A resistor restricts current flow; a variable resistor allows for finer control. Rheostats handle entire circuit currents, while potentiometers function as voltage dividers for precise control.
In an electrical circuit, a resistor acts as a fixed restriction on the flow of current without completely turning the current on or off. A variable resistor allows for finer control over current by changing the amount of resistance. As the resistance increases, the amount of current decreases. Some examples of variable resistors are the volume control knob on a radio and a dimmer switch used for a light. Rheostats and potentiometers are two common types of these devices.
Variable resistors have two basic components. The material that provides the resistance is called an element. The item has one of two types of tracks: linear or logarithmic.
A linear trace means that the change in resistance is constant throughout the element. In general, a linear rail variable resistor will have half the total possible resistance when the wiper is directly in the center of the rail. A logarithmic trace has a slow change in resistance on one side and a much faster change on the other. In a logarithmic trace, the midpoint of the element is not half of the total possible resistance.
The moving component, which is used to set the resistance, is called a wiper or brush. Depending on the construction of the resistor, the wiper will often be controlled with a slide switch or knob. To complete the circuit, the wiper makes contact anywhere along the length of the element. The resistance will change based on where the wiper makes contact with the element.
A rheostat is the simplest type of variable resistor, designed to handle the entire current in the circuit. It generally has only two terminals and is placed in series with the load it controls. The circuit is connected directly to the element and the wiper. Rheostats are usually larger than potentiometers due to the loads they must handle.
Visually, the element is the most recognizable part of a rheostat. The length of the wire affects the resistance, so coiling the wire increases the available length, thus increasing the possible resistance. As the wiper passes over the coil, the resistance increases or decreases.
A potentiometer is a variable resistor that allows for much more precise control than a rheostat, as it also functions as a voltage divider. It supplies a different voltage than that available to the circuit from the power supply. It usually has three terminals, one of which is used for the reference voltage, which increases accuracy. The element in a potentiometer is usually a carbon film.
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