A potentiometer is a resistor with three lugs that can serve as a voltage (or current) divider or as a variable resistor. The figure shows how the middle lug, called a wiper, is movable and moves in the range from the first to the third lug. By moving the wiper, we are changing the ratio between resistances. If we connect the wiper to lug 1, there is no resistance between the wiper and lug 1, and the resistance between the wiper and lug 3 is at the maximum. As we slide the wiper clockwise, the resistance between the wiper and lug 1 increases, while the resistance between the wiper and lug 2 decreases.
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If we connect the positive voltage to lug 1 and the ground to lug 3, the output of the wiper will become a voltage divider as presented in the following figure.
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The formula to calculate the output value of voltage is the following:
The voltage divider is extremely important in electronics because by choosing the right ratios between R1 and R2 resistances the output voltage can be adjusted to any fraction of the input voltage.
Finally, if we short one of the lugs with the wiper, and take the output of the wiper, we get a variable resistor also called a rheostat. Now, moving the wiper effectively changes the resistance applied in the circuit.
(Source: referenced)
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