact design and are manufactured in various forms, such as beads, discs, and rods. Despite their small size, they are highly durable and have a long service life.
When a device is powered on, it experiences a surge of current known as inrush current. Without proper protection, this surge can cause damage and adverse effects. NTC thermistors serve as inrush current limiters (ICLs) to safeguard sensitive circuits by controlling these surges. Inrush currents can harm capacitors, damage power switch contacts, and destroy rectifier diodes. PTC thermistors are also employed for limiting inrush current and providing overcurrent protection.
Inductive electrical devices like motors, transformers, and ballast lighting can experience inrush currents, which can be managed by using a series of thermistors to limit these initial currents to safe levels. NTC thermistors are preferred for this purpose because of their low cold resistance values.
In an electrical circuit, the flow of current generates heat, which is then dissipated. This heat raises the temperature of the resistor. With a thermistor, the resistance adjusts to a specific level, helping to manage and reduce the heat generated.
While thermistors are primarily recognized for their role in temperature measurement, they are also utilized for monitoring pressure, liquid levels, and power. Additionally, they serve as overload protectors and can issue warnings of potential malfunctions.
Thermistors are positioned at a specific distance from the circuit to prevent measurement inaccuracies caused by lead resistance. Because thermistors are designed for narrow temperature ranges, they provide highly precise readings. They also exhibit a rapid response to small and minor temperature fluctuations.
Thermistors are capable of detecting small incremental temperature changes, allowing them to provide immediate data with minimal delay. This quick response is partly due to their narrow operating temperature range.
Thermistors come in a wide range of types, sizes, and configurations, making them versatile for various temperature applications. Their adaptability allows them to be used effectively in diverse operations, conditions, and scenarios.
The International Electrotechnical Commission (IEC), established in 1881 during the International Electrical Congress in Paris, is a standards organization that sets guidelines for electrical, electronic, and technological devices. The IEC has played a crucial role in standardizing and categorizing the expanding electronics industry. It has introduced universally accepted units of measurement, such as Gauss, Hertz, and Weber, and has developed an international system of measurement standards used globally.
IEC 60539-1 is a contemporary standard specifically addressing negative coefficient thermistors constructed from transition metal oxides with semiconducting characteristics. It outlines the inspection procedures and testing methods for these directly heated negative temperature coefficient (NTC) thermistors. This latest edition supersedes all previous versions and incorporates technical updates.
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