A Complete Guide to Rheostats

Our rheostats guide explains their uses, how they work, the different types & how they differ from potentiometers.

Rheostats Banner

In this rheostats buyer’s guide, we will explain exactly what they are and how they are used.

We will provide readers with an introduction to how rheostats work, and how they differ from potentiometers.We will also cover the various main types of rheostats for sale online, and talk about rheostat wiring, voltages and testing.

What is a Rheostat?

A rheostat is a type of variable resistor. When placed into an electrical circuit, a rheostat can be used to adjust and control the amount of current flowing around it. They do this by varying the resistance on the circuit, without interrupting the supply of power.

Rheostats are one common type of electromechanical variable resistor. This means that they allow users to vary their ohmic resistance value manually when needed. The other common type of variable resistor is a potentiometer. While these two components are quite similar in form and function, they operate and behave somewhat differently depending on the specific use case.

Rheostat vs Potentiometer

Rheostats and potentiometers are both types of variable resistor. The two components are similar in both form and function but there are key differences between a rheostat and a potentiometer.

The main difference with potentiometers is that they operate as a potential divider, commonly referred to as a voltage divider. This means that they make use of the fact that voltages drop across resistors. In practice, a potential divider or potentiometer acts as two resistors in series, dividing the voltage and creating a new V-value for the section of the circuit in between.

Potentiometers are three-terminal components. There is a terminal at either end and a third one connected to the sliding wiper, which adjusts the resistance by moving along a coiled wire track.

On the other hand, a rheostat is a two-terminal variable resistor that usually adjusts current, rather than voltage. It is often used for controlling higher currents. This is largely because it is more mechanically robust when attached to a PCB due to having fewer terminals connected.

A rheostat is connected in series with the load, rather than in parallel with the voltage source (as in a potentiometer). It offers only linear taper, whereas a potentiometer offers both linear and logarithmic taper. Taper in this context refers to the relationship between wiper position and the resistance that the component is providing.

In summary, the easiest way to remember the difference between a rheostat vs. potentiometer is as follows:

Rheostat

Rheostats are two-terminal devices, used to control current across a circuit.

Potentiometer

Potentiometers are three-terminal devices, used to control voltage across a circuit.

What is a Resistor Used for?

A resistor is used to control the amount of voltage or current flowing across an electrical circuit. A variable resistor - such as a rheostat or potentiometer - can be easily altered by the user. The basic function of a rheostat in a circuit is to adjust the amount of current flow as and when required. As such, potential rheostat uses in electrical circuitry are many and varied.

By far their most common application is as power control devices. This covers a broad range of potential roles and functions. Resistors like rheostats can be used as dimmers for electric lights, volume control audio circuits, temperature adjustment dials, and much more.

While rheostats were once the go-to technology for these applications, many modern circuits are now being designed with high-speed electronic switching functions to control and adjust current or voltage flow. Even so, rheostats are still widely used in specialised roles like circuit tuning and calibration.

How Does a Rheostat Work?

If we want to alter the amount of current flowing across a circuit, we have two options. We can change either the resistance on the circuit or the voltage being applied to it.

This relationship is represented by the basic Ohm’s Law equation, commonly written as V = IR, where V = voltage, I = current, and R = resistance. Strictly speaking, V=IR is the formula for the definition of electrical resistance, although for most practical purposes this is functionally interchangeable with Ohm’s Law proper.

It should be noted that a rheostat does not change voltage directly; the applied voltage remains constant across both component and circuit, unlike in a potentiometer. Instead, a rheostat adjusts current (and thus in effect voltage) by increasing or decreasing resistance. Precisely how it does this will depend on the type of rheostat resistor being used. Different rheostats have varying maximum power and resistance ratings, stated in Watts and Ohms respectively.

A linear rheostat, for example, changes resistance by dragging a sliding wiper that is in contact with a straight wire coil. Meanwhile, controlling resistance using a rotary rheostat involves turning a knob or dial to sweep the wiper around a U-shaped wire coil. Other basic rheostat types use similar but slightly different methods.

To answer the question ‘how do rheostats work?’, it is important to understand how to wire a rheostat circuit. We need to highlight the role of the terminals and connections on the resistor:

  • A potentiometer uses three terminals - one at each fixed end, and a third connection at the sliding contact or wiper

  • A rheostat uses just two active terminals, one of which is fixed to either fixed end (the other can be left alone or grounded), while the second is connected to the wiper

  • As such, a potentiometer can be made to function as a rheostat by disconnecting one of the fixed terminals. A rheostat, however, cannot be made into a potentiometer by reversing the process

As the rheostat is connected via one fixed terminal and one sliding contact, the active length between these two connections can be altered when the wiper is moved along a wire coil. The path taken by the electricity bridging these connections is therefore either elongated or shortened.

In simple terms, this mechanical length adjustment is what creates variable resistance on the circuit, enabling the rheostat to alter and control current flow from A to B.

Types of Rheostats

When we answer the question ‘what is a rheostat?’, we need to mention various key types and styles of variable resistor. All do essentially the same thing, but their physical layouts and ideal uses are different.

Linear Rheostats

A linear rheostat is a two-terminal variable resistor in which the sliding contact, or wiper, is attached to a wire coil running in a straight line along an insulating cylinder. The mechanical operation of a linear rheostat thus takes the form of a two-way slider. They are often called slide rheostats (or sliding rheostats) for this reason.

Moving the wiper in one direction lengthens the electricity’s journey across the resistor, by forcing it along more of the wire coil due to the position of the sliding contact. This decreases current flow. Moving it in the opposite direction shortens the path, lowering resistance and increasing current.

Linear rheostats are often used in laboratory and testing environments, especially to run different currents across a circuit and check for stability.

Rotary Rheostats

A rotary rheostat works similarly to a linear version, except the resistive wire coil is arranged in a horseshoe or U-shape, and the contact wiper is controlled via a turning dial or knob. The basic operating principle is much the same in both cases.

For a rotary rheostat, turning the dial in either drags pulls the wiper back and forth around the wire coil it’s in contact with. This either extends or shortens the length of resistive wire sitting between the fixed and sliding terminals.

Current is affected in an inverse relationship with the length of wire exposed. A shorter path provides decreased resistance and thus higher current, while a longer path increases resistance for lower current.

Ohmite Rheostats

Ohmite is a leading brand name in the rheostat and variable resistor market. Ohmite rheostats are often preferred for their robust wire-wound designs, multiple shaft designs and adjustable arm technologies.

These engineering strengths result in a widely used rheostat in high power motor and current control applications, particularly where live circuit control is needed. Ohmite rheostats benefit from a UL rating with 11 standard sizes and a range of flexible configurations, making them a versatile choice for circuit control in situations where the live current is flowing during adjustment.

Plug-In Rheostats

A plug-in rheostat is a type of external rheostat switch that can be attached to certain appliances or circuits as an aftermarket addition. The aim is usually to give a device a range of adjustable power settings when it does not have this function built into its core circuitry.

Common examples include lamps, fans, and blowers. Plug-in rheostats are often sold as plug-in dimmers, even when they are intended for functions other than adjusting a light source.

Related Guides