Definition of a Thermistor
A thermistor is a type of resistor whose resistance is strongly dependent on temperature. The name is a portmanteau of 'thermally sensitive resistor'. Unlike standard resistors where temperature changes are an unwanted side effect, thermistors are specifically designed to exploit this characteristic for temperature sensing and control applications.
Types and Operating Principles
There are two main types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). NTC thermistors are the most common; their resistance decreases as temperature increases. PTC thermistors, conversely, show an increase in resistance with rising temperature. This behavior is due to the semiconductor materials used in their construction, which have energy bands that electrons can jump between when thermal energy is applied.
Practical Example: NTC Thermistor in a Digital Thermometer
Consider an NTC thermistor used in a digital thermometer. As the thermometer comes into contact with a warmer object, the thermistor's temperature rises. This causes its electrical resistance to decrease. An electronic circuit measures this change in resistance and converts it into a corresponding temperature reading, which is then displayed digitally. This rapid and accurate response makes thermistors ideal for many temperature-sensitive devices.
Applications of Thermistors
Thermistors are widely used due to their sensitivity, reliability, and cost-effectiveness. Common applications include household appliances (like ovens, refrigerators, and air conditioners), automotive systems (engine temperature sensing), medical devices (fever thermometers), and industrial equipment for process control. They are essential components wherever precise temperature monitoring or compensation is required.