A variable resistor is an essential electronic component used to adjust and control electrical resistance in a circuit. It is commonly found in devices like volume controls, dimmer switches, and tuning circuits. This article explains what it is, its types, and how to use it in a circuit.
What is a Variable Resistor?
Definition
A variable resistor is an electronic component. Its resistance value can be adjusted manually or electronically to control the flow of electric current in a circuit. Unlike fixed resistors, their resistance can be varied within a certain range to meet specific circuit requirements.
Basic Principle of Resistance Variation
A variable resistor operates on Ohm’s Law (V = IR). By changing the resistance in a circuit, it controls how much current flows:
Increase resistance → reduces current flow
Decrease resistance → allows more current to flow
The resistance is altered by moving a sliding contact (wiper) along a resistive path, which changes the effective length of the path that current must travel.
Physical Construction
A typical variable resistor consists of:
Resistive Track:
A strip of resistive material (carbon, metal wire, or conductive plastic).
Length: ~1–5 cm.
Forms the path current flows through.
Wiper:
A metal contact arm that slides along the track.
Position sets the effective resistance between the wiper and the terminals.
Terminals (3 in total):
Terminal A: One end of the resistive track.
Terminal B: Opposite end of the track.
Wiper Terminal: Connected to the sliding contact.
Example of Real-World Components
Type
Appearance & Use Case
Rotary Potentiometer
Round knob (dials, volume control)
Slide Pot (Fader)
Straight slider (audio mixers, light dimmers)
Trimpot (Preset)
Tiny screw-adjustable (inside circuit boards)
Thermistor
Button-shaped sensor (thermostats, chargers)
LDR (Photocell)
Disc with wavy lines (streetlights, garden lamps)
Functions of a Variable Resistor
A variable resistor serves multiple purposes in electrical and electronic circuits, allowing fine control over current, voltage, and signal levels. Its main functions include:
Controlling Current Flow:By increasing or decreasing resistance, it regulates how much current passes through a circuit.
Adjusting Voltage Levels: When used as a potentiometer, it acts as a voltage divider to provide a variable output voltage.
Calibrating and Tuning Devices: Helps in fine-tuning parameters in circuits for accurate performance.
Signal Level Adjustment: Modifies audio, radio, or sensor signal levels for desired output.
Speed and Power Control: Regulates the speed of motors or the output power of devices.
Protection of Circuit Components:Prevents overcurrent by limiting the maximum current flow.
Variable Resistor Symbol
In electrical schematics, a variable resistor is depicted by modifying the standard resistor symbol. The symbol includes an arrow, indicating that its resistance can be adjusted. The arrow represents the movable wiper contact that changes the resistance value.
Symbol Standards
There are two common standards for resistor symbols:
ANSI (American National Standards Institute)–Uses a zigzag line to represent resistance.
IEC (International Electrotechnical Commission)–Uses a rectangular box to represent resistance.
In both cases, an arrow drawn across or pointing to the resistor indicates it is variable.
Symbol Variations
Rheostat Symbol (Two-Terminal Use)
Based on the resistor symbol with a diagonal arrow across it.
Represents a variable resistor used to control current flow.
Only two of the three terminals are connected in the circuit.
Potentiometer Symbol (Three-Terminal Use)
A resistor symbol with an arrow pointing to the middle of the resistor.
Represents a variable resistor used as a voltage divider.
All three terminals (two ends of the resistive track and the wiper) are typically used.
Key Points About the Symbol
Arrow = adjustability: The presence of an arrow distinguishes a variable resistor from a fixed resistor.
Thermistors: Resistance changes with temperature, sometimes used as variable resistors in specific designs.
Variable Resistor vs Potentiometer
Though often used interchangeably, variable resistor and potentiometer have distinct meanings and typical uses in electronics:
Variable Resistor
Definition: A resistor whose resistance value can be adjusted to control current or voltage.
Terminals: Usually two terminals connected (either the wiper and one end of the resistive element).
Function: Primarily used to vary current in a circuit.
Example Use: Rheostats for controlling lamp brightness or motor speed.
Potentiometer
Definition: A three-terminal variable resistor used mainly as a voltage divider.
Terminals: All three terminals are typically used — two ends of the resistive element plus a movable wiper in between.
Function: Adjusts output voltage by varying the position of the wiper along the resistive path.
Example Use: Volume control in audio equipment, tuning sensors.
Key Differences
Feature
Variable Resistor (Rheostat)
Potentiometer
Number of Terminals
2
3
Primary Function
Varies current
Acts as voltage divider
Typical Application
Current control (e.g., dimmer)
Voltage control (e.g., volume knob)
Connection
Two terminals (wiper + end)
Three terminals (both ends + wiper)
Summary
A potentiometer is a specific type of variable resistor. It is designed for three-terminal voltage division. While a variable resistor (or rheostat) typically uses two terminals for current control. Understanding this helps in selecting the right component based on circuit needs.
Variable Resistor in a Circuit
Example 1: Current Control (Dimming an LED)
>>This circuit acts like a rheostat.
