This article takes an in-depth look at thermistors.
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A thermistor is a type of semiconductor with higher resistance compared to traditional conductors, designed to respond to changes in temperature. Its resistance varies based on the specific materials used in its construction. Typically, thermistors are made from a combination of metal oxides, binders, and stabilizers, which are molded into wafers and then sliced into chips. The proportions of these materials influence the thermistor's resistance characteristics and temperature response.
The term “thermistor” describes temperature-sensitive resistors known for their precision and efficiency in temperature measurement. There are two main types of thermistors: Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC). An NTC thermistor's resistance decreases with rising temperature, whereas a PTC thermistor's resistance increases as the temperature goes up.
Thermistors are passive components whose resistance varies with temperature fluctuations in a system. They offer a cost-effective, precise, and responsive means of temperature measurement.
Thermistors are employed to track the temperature around a device. The temperatures measured by thermistors impact equipment performance and are utilized for temperature sensing and overload protection. You can find thermistors in various circuits, devices, and equipment, offering an economical solution for temperature monitoring.
Thermistors come in various configurations, including commonly used types like hermetically sealed, flexible (HSTH series), bolt-on, washer types, and self-adhesive surface-mount styles. HSTH thermistors are encased in a plastic polymer jacket that fully seals the sensing elements, providing protection against moisture and corrosion.
Accurate temperature monitoring is vital in many manufacturing processes. The effectiveness of temperature control can greatly influence the outcome of an application. Thermistors play a key role by providing a consistent, predictable, and precise change in electrical resistance in response to temperature variations.
Thermistors are employed wherever temperature measurement is required, from industrial settings to everyday cooking. They are essential for determining, regulating, and monitoring temperature. A typical application of thermistors is in HVAC systems, where they play a crucial role in managing thermal control and air flow.
Thermistors operate effectively within a temperature range of 32°F to 212°F (0°C to 100°C). Class A thermistors provide the highest accuracy, whereas Class B thermistors are suitable for applications where precise measurements are less critical. Known for their stability, thermistors maintain their accuracy over extended periods without significant drift.
The central part of a thermostat is a highly sensitive thermistor. The temperature control on an HVAC system consists of simple circuit components that include an operational amplifier, thermistor, and a relay, with the thermistor being the main temperature sensor in the circuit.
In automobiles, thermistors are commonly used to monitor the temperatures of oil and coolant. They help alert the driver if the vehicle is overheating. Thermistors are linked directly to the dashboard instruments, providing critical data on the vehicle's operational efficiency.
Microwaves are equipped with thermistors to monitor and regulate their internal temperature. These thermistors help prevent the microwave from overheating and reduce the risk of fire.
During battery recharging, heat is generated that needs to be managed. Recharge units often incorporate a low-resistance thermistor to oversee the charging process. If the temperature rises excessively, the thermistor halts the charging to prevent potential accidents or damage.
As cell phones become increasingly compact and advanced, the risk of overheating grows. Thermistors are used to monitor the temperature inside the phone and transmit this information to the integrated circuit (IC). By detecting heat, thermistors help ensure that the phone's electrical components function efficiently and accurately.
In a washing machine, a thermistor’s role is to monitor and confirm that the optimal temperature for effective operation is achieved. If a heating error appears on the machine's display, it may signal an issue with the thermistor or the heating element. Thermistors are crucial for maintaining the correct temperature, making them essential for the proper functioning of washers and dryers.
Surge protectors are essential for preventing equipment damage caused by electrical overloads, which generate heat. Thermistors are integrated into surge protectors to manage these energy surges. When an overload causes heat accumulation, the thermistor detects this increase and interrupts the current flow to protect the equipment.
In a refrigerator, thermistors are used to gather temperature data from various components such as the freezer, evaporator, and main refrigerator compartment. They track temperatures and transmit this information to the control board. For instance, a thermistor is typically mounted on the top of the evaporator coil. A refrigerator may include between five and nine thermistors to oversee and regulate different aspects of its 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 (Al₂O₃), beryllium oxide (BeO), zirconium dioxide (ZrO₂), yttrium oxide (Y₂O₃), and dysprosium oxide (Dy₂O₃).
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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