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General PTC thermistor

PTCR(Positive Temperature Coefficient of resistance) 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 resistively of greater than 1012Ω.cm under normal temperature,through semi-conductor doping it has a strong PTC efficacy-----it has extremely low resistance under normal 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 both on industrial electronic equipment and household appliances.The field of its application is classified in light of the three basic electric performances of the PTC thermistor.The fundamental parameters add in the following diagram(Fig 1) as indicate.

General NTC thermistor

NTC(Negative Temperature Coefficient of resistance) is a kind of negative temperature coefficient thermistor, it is made through ceramic production technics and mainly made up of polycrystalline mixed metallic oxidematerial such as Mn, Co, Ni, Cu.These metallic oxide has semiconducting characteristics, because it has the same or similar electric conduction mode with the semiconducting material such as Ge, Si. When in low temperature, current carrier of oxide material (electronic and cavity) is in less number, it’s in high resistance; with the increase of temperature, quantity of current carrier increases,so resistance decreases accordingly. NTC thermistor can be widely used in surge current limiting,temperature measurement and control, temperature compensation etc..The resistance change range of NTC thermistor in room temperature is 10O~106Ω, temperature coefficient is -2%~-6.5%.

Glossary of NTC Thermistor:

1.Rt --- Zero Power Resistance Value: The resistance value measured at a fixed temperature by adopting sufficient low power consumption.
2. R25 ---  Zero-Rated Power Resistance Value : The resistance value measured at 25℃ by adopting sufficient low power consumption.
3. B value: is a constant, determined by the ceramic material and represents the slope of the R/T curve.Unless otherwise specified, B value is determined from measurements made at 25℃ (298.15K) and 50℃(323.15K). It's not a firm constant within the range of working temperature.
4. Zero-power Resistance-to-Temperature Coefficient: The temperature coefficient of resistance is defined as the relative change in resistance referred to the change in temperature.
5. Dissipation factor δ:  The dissipation factor δ is defined as the ratio of the change in power dissipation and the resultant change in the thermistor’s body temperature.δ=ΔP/ΔT. δ changes in response when the ambient temperature changes, within the operating temperature range.
6. Thermal Time Constant τ:  is the time required by a thermistor to change 63% of the difference between its initial and final temperature. τ= C/δ
7. Maximum Stable Current: Maximum successive current can be applied in NTC thermistor in the ambient temperature of 25℃.
8. Resistance-Temperature Characteristics: The dependence of the zero power resistance and body temperature of NTC thermistor.