Thermistors

Thermistors are ideal temperature transdusers. A thermistor can be defined as resistor made up of semiconductors having resistance that varies rapidly and predictably with temperature. The word thermistor is a portmanteau of Thermal & Resistors. 

In few cases the resistance of a thermistor may decrease upto 6 percent for 1⁰Celsius in temperature. Due to high sensitivity, thermistors are very useful for precision temperature measurement, control, and compensation. Thermistors are used in the temperature range of 100⁰  Celsius to 300⁰ Celsius. The resistance ranges from 0.552 to 72 Mega Ohms. Thermistors are made up of sintered mixture of metallic oxides such as cobalt, nickel, copper, manganese etc. they are available in the form of beads, rods or discs.Thermistors differ from Resistance Thermo Detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different.The characteristics of a Thermistor can be shown in the picture below:-

Classification of Thermistors:-

 Depending upon the sign of k the Thermistors can be classified into two types. They are:-

1. PTC Thermistors:-

 If k is positive, the resistance increase with the increasing temperature,& the device is called as Positive Temperature Coefficient(PTC) thermistor or Posistor. 

PTC Thermistor

2. NTC Thermistor:-

If k is negative,the resistance decreases with the increasing temperature,& the device is called as Negative Temperature Coefficient(NTC) thermistor.

NTC Thermistor

Thermistors have some advantages. They can be listed as below:-

1. PTC thermistors can be used as current limiting devices for circuit protection, as replacements for fuses.
2. PTC thermistors are used as timers in the degaussing coil circuit of most CRT displays & TVs.
3. NTC thermistors are used as resistance thermometers in low temperature measurements of the order of 10K.
4. NTC thermistors can be used as inrush current limiting devices in power supply.
5. NTC thermistors are regularly used in automotive applications.
6. NTC thermistors can be also used to monitor the temperature of an incubator.
7. Thermistors are commonly used in modern digital thermostats & to monitor the temperature of battery packs while charging.