A thermistor (thermal resistor) is a type of temperature-sensitive resistor made from semiconductor materials. Its electrical resistance varies significantly with changes in temperature. Thermistors are widely used in various applications for temperature measurement, temperature compensation, and temperature control due to their predictable and highly sensitive response to temperature changes.
Construction of Thermistors:
There are two main types of thermistors: negative temperature coefficient (NTC) and positive temperature coefficient (PTC).
NTC Thermistor (Negative Temperature Coefficient): In an NTC thermistor, the resistance decreases as the temperature increases. NTC thermistors are typically made from ceramic materials that exhibit a high temperature sensitivity. The basic construction involves mixing metal oxides like manganese, nickel, cobalt, and copper with ceramic materials. The mixture is then sintered to form a solid structure. The resistance-temperature relationship is nonlinear and can be described by the Steinhart-Hart equation.
PTC Thermistor (Positive Temperature Coefficient): In a PTC thermistor, the resistance increases with increasing temperature. PTC thermistors are usually made from barium titanate or other similar ceramic materials. These materials exhibit a transition point called the Curie temperature, above which their resistance increases rapidly. PTC thermistors are often used as self-resetting fuses or in applications where they need to limit inrush current.
Applications of Thermistors:
Temperature Measurement: Thermistors are commonly used for temperature sensing and measurement. They can be integrated into circuits to accurately measure and monitor temperature in various environments, such as industrial processes, HVAC systems, and electronic devices.
Temperature Compensation: Thermistors are used to compensate for temperature-induced variations in other electronic components, such as in temperature-compensated crystal oscillators (TCXOs) used in precision frequency control.
Thermal Protection: NTC thermistors are used for thermal protection in applications where excessive heat could cause damage. For instance, they can be placed in motors, transformers, and power supplies to monitor temperature and trigger shutdown or alarms if the temperature exceeds a safe limit.
Inrush Current Limiting: PTC thermistors are used in circuits to limit the inrush current during power-on. When the device is initially powered on, the resistance of the PTC thermistor is low, allowing a higher current to flow. As the temperature rises due to the current passing through it, the resistance increases, limiting the current to a safe level.
Liquid Level Sensing: NTC thermistors can be used to sense liquid levels by measuring the temperature difference between the thermistor in air and when it is immersed in the liquid. This principle is often used in applications like water level indicators.
Medical Devices: Thermistors are used in medical equipment for temperature monitoring in patient care, such as in thermometers, incubators, and blood temperature monitors.
Automotive Applications: Thermistors are used in automotive engines to monitor coolant temperature and in climate control systems to regulate cabin temperature.
Consumer Electronics: Thermistors are found in various consumer electronic devices, such as laptops, smartphones, and refrigerators, to monitor and control temperature.
These are just a few examples of the many applications of thermistors in various industries and technologies. They offer a cost-effective and reliable solution for temperature-related measurements and control.