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Types of Resistors-Special Resistors

Types of Resistors-Special Resistors 
Thermistors are special resistors whose resistance changes with the temperature. If the resistance increases with increase in temperature, then it is called positive temperature coefficient (PTC) or posistors. If the resistance decreases with the increase in temperature, then it is called a negative temperature coefficient (NTC).

An NTC can be replaced by a transistor with a trimmer potentiometer. PTCs are mostly used as current limiter for circuit protection. As the heat dissipation of resistor increases, the resistance is increased thereby limiting the current.
The NTCs are mostly used for temperature sensing, replacement of fuses in power supply protection and for low temperature measurements of up to 10K. These are constructed using sintered metal oxides in ceramic matrix.

B.Light dependent resistors (LDR):
LDRs have cadmium sulfide zigzag tack whose resistance decreases as the light intensity incident on it increases. In the absence of light, its resistance is in mega ohms but on the application of light, the resistance falls drastically. 
These resistors are used in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

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