What Is a Light Emitting Diode? Practical Circuit and Biasing Guide

What Is a Light Emitting Diode (LED)? The Practical Definition

A light emitting diode (LED) is a two-terminal semiconductor device that emits incoherent light when a forward-biased current passes through its PN junction. Unlike incandescent bulbs that rely on heating a filament until it glows, an LED generates photons through electroluminescence—specifically, when electrons recombine with electron holes within the semiconductor material, releasing energy in the form of light.

For the bench engineer or DIY builder, the critical takeaway is that an LED is fundamentally a diode. It has a highly non-linear, exponential current-voltage (I-V) curve. It does not obey Ohm’s Law like a resistor; a tiny increase in forward voltage past its threshold can result in a massive, destructive spike in current. According to the SparkFun Diode Tutorial, treating an LED like a passive resistive load is the most common reason hobbyists burn them out on the first power-up.

Schematic Symbol and Physical Pinout

On a schematic, the LED symbol is the standard diode triangle pointing toward a vertical line (the cathode bar), with two small outward-pointing arrows indicating light emission. The triangle side is the Anode (positive), and the bar side is the Cathode (negative).

Physically identifying the pins on a standard 5mm through-hole package relies on three visual markers:

• Lead Length: The Anode leg is manufactured slightly longer than the Cathode leg (before you trim them).
• Epoxy Flat Spot: The plastic lens has a flattened edge on the bottom rim; this flat side always denotes the Cathode.
• Internal Structure: Looking into the clear or tinted epoxy, the smaller internal post is the Anode, while the larger, anvil-shaped cup that holds the semiconductor die is the Cathode.

Selecting and Biasing an LED for Your Circuit

Because of the exponential I-V curve, you must never connect an LED directly across a voltage source. You must "bias" it using a current-limiting mechanism—most commonly a series resistor for low-power indicators, or a constant-current driver for high-power illumination.

Operation Regions and Typical Specifications

Different semiconductor doping materials (like Gallium Arsenide Phosphide for red, or Indium Gallium Nitride for blue/white) dictate the Forward Voltage ($V_f$) required to turn the junction on. Below is a reference table for standard 5mm indicator LEDs.

Complete Application Circuit: 5V Logic Indicator

Let’s design a basic indicator circuit driven by a 5V microcontroller GPIO pin (like an Arduino Uno or a 5V-tolerant ESP32 pin). We will use a standard Red LED.

1. Define the parameters: Source Voltage ($V_s$) = 5.0V. LED Forward Voltage ($V_f$) = 2.0V. Target Forward Current ($I_f$) = 15mA (0.015A). We choose 15mA instead of the 20mA max to extend the component's lifespan and stay well within the GPIO's sourcing limits.
2. Calculate the voltage drop across the resistor: $V_r = V_s - V_f = 5.0V - 2.0V = 3.0V$.
3. Calculate resistance using Ohm's Law: $R = V_r / I_f = 3.0V / 0.015A = 200\Omega$.
4. Select the standard E12 resistor value: The closest standard value is 220Ω. This slightly reduces the current to ~13.6mA, which is perfectly bright for an indicator.
5. Verify resistor power dissipation: $P = I^2 \times R = (0.0136)^2 \times 220 = 0.04W$. A standard 1/4W (0.25W) through-hole resistor is more than adequate.

Wiring Sequence: GPIO Pin → 220Ω Resistor → LED Anode (Long Leg) → LED Cathode (Short Leg) → System GND.

Safe Default Part Numbers and Ratings

When ordering components for a prototype or production run, relying on generic "red LED" listings from unbranded marketplace sellers often results in wildly inconsistent $V_f$ values and poor epoxy clarity. Stick to established optoelectronics manufacturers like Kingbright, Lite-On, or Cree.

• Standard 5mm Red (Kingbright WP710A104LSECK/J3): Hyper-red, water-clear lens, 2.2V $V_f$ at 20mA, 1500 mcd luminous intensity. Costs roughly $0.08 in bulk. This is the gold-standard default for 5V and 3.3V logic indicators.
• Standard 5mm Pure Green (Lite-On LTL-307EE): 3.2V $V_f$ at 20mA. Excellent for "system OK" status lights. Ensure your supply voltage is at least 4.5V to properly bias this.
• High-Power SMD White (Cree XLamp XP-E2): Designed for illumination, not indication. Rated for 3.0V $V_f$ at up to 1000mA (1 Amp). Requires a dedicated constant-current LED driver and a PCB with a thermal pad. Costs around $1.50 - $2.50 per unit.

How LEDs Fail and How to Test Them with a Multimeter

LEDs rarely fail spontaneously; they are victims of circuit abuse. The two primary failure modes are:

1. Thermal Runaway: As a semiconductor heats up, its internal resistance drops. If driven by a fixed voltage source without a current limiter, the LED draws more current, gets hotter, drops more resistance, and rapidly destroys the silicon die. The symptom is a sudden pop, a dark scorch mark inside the epoxy, and an open circuit.
2. Catastrophic Optical Failure (Wire Bond Rupture): Pushing a 20mA-rated indicator LED with 100mA of current (even briefly) can melt the microscopic gold wire bond connecting the anode lead to the semiconductor die. The LED will measure as an open circuit on a multimeter.

Testing an LED with a Digital Multimeter (DMM)

You do not need to solder an LED to a battery to test it. Most modern digital multimeters have a dedicated Diode Test mode that outputs enough current to faintly illuminate the junction while measuring the exact $V_f$.

Frequently Asked Questions

What is a light emitting diode used for in 120V AC indicator circuits?

You cannot wire a standard DC LED directly to 120V AC mains. The reverse voltage during the negative half-cycle will instantly shatter the 5V reverse-breakdown limit of the PN junction, and the forward half-cycle will deliver lethal current. To use an LED as a mains indicator, you must place a high-wattage series dropping resistor (e.g., 47kΩ, 2W) to limit current, and wire a standard 1N4007 rectifier diode in anti-parallel (reverse polarity) across the LED. The 1N4007 clamps the reverse voltage to a safe 0.7V, protecting the LED while the resistor limits the forward current to ~2.5mA. Always treat mains voltage with extreme caution and enclose the circuit in a rated, insulated housing.

How do I wire multiple light emitting diodes in series vs parallel?

When wiring multiple LEDs, series is always preferred over parallel if your voltage headroom allows it. In a series string, the exact same current flows through every LED, guaranteeing uniform brightness. You simply add the $V_f$ values together. For example, three 2.0V red LEDs in series require 6.0V just to turn on, meaning you need at least a 9V supply to leave room for a current-limiting resistor.

Wiring LEDs in parallel with a single shared resistor is a bad practice. Because no two LEDs have the exact same $V_f$ down to the millivolt, the one with the lowest $V_f$ will hog the majority of the current, overheat, fail open, and then pass its excess current to the next weakest LED, causing a cascading chain failure. If you must wire in parallel, use a dedicated current-limiting resistor for each individual LED.

Why does my light emitting diode glow faintly when the switch is off?

This "ghosting" effect is incredibly common in modern smart switches and solid-state relays. It happens because the switching device (like a TRIAC or a MOSFET) has a tiny amount of parasitic capacitance or intentional leakage current (often a few microamps) that bleeds through when in the "off" state. Because LEDs are hyper-efficient and can emit visible photons at currents as low as 50µA, this leakage is enough to cause a faint glow. To fix this, solder a 10kΩ to 47kΩ "bleeder" resistor in parallel with the LED. The resistor provides a low-impedance path for the leakage current to bypass the LED junction entirely, keeping it completely dark when switched off.