What are PTC Thermistors (POSISTOR)? Operating Principle | Basic Knowledge of PTC Thermistor (POSISTOR)

What are PTC Thermistors (POSISTOR)?

Barium titanate (BaTiO₃), discovered in the early 1940s in Japan, the United States, and the Soviet Union, is generally 10¹⁰ Ω・cm or more at room temperature. When trace amounts of rare earth elements (Y, Bi, Sb, etc.) are added, the specific resistance becomes 10 ~ 10⁶ Ω・cm, and the temperature characteristics of the product corresponding to the Curie point are shown in 1952 by Haayman et al. of Philips (Netherlands).

However, they did not publish literature, only applications for patents, so it became publicly known around 1954. In 1961, Murata Manufacturing Company, Ltd. began mass production for the first time in the world and acquired the registered trademark “POSISTOR”. From around 1963, industrialization progressed in European, American, and Japan companies, and it was applied as a temperature compensation, water level detection, motor overheating prevention, automatic temperature control heater, and degaussing circuit for color televisions.

Resistance-temperature characteristics of BaTiO₃ and PTC thermistors

Operating Principle

Barium titanate (BaTiO₃), the main component of ceramic PTC, changes its crystal structure around the Curie temperature. Below the Curie temperature, Crystal Structure of PTC is tetragonal and there are positive and negative electric dipoles. Above the Curie temperature, it changes to a cubic and the electric dipole disappears.

Oxygen ions are trapped at the boundary between the grains of the ceramic, which blocks the flow of electrons. Below the Curie temperature, electric dipoles electrically cancel out oxygen ions, and electrons flow smoothly. Above the Curie temperature, the electric dipole disappears, oxygen ions interfere with the flow of electrons, and the resistance value increases.

SEM image of ceramic PTC

State change before and after the Curie point

Applications

Like NTC thermistors, PTC thermistors can be used as temperature sensors by utilizing the change in resistance due to changes in temperature. It is characterized by being able to detect when the device is hot with a very simple circuit configuration.
However, there are also applications that utilize the “self-heating” unique to PTC thermistors. By applying a large current, the resistance value rises due to self-heating, which can suppress the current. They are used to protect ICs from abnormal currents flowing through circuits due to component failures, incorrect wiring, etc. It can be used as a resettable fuse that returns to the original resistance value when the abnormal condition is removed.
In addition, this self-heating has a self-control function to balance heat generation and heat dissipation by changing the resistance value depending on the state. Using this operation, it can also be used as a constant temperature heater.