Brief Introduction of PTC Thermistor

General

Thermistor can be classified into 2 category, PTC thermistor and NTC thermistor, according to its resistance-temperature characteristics. PTC, abbreviation of Positive Temperature Coefficient. NTC, abbreviation of Negative Temperature Coefficient. PTC thermistor is classified into 2 type: non-linear and linear PTC thermistor. Non-linear PTC thermistor can be further classified into ceramic PTC thermistor (CPTC) and polymer PTC thermistor (PPTC). Ceramic PTC thermistor is made of BaTiO₃,V₂O₅,BN adding with semiconducting elements. Mostly applied ceramic PTC thermistor is BaTiO₃series PTC thermistor. We now concentrate on BaTiO₃ series PTC thermistor, a typical type of non linear positive temperature coefficient thermistor, when temperature increases above curie (switch ) temperature, its resistance increases sharply. Temperature change can be obtained through current flowing through PTC thermistor, or environment heat input, or both 2 cases.

Application and advantage of PTC thermistor

o        As heater, ceramic PTC thermistor has temperature self-regulating characteristics, do not need temperature control circuit.

o        Apply in switching function, PTC thermistor has overcurrent and overheating protection function, can avoid equipment being destroyed, simple and reliable.

o        As temperature protection component, it has great resistance temperature coefficient near temperature control point. Precise temperature control function can be realized though simple configuration.

o        Wide switch temperature range: -40℃～320℃.

o        High resistance temperature coefficient: more than 40%/℃.

o        Wide resistance range: 0.1Ω～20kΩ.

o        Wide operating voltage: 3V～1000V.

3 Characteristics of PTC thermistor.

BaTiO₃ ceramics is a kind of typical ferroelectric material with the resistivity greater than 10¹²Ω.cm in room temperauture, relative dielectric constant 10⁴, good material for ceramic capacitor. With adding lanthanon Y, Nb, its resistivity can decrease below 10Ω.cm, and become a positive temperature coefficient semiconducting ceramic material. Within the temperature range of several dozens temperature range, its resistivity can increase sharply, create PTC efficacy. This PTC effect is a kind of crystal boundary efficacy, only the multicrystal ceramic material has this kind of characteristics. Thanks to this kind of PTC efficacy, PTC thermistor has been widely applied. PTC thermsitor has 3 characteristics: Resistance-Temperature characteristic, Voltage-Current characteristic, Current-Time characteristic.

Resistance-Temperature characteristic (R-T) characteristic

Resistance-temperature characteristic means the relation of zero power resistance of PTC thermistor to PTC thermistor body temperature under a specified voltage. (shown in the following fig.)

Voltage-Current characteristic (V—I characteristic)

Voltage-current characteristic means the relation of voltage across PTC thermistor to the current under thermal balance stable condition. (As shown in the following fig.)

Current-Time characteristic (I—T characteristic)

Current-Time characteristic means current change characteristics vs. time with voltage on. Instantaneous current with voltage starting is called initial current, Current in stable state is called residual current. (As shown in the figure)

Failed model of PTC thermistor

A. Voltage withstanding capacity—Exceeding stipulated voltage may lead to PTC thermistor short circuit and breakdown. Applying high voltage can eliminate inferior voltage withstanding product, and ensure the safety of PTC thermistor under maximum operating voltage.
B. Current withstanding capacity-Exceeding stipulated current or switching cycles may lead to non-resettable high resistance stage, and failure. Repeated ON/OFF test cannot discard all the early failed products.

Under stipulated condition, PTC thermistor presents high resistance state after failure. Long (generally larger than 1000 hours) applying voltage on PTC thermistor cause minor increase in room temperature resistance, while the resistance increase is more distinct on the PTC heater with curie temperature higher than200℃. The main failed reason of PTC thermistor, except PTC heater, is stress created in ceramic body center caused by switching operation, and craze. Please refer to the following figure, the uneven distribution of temperature, resistivity, electric field and power density in PTC ceramic bogy lead to higher central stress, and then layer splitting.

Distribution of temperature, resistivity, electric field and power density along PTC ceramic body thickness direction