Thermistors are ideal devices for monitoring temperature in electronic products because their resistance can change with temperature variations. Additionally, PTC thermistors can serve as resettable fuses, playing a crucial role in ensuring the safe operation of industrial automation equipment. This article will introduce the development and characteristics of thermistors, as well as related solutions offered by Murata.
Thermistors are ideal temperature sensors
A thermistor is a type of sensor resistor whose resistance value changes with temperature, and its size varies significantly with temperature compared to regular fixed resistors. Thermistors belong to the category of variable resistors and find widespread applications in various electronic components. They are primarily used for measuring, controlling, and monitoring temperature and have many applications such as surge current limiters, temperature sensors, resettable fuses, automatic heater control, and more.
Unlike resistance temperature detectors (RTDs) that use pure metals, the materials typically used in thermistors are ceramics or polymers. They also exhibit different temperature response characteristics, with RTDs suitable for a broader temperature range, while thermistors usually achieve higher accuracy within a limited temperature range, typically from -90°C to 130°C.
Thermistors are extensively employed in temperature sensors for measuring the temperature of the environment or objects. They are also used for temperature compensation in electronic devices to ensure correct operation of electronic components at varying temperatures, contributing to improved accuracy and stability of the equipment.
Thermistors can be utilized in temperature control systems, such as thermostats, heaters, and coolers, to maintain a stable temperature within a specific range. They are also employed in over-temperature protection applications within devices to prevent overheating and avoid damage.
With their temperature sensitivity, thermistors are ideal temperature sensors that provide high sensitivity for temperature measurement and can operate across a wide temperature range. Additionally, the resistance-temperature relationship of thermistors is typically nonlinear, requiring calibration and appropriate circuitry to convert resistance changes into temperature data. On the other hand, thermistors have relatively fast response times to temperature changes, making them suitable for applications that require real-time temperature monitoring and control.
NTC thermistors are widely used in various electronic products
The term "thermistor" typically refers to NTC (Negative Temperature Coefficient) thermistors. In 1833, while researching silver sulfide semiconductors, Michael Faraday discovered thermistors. Samuel Reuben commercialized them in the 1930s. NTC thermistors are made of an oxide semiconductor ceramic composed of manganese (Mn), nickel (Ni), and cobalt (Co).
NTC thermistors exhibit a resistance change of 3-5% per degree Celsius with temperature variation. They are commonly used as temperature sensors in electronic devices. NTC thermistors can be found in various applications in our daily lives. Due to their characteristic of decreasing resistance with increasing temperature, they are used as temperature sensors, temperature sensing devices in air conditioners, temperature control devices in smartphones, electric kettles, irons, and current control devices in power supply equipment.
Murata began mass-producing NTC thermistors around 1984 and has since built a product lineup primarily based on chip-type products. Murata's NTC thermistors use high-precision, highly sensitive ceramics and come in various types such as surface-mount (SMD) and leaded types, used for inrush current suppression, temperature compensation, and more. There are multiple product series available, including chip-type and leaded-type NTC thermistors.
The NCU series is a new addition to Murata's NTC thermistor lineup and serves as a surface-mount temperature sensor. It offers high reliability and is suitable for the automotive market, where high reliability is required, enabling temperature sensing and temperature compensation over a wide temperature range.
The NCP series is a chip-type SMD temperature sensor used for temperature sensing and temperature compensation. It comes in various sizes, ranging from 06 x 03 to 20 x 12mm, and finds applications in smartphones, computers, LED lighting, and more.
The NCG series is an SMD-type temperature sensor used for various temperature sensing and compensation applications. It is also suitable for automotive applications with high reliability requirements. The NCG series is a limited product for conductive glue mounting, and it is not compatible with solder mounting.
The NXF series is a flexible lead-type NTC thermistor and features a small-sized sensor heads for temperature sensing. It is known for its compact size and high sensitivity. Additionally, the NXF series offers good flexibility in the lead section, with product lengths ranging from 21mm to 50mm.
PTC thermistors can serve as resettable fuses
In the early 1940s, barium titanate (BaTiO3) was discovered by Japan, the United States, and the Soviet Union, which exhibited a high resistance of greater than 1010Ω･cm at room temperature. In 1952, researchers including Haayman from Philips (Netherlands) found that by adding small amounts of rare-earth elements (Y, Bi, Sb, etc.), the specific resistance changed to the range of 10～106Ω·cm, and the material's temperature characteristics corresponded to the Curie point. This marked the birth of PTC thermistors. In 1961, Murata took the lead in mass-producing PTC thermistors and registered the trademark POSISTOR.
