An **LED (Light Emitting Diode)** is a semiconductor light source that emits light when current flows through it. Electronically, it is a specialized type of P-N junction diode. ### 1. Key Internal Components An LED consists of several microscopic and structural parts that allow it to function efficiently: | Component | Description | | :--- | :--- | | **Anode (+)** | The positive terminal, usually the longer lead on a physical LED. | | **Cathode (-)** | The negative terminal, usually the shorter lead or marked by a flat edge on the casing. | | **Semiconductor Die** | The "heart" of the LED; a small chip made of materials like Gallium Arsenide (GaAs). | | **Reflective Cavity** | A small "cup" that holds the die and reflects light outward. | | **Epoxy Lens** | The plastic body that protects the internals and focuses the light beam. | | **Wire Bond** | A tiny gold wire connecting the Anode to the semiconductor die. | --- ### 2. Electrical Characteristics To use an LED in a circuit, you must understand its specific electronic requirements: * **Forward Voltage ($V_f$):** The minimum voltage required to turn the LED on. This varies by color: * **Red:** 1.8V – 2.2V * **Green/Blue/White:** 3.0V – 3.4V * **Forward Current ($I_f$):** Most standard 5mm LEDs operate safely at **20mA**. Exceeding this will burn out the component. * **Reverse Voltage ($V_r$):** LEDs have low tolerance for reverse voltage (usually ~5V). If connected backward with high voltage, they will be destroyed. --- ### 3. Essential Circuit Implementation Because LEDs have very low internal resistance once they reach their forward voltage, they require a **Current Limiting Resistor** in series to prevent them from drawing too much current. #### Calculation Formula (Ohm's Law): To find the correct resistor ($R$), use: `R = (V_source - V_forward) / I_forward` **Example Calculation:** If using a 9V battery, a Red LED (2V), and a desired current of 20mA (0.02A): ```text R = (9V - 2V) / 0.02A R = 7V / 0.02A R = 350 Ohms ``` --- ### 4. Types of LED Packages 1. **Through-Hole:** Standard LEDs with long legs (3mm, 5mm, 10mm). 2. **SMD (Surface Mount Device):** Tiny LEDs soldered directly onto the surface of PCBs (e.g., 0805, 5050 packages). 3. **COB (Chip on Board):** Multiple LED dies mounted together to create a high-intensity light panel. 4. **RGB LEDs:** Contains Red, Green, and Blue dies in one package to create any color via PWM (Pulse Width Modulation).