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ThermistorsThermistors are often classified as varistors, but they require a temperature change to produce the variations in resistance. Thermistors are made of semiconducting materials that have a large variation in resistance with change in temperature.43 Thermistors are made in many forms. In one type the semiconducting material is controlled by the surrounding temperature. In the directly heated type, the current in the circuit in which the thermistor is inserted passes through the semiconducting material and heats it. In the indirectly heated type the semiconducting material is heated by a separate heater that is placed in the controlling circuit.
The characteristics of a directly heated thermistor are shown in Fig. 44. As noted, this has a negative resistance characteristic, because the voltage drop decreases with increase in current. Thermistors possess many other interesting characteristics and are used in many ways.43,44
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Last Update: 2011-04-06