PTCR (Positive Temperature Coefficient of Resistance) is a kind of positive temperature coefficient thermistor which is mainly made up of BaTiO3 ceramics. BaTiO3 ceramics is a kind of typical ferroelectric material with the resistivity of greater than 10^１2Ω cm under normal temperature, through semiconductor doping it has a strong PTC efficacy——it has extremely low resistance under normal temperature but it experiences a sudden and large mutation around Reference temperature as ambient temperature rises. It is because a barrier layer induced by the surface state exists on the crystal particle border of the multicrystal BaTiO3 semiconductor material. Under Curie temperature, the high-resistance crystal boundary has ferroelectric characteristics with great dielectric constant and low potential barrier, electrons can easily penetrate the potential barrier and corresponding materials have low resistivity. When above Curie temperature, crystal lattice occurs at the high-resistance layer and the ferroelectric constant decreases quickly. The potential barrier increases because the dielectric constant drops in accordance with the Curie-Weiss Law. With the dramatic increase of the barrier height, it becomes difficult for electrons to pass potential barrier and resistivity of the corresponding materials rises dramatically. This behaves as the PTC efficacy of the material in a macro manner. Due to this property, the PTC thermistor is widely used both on industrial electronic equipment and household appliances. The field of its application can be classified in light of three basic electric performances of the PTC thermistor. The fundamental parameters are indicated in the following diagram.

   Resistance-temperature characteristic means the relation of zero-power resistance of PTC thermistor body temperature under a specified voltage shown in Fig.1. Zero-power resistance should be measured in super slot by using pulse power supply with low output impedance and stable output amplitude. Temperature rise of PTC thermistor induced by measuring current should be limited that it could be ignored. Shown in fig. 2、3 are resistance-temperature characteristic curves for various voltages and frequencies. As shown in fig. 2、3, at the same temperature resistance value decreases dramatically as measuring voltage and voltage frequency increase.

    Voltage-current characteristic normally means the relation of voltage across PTC thermistor to current under stable condition with thermal balance in static air of 25℃, that is ,voltage-current characteristic of PTC thermistor in actual operating state as shown in Fig.4. Ⅰstands for non-operating area or linear area , Ⅱ stands for jumping area, Ⅲ for breakdown area. When measuring the characteristic, ambient temperature should be kept constant and current readings should be taken after thermistor body temperature is balanced, and measuring circuit is shown in Fig.6. Fig.5 are voltage-current characteristic curves for various ambient temperatures of PTC thermistor.

   Current-time characteristic means current change characteristic vs. time(as shown in Fig.8). It is normally measured with memory oscilloscope by using measuring circuit as shown in Fig.7.
   Shown in Fig.9. is the relation curve of operating current to operating time. Operating time of PTC thermistor decreases dramatically as initial current increases. Furthermore the operating time is related to resistance temperature coefficient, applied voltage and heat capacity of PTC thermistor.