The word thermistor is derived from its description: “thermal sensitive resistor”. Thermistors are passive semiconductors, which produce resistance values dependent on temperature.
A Negative Temperature Coefficient (NTC) thermistor decreases in resistance as its body temperature increases. In fact, NTC thermistors exhibit two characteristics, which make them extremely useful in a variety of applications. Their change in resistance is predictable and relatively large per degree change in temperature.
Resistance-Temperature (R/T) Curves and Negative Temperature Coefficient
Nine different materials are made, each with its own unique and predictable resistance-temperature characteristics. These characteristics are called “curves”. Thermistors are most often specified by their curve and by their resistance value at 25 0C.
The NTC (Negative Temperature Coefficient) is the negative percentage resistance change per degree C. Our thermistors have NTC values at 25 0C ranging from -3.9%/0C to -6.4%/0C. Resistance values at 25 0C range from 300 ohms t 40 Meg Ohms.
Thermal Time Constant
The time constant, expressed in seconds, is the time required for a thermistor to indicate 63.2% of a newly impressed temperature. The time constant of a thermistor is you directly affected by the mass of the thermistor and thermal coupling to the environment. An epoxy or phenolic coated thermistor with a 0.095” O.D. will typically have a time constant of 0.75 seconds in stirred oil and 10 seconds in the still air.
Dissipation constant is the power required to raise the temperature of a thermistor 1 0C above the surrounding environment. Power is expressed in watts. The dissipation constant of a thermistor with a 0.095” O.D., coated with epoxy or phenolic, is typically 13 mW/0C in stirred oil and 2 mW/0C in still air.
Very low current is required for a thermistor being used in temperature measurement, control or compensation applications. Current levels should typically be less than 100 mA for the thermistor to dissipate “zero power”. As previously discussed, power dissipation for a thermistor in still air is approximately 2mW/0C. Therefore, in order to keep the thermal error (self-heat) below 0.1 0C, the power dissipation must be less than 0.2mW.
Self-heating is desirable in applications such as air flow measurement and liquid level control. Standard epoxy or phenolic coated thermistors with 0.095” O.D. has a maximum power rating of 30 milliwatts at 25 0C to 1 milliwatt at 100 0C.