Components:
Battery (3-9V)
Fixed Resistor (e.g., 330Ω)
LED (any color)
Variable Resistor (e.g., 10kΩ Potentiometer used as a variable resistor)
Wiring:
Connect the battery's positive (+) terminal to one fixed leg of the variable resistor.
Connect the variable resistor's wiperterminal to one leg of the fixedresistor.
Connect the other leg of the fixed resistor to the anode(long leg) of the LED.
Connect the cathode(short leg) of the LED back to the battery's negative (-) terminal.
How It Works:
When you turn the knob on the variable resistor, you change how much resistance is in the path beforethe LED and fixed resistor.
High resistance = less current flows = dim LED.
Low resistance = more current flows = bright LED.
The fixed resistor prevents the LED from blowing out if the variable resistor is turned to near zero resistance.
Circuit Diagram
Example 2: Voltage Division (Simple Volume Control)
>>This uses the potentiometer in its true three-terminal voltage divider mode.
Components:
Audio Signal Source (like a phone headphone jack)
Potentiometer (e.g., 10kΩ Linear)
Amplifier/Speaker (or just headphones)
Wiring:
Connect the input audio signal wire to the potentiometer's Aterminal.
Connect the Bterminal to ground (GND).
Connect the Wiperterminal to the input wire of your amplifier or headphones.
The return wire (ground/shield) from the input signal and the amplifier should also connect to the potentiometer's Bterminal (ground).
How It Works:
The full audio signal voltage exists between A(signal) and B(ground).
As you move the wiper (Wiper), you are effectively picking off a fraction of that voltage relative to ground.
At the Bend, the wiper output is near 0V (silence).
At the Aend, the wiper output is near the full input voltage (loudest).
Positions in between give different volume levels.
Circuit Diagram
Real-World Applications
Dimmer Switches: Controlling light levels.
Volume/Tone/Balance Knobs: On stereos, radios, TVs, mixers.
Fan Speed Controls: Ceiling fans, computer case fans.
Speed Controls: Small DC motors in toys, power tools.
Game Controllers: Analog joysticks and triggers use potentiometers to sense position.
Calibration Trimmers: Inside almost every complex electronic device (TVs, computers, power supplies).
Variable resistor perform several critical functions: controlling current, dividing voltage, adjusting signal levels, fine-tuning circuits, and sensing environmental changes.
Whether it's the rotary potentiometer on your stereo, the tiny preset trimmer built into your phone, or thermistors monitoring temperature, the ability to change resistance is crucial to building controllable and responsive electronic systems.
Frequently Asked Questions
Is a potentiometer a variable resistor?
Yes, a potentiometer is a type of variable resistor. It is a three-terminal resistor with a sliding or rotating contact. You can adjust the resistance value within a circuit.
What are the three common types of variable resistors?
The three common types of variable resistors are potentiometers, rheostats, and trimmer resistors.
What is another term for a variable resistor?
Another term for a variable resistor is a rheostat or potentiometer, depending on its specific use. Both terms refer to resistors with adjustable resistance, but they are often used in slightly different contexts.
What is the purpose of a variable resistor?
A variable resistor is used to adjust electrical resistance in a circuit. It allows for adjustment of current, voltage, or signal strength within a circuit.
How to connect a variable resistor on a breadboard?
To connect a variable resistor on a breadboard, insert its three pins into different rows. Connect the fixed ends to power and ground. And link the sliding end to the load or signal output.
What is the difference between a potentiometer and a variable resistor?
A potentiometer is a three-terminal variable resistor used as a voltage divider to control voltage. While a variable resistor generally refers to any device that adjusts resistance, including rheostats for controlling current.
Why would you use a variable resistor?
You would use a variable resistor to dynamically adjust resistance in a circuit, enabling control over current, voltage, or signal levels. This adjustability is valuable for a variety of applications where fine-tuning or dynamic control is needed.
What is a 3-wire variable resistor called?
A 3-wire variable resistor is commonly called a potentiometer. It consists of a resistive element and a movable wiper.
How to test a variable resistor?
To test a variable resistor, use a multimeter to measure resistance between its terminals. Turn the knob to ensure the resistance changes smoothly and matches its rated value.
Where would you find a variable resistor?
You would find a variable resistor in volume controls, dimmer switches, sensor calibration devices, radios, TVs, and lighting controls.
What is a substitute for a variable resistor?
A substitute for a variable resistor could be a fixed resistor network with switches or a digital potentiometer for electronic control.
Liam Carter is an accomplished Senior Electronic Engineer with over a decade of expertise in the design, development, and optimization of core electronic components. His career has focused on pioneering advancements in semiconductor devices, including precision resistor networks, high-frequency transistor architectures, and innovative IC packaging solutions. With extensive experience in circuit simulation, failure analysis, and thermal management strategies, he has successfully led cross-functional teams in delivering robust electronic systems for industrial automation and IoT applications. His technical leadership in material selection, signal integrity validation, and miniaturization techniques has consistently elevated product performance while reducing manufacturing costs, solidifying his reputation as a forward-thinking innovator in electronic component engineering.
