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Startup rheostat of VL10 3 kV DC electric locomotive
Startup rheostat of VL10 3 kV DC electric locomotive

Variable resistor //

A variable resistor can be made from a wirewound element, rather than a solid strip of material.

This is called a rheostat.

A rheostat can have either a rotary control or a sliding control. This depends on whether the nichrome wire is wound around a dough-nut-shaped form (toroid) or a cylindrical form (solenoid).

Rheostats always have inductance, as well as resistance. They share the advantages and disadvantages of fixed wirewound resistors.
Toroid and slide wire rheostats
Left: Toroid wire rheostat; Right: Slide wire rheostat

A rheostat is not continuously adjustable, as a potentiometer is. This is because the movable contact slides along from turn to turn of the wire coil. The smallest possible increment is the resistance in one turn of the coil. The rheostat resistance therefore ad-justs in a series of little jumps.

Connection of a rheostat in a variable-voltage power supply
Figure 1 – Connection of a rheostat in a variable-voltage power supply

Rheostats are used in high-voltage, high-power applications. A good example is in a variable-voltage power supply.

This kind of supply uses a transformer that steps up the voltage from the 117-V utility mains, and diodes to change the ac to dc. The rheostat can be placed between the utility outlet and the transformer.

This results in a variable voltage at the power-supply output. A potentiometer would be destroyed instantly in this application.

Rheostat Working Principle (VIDEO)

Resource: Teach Yourself Electricity and Electronics – Stan Gibilisco

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

One Comment

  1. Thembinkosi Dyeyi
    Sep 16, 2019

    How do you decide which rheostat to use in a circuit, as they have various resistance ratings?

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