Light Emitting Diodes (LEDs) are one of the most commonly used electronic components today. From tiny indicator lights on electronic devices to bright household lighting, LEDs have almost completely replaced traditional incandescent bulbs because they consume less power, last much longer, and generate very little heat.
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If you’re just getting started with electronics, understanding how LEDs work is essential. In this article, you’ll learn what an LED is, how it works, how to identify its terminals, why LEDs require a resistor, and the different types of LEDs commonly used in electronic circuits.
From incandescent bulbs to LEDs
Before LEDs became popular, incandescent bulbs were the primary source of artificial light.
An incandescent bulb contains a thin metal filament enclosed inside a glass bulb. When electric current passes through the filament, it becomes extremely hot and produces light. However, most of the electrical energy is converted into heat rather than light, making incandescent bulbs relatively inefficient.
LED technology solved this problem by producing light much more efficiently. As a result, LEDs consume less power, generate less heat, and have a much longer lifespan than incandescent bulbs.
What is an LED?
LED stands for Light Emitting Diode.
It is a semiconductor device that emits light when electric current flows through it in the forward direction. Since an LED is a type of diode, it allows current to flow primarily in one direction.
Compared to incandescent bulbs, LEDs offer several advantages:
- Lower power consumption
- Longer lifespan
- Smaller size
- Faster switching speed
- Higher efficiency
- Greater reliability
Because of these advantages, LEDs are used in almost every modern electronic device.
Common applications of LEDs
Once you start looking for them, you’ll notice LEDs almost everywhere.
Some common examples include:
- Power indicators on electronic devices
- Laptop Caps Lock and Num Lock indicators
- Notification LEDs on some smartphones
- TV remote controls, which use infrared (IR) LEDs
- Flashlights
- LED bulbs
- Arduino, ESP32, and other electronics projects
Although you cannot see the light emitted by an infrared LED, your TV can detect these infrared signals to receive commands from the remote.
LED terminals
An LED has two terminals:
- Anode (+) – Positive terminal
- Cathode (-) – Negative terminal
To make an LED glow, connect the anode to the positive supply and the cathode to the negative supply.
Since an LED is a diode, it blocks current in the reverse direction. Therefore, it normally does not light up when connected with reverse polarity.
How to identify the anode and cathode
There are several ways to identify the terminals of a through-hole LED. The easiest method is to look at the leads. The longer lead is the anode, while the shorter lead is the cathode.
If the leads have been trimmed to the same length, look at the flat edge on the plastic body. The flat edge indicates the cathode. You can also look inside the transparent package. The larger internal metal piece is connected to the cathode, while the smaller metal piece is connected to the anode.
How an LED works
When an LED is forward biased, meaning the anode is connected to the positive terminal and the cathode to the negative terminal, current flows through its semiconductor junction.
As electrons recombine with holes inside the semiconductor material, energy is released in the form of light. Different semiconductor materials produce different colors, which is why LEDs are available in red, green, blue, yellow, white, and many other colors.
Forward voltage of an LED
Every LED has a characteristic known as its forward voltage (VF), which is the voltage across the LED when it operates normally.
The forward voltage depends on the LED’s color and semiconductor material.
| LED Color | Typical Forward Voltage |
|---|---|
| Red | 1.8–2.0 V |
| Yellow | 2.0–2.2 V |
| Green | 2.0–3.2 V |
| Blue | 3.0–3.3 V |
| White | 3.0–3.3 V |
For example, a blue or white LED generally requires a higher forward voltage than a red LED.
Why does an LED need a resistor?
Unlike a resistor, an LED does not automatically limit the amount of current flowing through it. Once the forward voltage is reached, even a small increase in voltage can cause a large increase in current.
For example, suppose you connect a blue LED with a forward voltage of 3 V directly to a 9 V battery. The LED will drop approximately 3 V, leaving around 6 V across the rest of the circuit. Since there is almost nothing to limit the current, a very large current can flow, permanently damaging the LED.
To prevent this, a resistor is connected in series with the LED to limit the current.
For example, using a 220 Ω resistor with a 9 V supply and a 3 V LED gives:
Current = (9 V − 3 V) ÷ 220 Ω = 27 mA
This current is within the safe operating range for many standard indicator LEDs.
Types of LEDs
Through-hole LEDs
These are the traditional LEDs with long leads that are inserted through holes in a printed circuit board (PCB).
The most common sizes are 3 mm, 5 mm, and 8 mm. They are available in a variety of colors and are widely used in breadboard circuits and hobby electronics.
SMD LEDs
Surface Mount Device (SMD) LEDs are designed to be soldered directly onto the surface of a PCB.
Because of their compact size, they are commonly used in smartphones, laptops, LED strips, televisions, and other modern electronic devices. SMD LEDs are generally identified by their package size, such as 0603, 0805, 2835, or 5050.
High-power LEDs
High-power LEDs are designed to operate at much higher currents than standard indicator LEDs. Common power ratings include 1 W, 3 W, 5 W, and higher.
Since they generate significantly more heat, they usually require a heat sink for proper operation. These LEDs are commonly used in flashlights, spotlights, automotive lighting, and high-brightness lamps.
For comparison, a standard 3 V LED operating at 30 mA dissipates only 90 mW of power, whereas a high-power LED typically dissipates 1 W or more.
Summary
LEDs are among the most important components in electronics. They are efficient, inexpensive, reliable, and available in a wide variety of shapes, sizes, and colors.
Before using an LED in a circuit, always remember these key points:
- Connect the anode to positive and the cathode to negative.
- Never connect a standard LED directly across a power supply.
- Always use a suitable series resistor to limit the current.
- Different LED colors have different forward voltages.
- Choose the appropriate type of LED based on your application.
Understanding these basics will help you confidently use LEDs in breadboard circuits, Arduino projects, ESP32 projects, and countless other electronics applications.