Thermistors are thermally sensitive resistors and have either a negative temperature coefficient (NTC) or positive temperature coefficient (PTC)which means, in NTC as temperature increases, resistance decreases; and in PTC as temperature increases as temperature increases resistance increases.
Thermistors term is a combination of temperature and resistor.
Thermistors are made from metallic oxides. Thermistors are easy to use and less costly. Thermistors exhibit non linear characteristics.
NTC thermistors are made from metal oxides like specific mixtures of pure oxides of nickel, manganese, copper, cobalt, tin, uranium, zinc, iron, magnesium, titanium, and other metals where as PTC are made from semiconductor like silicon (silistors) or barium, lead, and strontium titanates with the addition of yttrium, manganese, tantalum, and silica. The negative exponential function that best describes the resistance-temperature (R(T)) characteristic of an NTC is given by Steinhart-Hart equation:
Where T is the absolute temperature in Kelvin and R the thermistor resistance at temperature T.
The graph is as shown below
Thermistors have the desirable characteristics of small size,narrow spans, fast response (their time constant can be under 1 s), and a very high sensitivity (about 2%/°F [4%/°C]), which usually increases as the measured temperature drops. Thermistors do not need cold junction compensation because their resistance is a function of absolute temperature, and errors due to contact or lead-wire resistance are insignificant because of their relatively small values. Unlike RTDs and TCs, they are well suited for remote temperature sensing. Thermistors are available in a great variety of configurations, are inexpensive, are not affected by polarity, and their stability increases with age. At present, they are typically more rugged and better able to support mechanical and thermal shock and vibration than other temperature sensors.They are the most sensitive differential temperature detectors available.