PTC thermistors are characterized by their ability to detect when devices overheat through a very simple circuit configuration. On the other hand, they can also be used in applications that leverage PTC thermistors' unique "self-heating" properties. By applying a large current, their resistance increases due to self-heating, which can suppress the current. They are used to protect integrated circuits (ICs) from abnormal currents flowing through the circuit due to component failures, incorrect wiring, and other reasons. PTC thermistors can be used as resettable fuses and return to their original resistance value once the abnormal condition is removed.
Murata's POSISTOR PTC thermistors are made from ceramic materials with excellent reliability and performance. The comprehensive product line covers various packaging forms (surface mount type, leaded type) and is suitable for different applications such as overcurrent protection, overheat protection, and inrush current suppression.
Murata's PRF series of PTC thermistors are SMD type and can be used for overheat sensing. These thin film PTC thermistors for temperature sensing take advantage of their rapid increase in resistance above a certain temperature, making them suitable for overheat sensing in FETs, power ICs, and other heat generating areas. The PRF series offers excellent noise resistance due to the rapid change in electrical impedance. Additionally, because of the rapid resistance change, even in simple circuits with multiple PTCs connected in series for overheat sensing, the detection can be performed correctly. This reduces the number of IC ports and contributes to downsizing equipment.
The PRG series (resettable fuses) is used for overcurrent protection devices and features fast operation. It protects circuits in case of overcurrent caused by short circuits and automatically returns to its initial state after overcurrent elimination, making it can be used repeatedly. The use of ceramic materials ensures high reliability and rapid protection after a short circuit, allowing customers to make their equipment safer and maintenance-free.
Multi-layer structured PTC thermistors exhibit excellent characteristics
PTC thermistors are chip components suitable for factory automation, and maintenance-free PTC thermistors are used for overcurrent protection caused by I/O sequence output circuit errors and short circuits in factory automation equipment and sensors. In addition, PTC thermistors are widely used for I/O circuit protection in machine controllers and robot controllers.
Currently, there are three types of resettable fuses: polymer PTC, ceramic PTC, and Murata ceramic PTC. Polymer PTC has unstable characteristics and can support higher current and voltage. Legacy ceramic PTC technology has stable characteristics and can support lower current and voltage, but it cannot meet the electrical specifications required by factory automation standards (24V voltage and over 100mA). Murata's ceramic PTC technology has stable characteristics, supports higher current and voltage, which means Murata can solve the technical problem of unstable characteristics in polymer PTC.
Murata's ceramic PTC uses a multi-layer structure, significantly reducing resistance and achieving high withstand voltage (maximum over 30V) and high power capacity, as well as a higher hold current (260mA). It uses ceramic materials, ensuring good reliability, short overcurrent protection time, and maintenance-free operation, improving safety. Compared to legacy ceramic PTC technology, which only supports lower voltage and lower hold current (up to 20mA with 30V).
In factory automation applications, the PRG series protects factory automation equipment from overcurrent failures caused by misconnection or short circuits. For example, in motor drives, it can protect I/O line transistors and other switching devices from overcurrent. When used for digital output lines in motors, a open-collector- circuit is used, supporting 24VDC voltage, with the current depending on equipment. The PRG series is suitable for circuit protection in motor drives.
Murata's PRG series PTC thermistors used for overcurrent protection act as "resettable fuses" and provide overload protection similar to polymer PTC resettable fuses. They comply with RoHS/ELV standards and are UL/cUL and TÜV certified (all lineup). They have fast response, prevent abnormal currents, and support a maximum voltage range of 6V to 32V. Compared to polymer PTC, the PRG series has stable resistance characteristics after soldering, load testing, and thermal shock, can reliably operate under PCB coating or molding, and has compact dimensions, saving PCB space, among other benefits.
Conclusion
The temperature sensitivity of thermistors makes them an ideal choice for temperature sensors. When applied in factory automation environments, they can monitor equipment temperatures in real-time, ensuring the stable operation of the equipment. Additionally, PTC thermistors can also function as resettable fuses, protecting factory automation equipment from overcurrent failures caused by misconnection or short circuits. Murata offers a complete line of thermistors known for their high reliability, cost-effectiveness, and small footprint. Using these thermistors in your sensor design can enhance reliability and potentially increase profits.