THERMISTOR

Specifications & Properties

A Thermistor is a semieonductor device whose resistance varies with temperature but change in temperature is non-linear. Conductor shows very small change in resistance with temperature change but Thermistor can increase or decrease its resistance by more value. Hence Thermistor can be NTC or PTC. When resistanee decreases with increase in temperature then it is called as “Negative Temperature Co-efficient" (NTC). If it increases then it is called as “Positive Temperature Co-efficient" of resistance (PTC). Thermistors are used in many applications like measurement by temperature, temperature control, temperature compensation etc.

Thermistors can be directly heated or indirectly heated when they are vacuum mounted in a glass envelope. It is often necessary to solder mount brackets or connect leads to provide efficient thermal contact. Thermistors are used in an application as excess current limiters, temperature sensors and protection devices against overheating. The thermistors are used in all kind of apparatus such as electric motors, washing rnachines and Alarm installations.

They are prepared from the oxides of the iron group of transition elements like Cr, Mn, Fe, Co or Ni. These oxides have a high resistivity in the pure state but can be transformed into Semiconductors by adding small amounts of foreign ions.

1. PTC
   Positive Temperature Coefficient Thermistor
   With this type, the resistance value increases suddenly when the temperature rises above a specific point.
   
2. NTC
   Negative Temperature Coefficient Thermistor
   With this type, the resistance value decreases continuously as the temperature rises.
   
3. CTR
   Critical Temperature Resister Thermistor
   With this type, the resistance value decreases suddenly when the temperature rises above a specific point.