MCU Examples.com
PIC Microcontroller Project Examples, free source codes and resources collection.
Thermistors
Like the RTD, the thermistor is also a temperature sensitive resistor. While the thermocouple is the most versatile temperature transducer and the PRTD is the most stable, the word that best describes the thermistor is sensitive. Of the three major categories of sensors, the thermistor exhibits by far the largest parameter change with temperature.
Thermistors are generally composed of semiconductor materials. Although positive temperature coefficient units are available, most thermistors have a negative temperature coefficient (TC); that is, their resistance decreases with increasing temperature. The negative T.C. can be as large as several percent per degree Celsius, allowing the thermistor circuit to detect minute changes in temperature which could not be observed with an RTD or thermocouple circuit.
The price we pay for this increased sensitivity is loss of linearity. The thermistor is an extremely non-linear device which is highly dependent upon process parameters. Consequently, manufacturers have not standardized thermistor curves to the extent that RTD and thermocouple curves have been standardized.
The NTC (negative temperature coefficient of resistance) thermistors which are discussed herein are composed of metal oxides. The most commonly used oxides are those of manganese, nickel, cobalt, iron, copper and titanium. The fabrication of commercial NTC thermistors uses basic ceramics technology and continues today much as it has for decades. In the basic process, a mixture of two or more metal oxide powders are combined with suitable binders, are formed to a desired geometry, dried, and sintered at an elevated temperature. By varying the types of oxides used, their relative proportions, the sintering atmosphere, and the sintering temperature, a wide range of resistivities and temperature coefficient characteristics can be obtained.
The PTC (positive temperature coefficient of resistance) thermistors have been used in a wide variety of applications over the years. PTC thermistor applications make use of the characteristics inherent in their composition. Generally, applications are broken up into two distinct categories that utilize different characteristics of the PTC. The first category is those applications that utilize the voltagecurrent or current-time characteristics of the PTC thermistor. These are sometimes known as selfheated applications. The other general category is zero power or sensing applications. Unlike the NTC thermistor, the resistance versus temperature characteristic of a PTC thermistor is not well defined and applications that utilize the resistance versus temperature characteristic tend to utilize only a small portion of the R vs T curve and utilize broader tolerances than that of NTC thermistors.
