A variable resistor is an electronic component whose electrical resistance value can be adjusted or changed. Unlike a standard fixed resistor that has a constant resistance, a variable resistor allows you to control the amount of current flowing through a circuit or to adjust the voltage level within it.
How Does a Variable Resistor Work?
A variable resistor typically consists of three main parts:
- A Resistive Element: A track or a wire made of a resistive material (like carbon or a metal alloy).
- A Wiper (or Slider): A movable contact that slides along the resistive element.
- Terminals: Usually three terminals. Two are connected to the ends of the resistive element, and the third is connected to the wiper.
The working principle is simple. By moving the wiper along the resistive track, you change the length of the resistive material through which the current has to flow. A longer path means higher resistance, and a shorter path means lower resistance. This allows for smooth, continuous adjustment of the resistance between a minimum and a maximum value.
The Circuit Symbol for a Variable Resistor
In electronic circuit diagrams, a variable resistor has a standard symbol that distinguishes it from a fixed resistor. There are two common symbols:
- IEC Symbol: A rectangular box (representing the resistor) with an arrow drawn through it diagonally.
- American Symbol: A zigzag line (representing the resistor) with an arrow drawn through it.
The arrow signifies that the resistance is adjustable.
Types of Variable Resistors
While the general principle is the same, variable resistors come in different forms, each designed for a specific application. The two most common types are the potentiometer and the rheostat.
| Type | Symbol | Primary Function | Connection | Common Applications |
|---|---|---|---|---|
| Potentiometer (‘Pot’) | A resistor symbol with a third terminal connected by an arrow. | To function as a **voltage divider**. It is used to control voltage in a circuit. | All three terminals are used. An input voltage is applied across the two fixed terminals, and a variable output voltage is taken from the wiper terminal. | Volume controls in audio equipment, dimmer switches for lights, joystick controls. |
| Rheostat | A resistor symbol with an arrow, where the third terminal is either connected to one of the fixed terminals or left unused. | To control **current**. It is used to vary the amount of current flowing through a circuit. | Only two terminals are used (one fixed terminal and the wiper). It is connected in series with the load. | Controlling motor speed, high-power applications. |
| Trimpot (or Trimmer Potentiometer) | Similar symbol to a potentiometer, often smaller. | Used for fine-tuning or calibrating a circuit. | All three terminals are used. | Used inside electronic devices for adjustments that are made infrequently, such as during manufacturing or by a technician. |
Practical Applications
Variable resistors are everywhere in the electronic devices we use daily.
- Volume Knobs: The volume control on your stereo is a potentiometer. Turning the knob moves the wiper, adjusting the voltage sent to the amplifier, which changes the volume.
- Light Dimmers: A rotary dimmer switch uses a variable resistor to control the amount of current flowing to the light bulb, thereby changing its brightness.
- Fan Speed Regulators: Old-style fan regulators use a rheostat to control the fan’s speed by varying the current.
- Joysticks: The movement of a joystick is translated into changing resistance values by two potentiometers (one for each axis), which the game console then interprets as movement.
Understanding components like a variable resistor is fundamental to electronics, a key subject in any course related to automobile engineering (for modern vehicle electronics) or printing engineering (for machine control).
Frequently Asked Questions (FAQs)
What is a variable resistor?
A variable resistor is an electronic component that allows you to change its resistance value. It is used to control the flow of current or to adjust the voltage in a circuit. Common examples include potentiometers and rheostats.
What is the difference between a potentiometer and a rheostat?
A potentiometer is a three-terminal device typically used as a voltage divider to control voltage. A rheostat is a two-terminal device used in series with a load to control current. You can use a potentiometer as a rheostat by using only the wiper and one of the end terminals.
What is the symbol for a variable resistor?
The standard symbol for a variable resistor is the symbol for a fixed resistor (a rectangle in the IEC standard or a zigzag line in the American standard) with an arrow drawn through it to indicate that it is adjustable.
Where are variable resistors used?
Variable resistors are used in a wide range of applications where adjustment is needed. Common examples include volume controls on audio devices, dimmer switches for lights, speed controls for motors and fans, and joysticks for gaming.
How does a variable resistor control volume?
A volume knob uses a potentiometer. The audio signal is an input voltage. By turning the knob, you move the potentiometer’s wiper, which ‘taps off’ a variable amount of that voltage. This adjusted voltage is then sent to the amplifier, making the sound louder or softer.