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What is the main difference between and potentiometer, rheostat and a varistor? And how do the application of the three in a circuit differ? Or to be more concise, in a variable power supply why would you want to use each one rather than the others, or what benefits would one give over another in a variable power supply circuit.

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A Varistor is a voltage-dependent-resistor. Its resistance depends on the voltage applied to it, and it is a 2-terminal device.
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A Potentiometer is a fixed resistance with a mechanically adjustable wiper which can be moved from one end of the fixed resistance to the other, forming an adjustable voltage divider. It is a 3-terminal device.
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A Rhoestat is not the same thing as a Potentiometer, since a Rheostat is typically considered to be mechanically variable resistor and is a 2-terminal device.
You can use a Potentiometer as a Rheostat by simply ignoring one of its fixed-resistor terminals.
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The application in-circuit of a Varistor vs a Potentiometer is not really comparable as they are completely different devices.

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  • \$\begingroup\$ Ok, thanks for your answer. Can you elaborate by referencing in what type of power circuit you would want to use each one and why. \$\endgroup\$ – RedDogAlpha Mar 9 '16 at 19:14
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    \$\begingroup\$ My layman's understanding is that potentiometers are used in low-power applications - typically to control a signal amplitude. Conversely, rheostats are used in higher power applications to actually control power, e.g. a light dimmer switch. \$\endgroup\$ – Digital Trauma Mar 9 '16 at 19:59
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    \$\begingroup\$ Calling a rheostat a potentiometer is pretty damn common, however. \$\endgroup\$ – hobbs Mar 9 '16 at 23:13
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    \$\begingroup\$ @hobbs Probably because a potentiometer with one end disconnected is a rheostat. \$\endgroup\$ – user253751 Mar 10 '16 at 2:54

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