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Types of Resistors-Variable resistors

Types of Resistors-Variable resistors
Pre-sets and potentiometers are commonly used types of variable resistors. These are mostly used for voltage division and setting the sensitivity of sensors. These have a sliding contact or wiper which can be rotated with the help of a screw driver to change the resistance value. In the linear type, the change in resistance is linear as the wiper rotates. In the logarithmic type, the resistance changes exponentially as the wiper slides. The value is meant to be set correctly when installed in some device, and is not adjusted by the device's user.

The variable can have one or two switches in-built where the resistor operates for the ON state of the switch(s). Such resistors were mostly used for volume control in older TV and radio circuits.
There may also be four-tab variables where the fourth lead is for feedback signal and placed near the first tab. Wire wound variable resistors are used for very precise control of resistance.

The wiper may also be rotary (as in most pre-sets), sliding or disc shaped (as used in pocket radios for volume control).

Semi variable resistors
These are two terminal variable resistors designed for handling higher voltages and currents. These are constructed by resistive wire wrapped to form a toroid coil with the wiper moving over the upper surface of the toroid, sliding from one turn of the wire to the next. 
A rheostat is also made from resistance wire wound on a heat-resisting cylinder with the slider made from a number of metal fingers. The fingers can be moved along the coil of resistance wire by a sliding knob, thus changing the tapping point.

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