Understanding Rheostats: A Comprehensive Guide to Their Function and Mechanics

A rheostat is a variable resistor designed to control the flow of electrical current in a circuit. By adjusting resistance, rheostats manage current without interrupting the circuit.

Rheostats are a widely used type of electromechanical variable resistor, enabling users to manually adjust their resistance value as needed. Another common variable resistor is the potentiometer. Although rheostats and potentiometers share similarities in form and function, they differ in operation and behaviour depending on the specific application.

This diagram provides a clear representation of how rheostats are integrated into circuits, illustrating their components and connections. The terminals serve as the input and output points for electrical flow, while the sliding contact adjusts resistance by moving along the resistive element. The resistive element, which is a core component of the rheostat, is the material responsible for limiting the flow of current.

To help you fully understand the diagram, we give you a detailed breakdown of the key components found in a rotary rheostat:

• Terminals: Typically, rotary rheostats have two or three terminals. One terminal connects to the start of the resistive element and another to the wiper, which moves along the element to adjust resistance. The third terminal may be used for additional circuit configurations.
• Wiper (Adjustable Contact): The wiper is the movable contact that slides or rotates over the resistive wire. Its position determines the length of the resistive path and, consequently, the resistance value.
• Resistive Wire: This is the element that provides resistance. It is usually made from materials like nickel-chromium alloy or similar high-resistance materials, designed to handle various power ratings.

This is the IEC (International Electrotechnical Commission) standard version of a rheostat circuit symbol. It features a resistor line with an arrow, signifying adjustability. In circuit diagrams, this symbol helps identify its function and orientation. They are typically prevalent in industrial circuit diagrams and educational resources, often appearing in electric motor control setups, lighting systems, and laboratory equipment.

Sometimes, the symbol used for rheostats is mistaken for the potentiometer's IEC symbol. However, the potentiometer symbol adds a third terminal and sometimes a voltage-divider configuration.

A linear rheostat is a two-terminal variable resistor with a sliding contact or wiper that moves along a wire coil arranged in a straight line on an insulating cylinder. This sliding motion operates as a two-way mechanism, which is why these devices are often referred to as slide or sliding rheostats.

Moving the wiper in one direction increases the length of the resistive path, forcing electricity to travel through more of the wire coil and reducing the current flow. Conversely, moving the wiper in the opposite direction shortens the path, decreases resistance, and allows more current to flow.

A rotary rheostat functions similarly to its linear counterpart but features a circular or U-shaped resistive wire coil. The contact wiper is moved using a rotating dial or knob. As the dial is turned, the wiper moves along the wire coil, either extending or reducing the resistive path between the fixed and sliding terminals.

Ohmite, mainly known as Acrol Ohmite, is a leading manufacturer of rheostats and variable resistors, known for their durable wire-wound designs, innovative shaft options, and adjustable arm technologies. These features make Ohmite rheostats ideal for high-power motor control and current regulation, particularly in scenarios requiring adjustments to live circuits.

Ohmite rheostats are UL-rated and available in 11 standard sizes with a variety of configurations, making them a reliable and versatile choice for demanding circuit control applications.

A plug-in rheostat is an external variable resistor designed to be attached to existing appliances or circuits. It is typically used as an aftermarket solution to add adjustable power settings to devices that do not have this capability built into their original circuitry.

Common examples include plug-in dimmers for lamps, fans, or blowers. These rheostats allow users to control power levels with ease, even in devices not initially designed for variable operation.

When choosing the type of rheostat, you need to consider the following:

• Power Ratings: Rheostats are subject to power ratings that define the maximum amount of current they can safely handle. Exceeding this rating can cause overheating or damage to the device.
• Heat Generation: High resistance levels in a rheostat can result in significant heat generation. Therefore, if you’re using them in particularly humid and dry areas in Australia, proper ventilation or cooling may be necessary to prevent equipment failure.
• Application Suitability: Make sure that the type of rheostat you choose will suit your specific application. For example if you’re planning to use it for testing electrical equipment, then linear rheostats would be appropriate. For compact devices, rotary rheostats may be suitable due to their ergonomic design.
• Adherence to Australian Standards: Additionally, it's crucial to confirm that the rheostat complies with Australia's electrical equipment standards to guarantee safety and regulatory adherence.

Rheostats are essentially two-terminal variable resistors where the current flows through the entire resistive element. These components are constructed with wire-wound resistance elements, often made of materials like alloys of copper and nickel, which can handle high currents and dissipate significant heat. Their robust construction makes them suitable for demanding applications where durability and power handling are critical.

Compared to rheostats, potentiometers are three-terminal devices primarily used for voltage control. They function as voltage dividers, allowing you to tap off a variable portion of the applied voltage. The potentiometer has a resistive element with a wiper that slides across it. By adjusting the wiper's position, you change the ratio between the two segments of the resistive element, effectively altering the voltage at the output terminal. These components are available in various types and constructions, including a carbon film, cermet, and wire-wound. Carbon film potentiometers are common in consumer electronics due to their low cost, while cermet and wire-wound types offer better precision and stability. For further insight, check our guide on how different potentiometer types work.