Types and Trends of Resistance Temperature Sensors
Temperature sensors, whose resistance value changes against the temperature, are mainly categorized as thermistors, linear resistors, or platinum resistance thermometer sensors. Their temperature characteristics are shown in Figure 1.
Thermistors are divided into positive temperature coefficient (PTC) and negative temperature coefficient (NTC). For PTC type, the resistance value rapidly increases at a constant temperature. On the basis of these characteristics, PTC type is used for overcurrent protection during thermo-runaway of semiconductors. NTC type, on the other hand, has resistance value that decreases in exponentially when temperature increases. In general, NTC thermistors are often used for the detection of temperature because resistance value is high at room temperature and changes remarkably against temperature change. A thermistor is usually referred to as NTC type, so NTC type is hereinafter referred to as thermistor.
Linear resistors have resistance value that linearly increases when temperature rises. They have been often used for temperature compensation of the winding of motors. In recent years, they are also used for temperature compensation of high-frequency circuits or displays.
Meanwhile, platinum resistance thermometer sensors also have resistance value that linearly increases when temperature goes up. However, they have higher linearity of temperature and more excellent in long-term stability than linear resistors. They are available for wide temperature range and have high precision TCR tolerance as well as resistance tolerance. Platinum resistance thermometer sensors Pt100(100Ω at 100°C) is conformed to the Japan Industrial Standard (JIS) and is normally used but there are other variations of resistance between 10Ω to 1kΩ.
Among these resistance temperature sensors, the demand for SMD type thermistors has been rapidly increasing as the manufacturing quantity of mobile phones increases. Therefore, the quantity of thermistors used is remarkable compared to other products.
Thermistors offer advantages of large resistance change, low price, and detection circuit that is easy to configure. However, general thermistors are limited by lack of compatibility for precision applications. It is easy to configure circuits that turn ON/OFF itself at a certain point of temperature change. However, it is difficult to perform high-precision detection in a circuit for detecting the absolute value of temperature and feeding it back to the circuit. They are suitable for the set with short-term guarantee, but not suitable for the set that requires long-term guarantee or whose temperature sensor shall be replaced as occasion needs.
In reality, linear resistors have been applied instead of thermistors so as to change the number of fan rotations according to the temperature of cooling fan. Or platinum resistance thermometer sensors have been applied for the cold junction compensation of thermo-couples because there is no long-term stability. To enhance world wide compatibility, however, thermistors have been replaced with platinum resistance thermometer sensors, Pt 100 types, for the temperature compensation of measurement equipment.