Thermistors

Thermistors are temperature-sensitive resistors made from ceramic semiconductors, typically metal oxide blends. Their resistance changes much more with temperature than that of standard resistors, allowing compact and inexpensive temperature sensing, compensation, and control in a wide range of devices.

There are two main types of thermistors. Negative Temperature Coefficient (NTC) thermistors have resistance that decreases as temperature rises, making them useful for temperature sensing and compensation in circuits. Positive Temperature Coefficient (PTC) thermistors increase in resistance with temperature and are often used for overcurrent protection, self-regulating heating elements, and inrush current limiting. PTC thermistors were historically used in CRT degaussing circuits and are now common as resettable fuses in electronic devices.

Thermistors are typically manufactured from doped metal oxides formed into beads, disks, rods, or surface-m mountable chips. The temperature dependence is strongly nonlinear, and a parameter such as the Beta value summarizes the resistance change over a temperature range for NTC devices. They span a wide resistance range, from a few ohms to several hundred kilohms (and higher in some cases). Key characteristics include sensitivity, response time, tolerance, and potential self-heating if high current flows through the element, which can affect readings.

Applications include temperature measurement and control in household appliances, automotive sensors, and industrial electronics; temperature compensation for other components; and inrush current limitation or overcurrent protection using PTC variants. Thermistors offer high sensitivity and low cost but may require calibration and linearization for precise measurements, and they can drift with age and exposure to harsh conditions.