A thermistor is a semiconductor device with an electrical resistance proportional to temperature. The name is formed from a combination of the words "resistor" and "thermal." Thermistors come in two types: one type is PTC, which stands for positive temperature coefficient of resistance; the other is NTC, which stands for negative temperature coefficient of resistance.
All conductors have an electrical resistance proportional to their temperature. Everyday devices such as toasters, heaters, hair dryers and light bulbs operate on this principle. Thermistors are created to deliberately optimise this property and are more temperature sensitive than regular resistors. This sensitivity enables them to determine thermal measurements to within ranges of plus or minus .05 per cent to plus or minus .02 per cent. This accuracy makes them very effective as thermometers and sensors.
Thermistors make excellent thermometers because they are small, very stable and long lasting, in addition to being accurate. They are better at measuring certain temperatures than thermocouples. The disadvantage is that like typical semiconductors they are non-linear, so circuits must be designed with this limitation in mind. Also, unlike thermocouples, they cannot be used at extreme temperatures and are usually confined to those from zero to one hundred degrees Celsius.
NTCs are the most common type of thermistor. Their temperatures vary inversely with their resistance. When the temperature increases, their resistance decreases, and when the temperature decreases, their resistance increases. Another role is that of current suppressors. Additional tasks include temperature compensation, control and indication. NTCs are made from oxides of materials such as nickel, manganese, copper, iron and cobalt. Some are also made from silicon, germanium or a combination of both.
PTC thermistors are the opposite of NTCs. When the temperature increases so does their resistance; when the temperature decreases, their resistance does the same. They are used as thermometers or switches and also for circuit overload protection. Other uses are for light switching, heating regulation, starting motors and telecoms. PTCs are constructed using semiconductors combined with ceramics or polymers.
Thermistors have important roles in circuit operations. They can supply temperature information for Wheatstone bridges. They can help with meter compensation, because the coil resistance of meter movement is temperature dependent. In medicine, thermistors are used as clinical temperature sensors, in stethoscopes and as probes during surgery. They are found in other devices where temperature detection and control is vital, such as in controlling water, steam and air. In the food service industry, this ability is used in food safety and cooking equipment. Since they regulate temperature and provide protection against overheating in circuits, they can be found in cars, cell phones, computers, amplifiers, home appliances and more.