Thermistor 
The Thermally sensitive resistors(Thermistors) are similar to electrical resistors whose resistance changes with temperature. Thermistors are made of semiconductor material. There are 2 types of thermistors:
1. PTC (positive thermal coefficient) which increases it's resistance with increasing temperature.
2. NTC(negative thermal coefficient) which decreases it's resistance with increasing temperature. The NTC is the most common among the two types of thermistors.
Advantages of thermistors
• more sensitive than the RTD's and thermocouples
• Small sizes
• Quick response
• Low cost
• Suitability for narrow spans
Disadvantages of thermistors
• Linearity is not good
• More susceptible to permanent decalibration at high temperatures
• Use is limited to a few hundred degrees Celsius
• Respond quickly to temperature changes, especially susceptible to self-heating errors
• Very fragile
Applications of thermistors
Due to it's low cost and high sensitivity,it is widely used in the applications such as the winding temperature of motors, or the temperature of bearings, or the temperature of transformer windings in which the actual temperature may not be of as much concern as the change or rapid increase in temperature . Many consumer-grade devices use thermistors for temperature sensors too.
A PTC is used as a current limiter. As the current increases the temperature increases. hence the resistance increases limiting the current.
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