Potentiometer vs. Rheostat: Which Should You Use?

I remember as a young freshman college student playing with potentiometers for the first time in my introductory electronics class. You suddenly start to realize where these components get used in everyday devices and you never knew it. Potentiometers and rheostats are two versions of a variable resistor that provide a simple way to control current flow and voltage drop in different legs of a system.

Although these components provide the same basic function, they are quite different and are not always perfect replacements for each other. So what makes these components so different, and why are they preferred in different applications? Here’s when you should use a potentiometer vs. rheostat and what to look for in datasheet specs to select the right component.

Potentiometer vs. Rheostat Construction

Potentiometers and rheostats have similar construction and function in similar ways: by turning a knob you can control the current and voltage distribution in a circuit. Seems simple enough, but the way these components are built makes them better options for different applications.

The most obvious difference between a potentiometer vs. rheostat is the number of terminals; potentiometers have three (input and 2 outputs) while rheostats have two (1 input, 1 output). Thanks to the potentiometer’s three terminals, it can be used as a rheostat when one of the terminals is left floating. However, the reverse is not entirely true; a rheostat cannot be used as a potentiometer unless you add at least one other resistor and ground the rheostat’s output.

A potentiometer is basically a variable voltage divider; as the knob on the device is turned, a sliding contact creates a voltage divider between the input and the two outputs. In other words, a potentiometer is simply meant for voltage division. With a rheostat simply being a potentiometer with infinite resistance at the second output, it modulates total power received by the load connected to the output. If the rheostat and load resistance are equal, then maximum power is transferred to the load component.

There are digital potentiometer ICs that provide digital control over the voltage being split between a voltage divider. These ICs provide a voltage output across the bottom half of a voltage divider in discrete steps (defined by some number of bits). These ICs can be set to operate in rheostat mode, giving a digitized voltage output rather than continuous. A great example is the MCP40D19T-503E/LT from Microchip, which provides 7-bit output over an integrated resistor network via an I2C interface.

These days, the term “rheostat” is sometimes used interchangeably with “2-terminal potentiometer” or something similar, but some component manufacturers still make a specific distinction between these components. An example is the RT025AS1501KB by Vishay, which provides the type of precise mechanical resistance tuning a designer would expect from a typical potentiometer.

Selecting a Mechanical Potentiometer vs. Rheostat

When selecting these components, it’s important to note that they have slightly different specifications, even though a potentiometer can be configured to provide the same functions as a rheostat. Here are some of the important specifications:

• Maximum Resistance. Pay attention to the maximum resistance of the device as this will determine your voltage/current limit delivered to the load. Typical values range from 10’s of Ohms to kOhms. This should be carefully paired with your power source and current requirements. 

• Wiper Resistance. The wiper resistance is quite important in a rheostat and should be very small to provide an accurate amount of current to the load. For a potentiometer, if we are generally delivering some voltage to a high impedance load, the wiper resistance is not so important as it will always be much smaller than the load resistance. In any case, you need to carefully analyze how large of a wiper resistance you can tolerate as the wiper resistance appears in series with the load resistance. 

• Taper style. The taper tells you how the resistance of the device, or rather the resistance division across the slide, varies as the wider is moved. Linear taper is great when you need control functions that are linear functions of the voltage/current delivered to the load. An example might be an operational amplifier with the component placed in the feedback loop. Other taper styles are logarithmic and an audio taper that is specifically defined for audio systems. 

• Power rating. Just as regular resistors have a power rating, so do potentiometers and rheostats. Exceeding the power rating can damage the component. 

• Tolerance and hysteresis. Because potentiometers and rheostats are electromechanical components, the resistance tolerance can vary by 10-20% in some cases due to the winding construction and hysteresis. Precision components will have smaller tolerances and higher reproducibility (lower hysteresis).

Any electronics lab should keep some potentiometers or rheostats lying around for prototyping or breadboarding. They are quite useful for designing simple circuits before you start creating schematics or a PCB layout. Once you’ve designed a basic circuit with the desired potentiometer/rheostat resistance value, take an ohmmeter and measure across the terminals; this tells you the resistance values you need in your circuit.

When you’re ready to decide between a potentiometer vs. rheostat for your analog circuits, try using the only electronics search engine with advanced search and filtration features. Octopart gives you a complete solution for sourcing electronic components and helps you drill down to exactly the components you need. Take a look at our electromechanical components page to find more components for your analog system.

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