A Summary of Rheostats

Rheostats are variable or adjustable resistors used to divide voltage in applications that require the adjustment of resistance in an electrical circuit. Similar to potentiometers, rheostats are commonly used in applications that call for variable control for higher amounts of voltage. In this blog, we will discuss the construction, operation, and applications of rheostats.

Construction

Many rheostats are wire-wound and feature a lengthy conductive material wrapped into a spiral. The resistance element within rheostats can be a ribbon or metal wire, a conducting liquid, or carbon, depending on the application’s current flow. For average current flows, the metallic type is used. For small current flow, the carbon form is used, and for large current flow, the conducting fluid is most ideal. To prevent them from contaminants, the contacts and coils within rheostats are sealed within a casing. Rheostats commonly have a ceramic center and feature three terminals made from resistive wire wrapped to create a coil and wiper that glides along the coil.

When choosing a rheostat, the current rating is typically a more important factor than the wattage rating. If you are using a rheostat for motor control, it is crucial to understand that while all types of DC motors can be speed controlled, only some AC motors are controllable via a rheostat. Therefore, you must acquire the appropriate type of AC motor when speed control is necessary. Most rheostats consist of a round or flat shaft allowing users to attach a knob, and smaller-sized rheostats have screw-driver slots that make for easy adjustments.

Operation

The basic principle related to the effect rheostats have within a circuit is Ohm’s law, which states the current is inversely proportional to resistance for any given voltage. This means that the current decreases as the resistance increases, and vice versa. Current enters the terminal of the rheostat, flows through the wire coil and contact, and exits through the other terminal. Rheostats do not have polarity and behave the same when the terminals are reversed.

Applications

Many light dimmers utilize rheostats to limit the current flowing through a lightbulb, therefore varying the illumination. The higher the resistance, the dimmer the bulb will glow. However, certain types of lights, such as fluorescent and gas discharge lamps, will not work effectively with rheostats. This is because these lights use resistive, inductive, or electronic ballasts which maintain a constant current for the lighting circuit. Rheostats would have no effect on these lights and could potentially harm the ballasts. Rheostats are also used in motor controllers, where they control a motor’s velocity by limiting the flow of current, and in many small devices such as blenders, mixers, fans, power tools, and more.

Rheostats remain a common electronic component used to control the outflow of current in a circuit. Despite this, they are becoming less common as the use of solid state devices like triacs and silicon controlled rectifiers (SCR) grows. Generally speaking, triacs are more energy competent and reliable due to the lack of mechanical components. Rheostats can fail if their contacts become dirty or the coil wire corrodes and breaks, while solid state devices are relatively immune to both of these conditions.