Variable Resistor: Definition, Working, Formula, and Applications

A variable resistor is a type of resistor whose resistance can be adjusted according to the requirement. It permits the change in the current or voltage in an electric circuit as per Ohm’s law.

The term ‘variable’ means ‘changing.’ Thus, the resistance of a variable resistor can be varied as per the requirements of the circuit. Variable resistors are important components for various electronic devices like volume controllers, voltage regulators, and calibration devices.

In this article, we will discuss variable resistors and will cover their various concepts, such as definition, working principle, circuit symbols, formula of resistance, and applications.

What is a Variable Resistor?

A variable resistor is a passive circuit element used in electrical and electronic circuits and devices to insert a resistance that can be changed as per requirements. It is simply a resistor with an adjustable resistance mechanism. By adjusting its resistance, it regulates the electric current in the circuit.

A three-terminal variable resistor (two fixed ends and a movable wiper) used as a potential divider is called a potentiometer. If two terminals of the potentiometer are used to get the variable resistance, it is called a rheostat. The resistance can be adjusted mechanically or electronically. The potentiometer whose resistance can be changed electronically is called a digital potentiometer.

Variable Resistor Symbol

The circuit symbol of a variable resistor is shown in the following figure.

Working Principle

We get variation in the resistance by changing the effective length. This means the resistance of a variable resistor is changed by varying its length by some means.

A variable resistor typically consists of a long resistive element and a movable contact terminal called a wiper. The wiper can be moved along the length of the resistive element to adjust its effective length to change its resistance.

How to Calculate Resistance of Variable Resistor?

The value of the resistance depends on the position of the wiper on the resistive element.

We can use the following formula to compute the resistance of a variable resistor for a given wiper position on the element:

Where R is the resistance of the variable resistor in ohms for a given wiper position, R1 is the resistance of the element between ‘x’ and ‘y’, l is the total length of the element, and l1 is the distance between the wiper and end ‘y’ of the element.

For example, if a variable resistor has 100 Ω between ends ‘x’ and ‘y’. The total length of the element is 10 m. If the wiper is placed at a position on the element, where the distance between the point ‘y’ and the wiper is 4 m. Then, calculate the effective resistance.

Here, R1 = 100 Ω, l = 10 m, l1 = 4 m. Then, the resistance R of the resistor for the given wiper position will be,

Hence, it offers a resistance of 60 Ω for the given wiper position.

Types of Variable Resistor

Variable resistors, also known as adjustable resistors or potentiometers, come in several types, each suited for different applications. Here are the main types:

It functions as a resistive divider and is typically used to generate a voltage signal depending on the position of the potentiometer.

1. Potentiometer: A three-terminal resistor with a sliding or rotating contact forms an adjustable voltage divider. The potentiometer is the most commonly used variable resistor. It functions as a resistive divider and serves as a voltage divider. and is commonly utilized to produce a voltage output based on the potentiometer’s position; typical applications are volume controls in audio equipment, tuning circuits, and adjustable power supplies.
2. Rheostat: A two-terminal potentiometer is used as a variable resistor to adjust current. It is used in light dimmers, motor speed control, and heater controls.
3. Trimpot (Trimmer Potentiometer): A small potentiometer typically used for fine-tuning and calibration in circuits. It is often adjusted with a small screwdriver. Common applications include instrument calibration, setting bias in amplifiers, and adjusting sensor sensitivity.
4. Digital Potentiometer (Digital Pot): A digital potentiometer is a type of potentiometer that is controlled electronically rather than mechanically and is typically used in digital circuits. Common applications include Digital volume controls, automatic gain control, and programmable filters.
5. Slider Potentiometer: A sliding potentiometer is adjusted by sliding contact along a track rather than rotating a knob. It is often used in audio mixing consoles, equalizers, and other audio equipment.
6. Multiturn Potentiometer: A potentiometer that provides precise control through multiple turns of the adjustment knob to cover the entire resistance range. Ideal for precision tuning applications, such as in lab instruments and measurement devices.
7. Variable Resistor with Switch: A variable resistor that incorporates a switch to turn the circuit on or off. It is used in devices where both variable resistance and an on/off switch are required, such as in some power tools and appliances.

The following are some important advantages.

• They provide flexibility in the circuit, as they have variable resistance that can be changed as needed.
• It provides precise and continuously adjustable resistance in the circuit.
• They can be fixed easily into an electronic circuit.
• They provide a cost-effective solution for adjustable resistance in a circuit.
• It is a highly versatile component, as it can be used in various types of electronic circuits and devices.

Applications of Variable Resistors

The following are some key applications.

• Used in electronic circuits to control the strength of the electric current. They act as a potentiometer and control the output voltage across the audio device. Logarithmic-type resistors are better than linear ones because they exactly match the human perception of loudness.
• Used in fan regulators for speed control purposes.
• They are also used in light dimmer switches to control the intensity of light produced by the lamps.
• Applications in power supplies to regulate their output voltage and currents.
• Used in audio devices and systems to control the volume, bass, and tone of the output sound.
• Applications in various types of calibration devices and sensors for fine calibration of different types of instruments.
• Used for adjusting the color, brightness, contrast, and position of the picture on a television screen.
• They are used to control the speed, torque, and direction of motors, servos, fans, pumps, etc.

Conclusion

In conclusion, in this article, we have described all the major concepts of variable resistors, including definition, working principle, formula, and applications. This resistor is a crucial circuit element used in various electronic circuits and devices to adjust the amount of voltage and current and its application increases the flexibility in circuit design by providing an easily adjustable resistance.