Posistors - An Introduction

Posistors (PTC thermistors) are elements based on ceramics whose resistance rises with an increase in temperature!

Positors find use in applications such as temperature sensing, overcurrent protection, and inrush current limiting, etc.

According to a Murata documentation, Barium titanate (BaTiO), discovered in the early 1940s in Japan, the United States, and the Soviet Union, is generally 10^10 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 ~ 10^6 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.

Like NTC thermistors, PTC thermistors can also be used as temperature sensors by utilizing the change in resistance due to changes in temperature. 

Note at this point that the posistor has resistance-temperature characteristics that cause its resistance to exponentially increase when the part’s temperature exceeds its Curie Point, the critical temperature where the resistance value increases dramatically.

And, typically above the Curie Point, the resistance raise at rate of 15% to 60% per °C.

Coming to applications that utilize the self-heating property unique to a PTC thermistor, it can be used as a resettable fuse (to suppress abnormal current flowing through a circuit due to component failures) that returns to the original resistance value when the abnormal or overload condition is removed.

Now let's look at a PDF Datasheet ↗

Nevertheless a PTC thermistor can also be used to form a constant temperature heater plate (see below).

As an aside, a posistor, as you can see in a cathode ray tube (CRT) color television (CTV) circuit, is a clever combination of a positive temperature coefficient (PTC) resistor and another resistor-element to heat it up and keep it hot.

All color CRTs include an inbuilt degaussing coil, and the posistor activates that coil each time the system is powered up from cold.

The heater element in such a 3-pin PTC thermistor is a disk-shaped resistor across the power line and the thermistor is a disk shaped device in series with the degaussing coil (both are in clamped together to be in close contact thermally).

To sum up, posistor is positive temperature coefficient thermistor (PTC Thermistor).

Normally when the temperature increases, the resistance of a conventional negative temperature coefficient thermistor (NTC Thermistor) decreases. But, the resistance of a PTC thermistor rises sharply when its temperature exceeds a specific level. A posistor can also provide over-current protection if it is series-connected in a circuit.

Last but not the least, we are grateful to the Murata team for the invaluable free documentation on their posistors, without which coming up with this short post would never have been possible.

That's all for now. Any insights you may have will be appreciated!