Structure and Function of PTC Thermistor

PTC (Positive Temperature Coefficient of Resistance) thermistor is a kind of positive temperature coefficient thermistor, mainly made up of BaTiO3 ceramics. BaTiO3 ceramics is a kind of typical ferroelectric material with the resistivity of greater than 10¹²Ω.cm under normal temperature, through semi-conductor doping, it has a strong PTC efficacy, that means it has extremely low resistance under normal temperature, but it experiences a sudden and large mutation around Curie temperature (also called switch temperature, or reference temperature) as ambient temperature rises. This 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 the 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 in industrial electronic equipment and household appliances. The field of its application is classified in light of the three basic electric performances of the PTC thermistors. The fundamental parameters are as indicated in the following matrix diagram (Fig.1).

Fig.1 PTC Thermistor Resistance vs. Temperature Curve

Rn - Room temperature zero-power resistance

Rmin - Minimum zero-power resistance

Tb - Curie temperature

Rb - Switch resistance value Rb=2Rn

Rmax - Maximum resistance

Tp - Poise point temperature

β - Lift-drag ratio is LgRmax/Rmin

date: 2007-7-8