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What Are Rheostats?

A rheostat is a type of variable resistor that, when placed into an electrical circuit, can be utilized to adjust and control the quantity of current flowing around it. It achieves this by varying the resistance on the circuit, without interrupting the power supply. Rheostats are considered a common type of electromechanical variable resistor, meaning that they enable users to vary their ohmic resistance manually when necessary.

Generally, rheostats consist of a coil of wire, a sliding contact, and two terminals. Similar to other circuits, rheostats are represented by a particular symbol. Delineated by the IEC (International Electrotechnical Commission), the standard version of a rheostat circuit symbol looks like a rectangle positioned in the center of a single horizontal line, and a vector cutting diagonally across the rectangle with its head pointing to the top right corner. 

Another common type of variable resistor is a potentiometer that, while similar in form and function to rheostats, operates and behaves differently. The central difference between them is that potentiometers work like voltage dividers, meaning that they take advantage of the fact that voltages drop across resistors. Essentially, a potentiometer acts as two resistors in series by dividing the voltage and creating a new V-value for the section of the circuit in between.

Potentiometers are three-terminal components, wherein there is a terminal at each end and a third one connected to the sliding contact. As previously mentioned, rheostats are two-terminal resistors that adjust current, rather than voltage. In fact, a rheostat is usually utilized for controlling higher currents because it is more mechanically robust when attached to a PCB due to having fewer terminals connected.

Rheostats are typically connected in series with the load, instead of in parallel with the voltage source as is the case with potentiometers. More than that, they only offer linear taper, whereas potentiometers offer both linear and logarithmic taper. For additional context, taper is the relationship between the wiper position and the resistance that component is supplying.

Put simply, a rheostat is a two-terminal device that controls current across a circuit, while a potentiometer is a three-terminal device that controls voltage across a circuit.

By far, the most common application of rheostats are as power control devices. For example, they can be used as dimmers for electric lights, volume control for audio circuits, temperature adjustment dials, and more. While many modern circuits are designed with high-speed electronic switching functions, rheostats still find use for circuit tuning and calibration. There are a wide range of rheostat types, some of which include linear rheostats, rotary rheostats, ohmite rheostats, and plug-in rheostats.

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