What is a Resistor? Definition, Types, Symbols & Applications Explained

Resistors are one of the most widely used components in electronics. No matter how complex or straightforward a circuit is, you will almost always find resistors in it. Their primary purpose is to control or limit the flow of electrical current to ensure that other components operate safely and effectively.

In simple terms, 

A resistor resists the flow of electric current and reduces voltage levels in a circuit.

They are available in different sizes, shapes, and resistance values to suit a wide variety of applications — from LED circuits to high-power industrial systems.

How Does a Resistor Work?

Electric current flows through a circuit because of the movement of electrons. A resistor is made from materials that oppose this movement, such as carbon, metal-film, or wire-wound alloys.

When electrons encounter resistance:

• The current decreases
• Some energy is converted into heat

This is based on Ohm’s Law:

V = I × R

Where:

V = Voltage

I = Current

R = Resistance

So, by changing R, we directly control the current I.

Resistor Circuit Symbols

When drawing circuit diagrams, resistors are shown using standardised symbols:

Symbol	Region Used	Appearance
Zig-Zag Line	US/ANSI	\/\/\/\/
Rectangle	Europe/IEC	[ ]

Both symbols represent the same component — the difference is only based on drawing standards.

Types of Resistors

Resistors can be classified into fixed, Variable, and special-purpose types.

1. Fixed Resistors

These have a constant resistance value.

Common types:

• Carbon Film Resistor
• Metal Film Resistor
• Wire-Wound Resistor

Applications: General circuits, sensors, power supplies.

2. Variable Resistors

Their resistance can be adjusted manually.

Examples:

• Potentiometer → used for volume control, brightness control.
• Rheostat → used for controlling current in powerful circuits.

3. Special Resistors

These resistors change resistance depending on environmental conditions.

Type	Full Name	Behavior
Thermistor	Temperature Resistor	Resistance changes with temperature
LDR	Light Dependent Resistor	Resistance decreases in light
Varistor	Voltage Dependent Resistor	Protects circuits from voltage spikes

Resistor Colour Code Chart

Small resistors use a colour code to represent their resistance value.

Example of colour bands:

Color	Digit
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Gray	8
White	9

To decode:

• First two bands → digits
• Third band → multiplier
• Fourth band → tolerance (% accuracy)

Applications of Resistors

Resistors are used in almost every electronic device. Common applications include:

• Limiting current (e.g., prevent LEDs from burning out)
• Voltage division in power supply circuits
• Setting operating conditions for transistors and amplifiers
• Pull-up/pull-down in digital circuits
• Heat generation in heaters and appliances

Example: LED Circuit

Without a resistor, an LED would receive too much current, burning out. A resistor is used in series to protect the LED and extend its lifespan.

Advantages of Resistors

• Simple to use
• Low cost
• Available in many sizes and values
• Stable electrical behaviour

Limitations

• Produce heat when current flows
• Limited power handling in small sizes
• Cannot amplify or store energy (passive component)

Conclusion

Resistors are foundational electronic components that are essential for controlling current and voltage in circuits. Understanding their types, operation, and applications is key to working safely and effectively with electronics — whether you are a beginner hobbyist or a professional engineer.