Thermistor is a semiconductor device passive in nature which exhibits a large change in resistance with a change in the applied temperature. There are two types of thermistors- positive temperature coefficient (PTC) and negative temperature co-efficient (NTC) thermistors. In NTC thermistor construction, two or more metal oxides are mixed in the presence of some binding materials. This mixture is then sintered at a high temperature and then hermetically sealed in a glass enclosure. The two popular NTC thermistors are bead type thermistor and metalized-surface-contact thermistor. The two types of PTC thermistors are Silistors and Switching PTC thermistors. Silisters are silicon resistors. Switching PTC thermistors are made of polycrystalline ceramic materials which by the addition of appropriate dopants can be converted into semiconducting materials. The ceramic materials commonly used here are barirm titanate, lead titanate and Strontium titanate. Examples for doping materials are Silica, Yttrium, Manganese and Tantalum. The main application of PTC thermistor is in resettable fuses.
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