What Is an LED?

An LED is a semiconductor diode designed to convert electrical energy into light.

Like other diodes, an LED has two electrical terminals:

• Anode
• Cathode

When the LED is forward biased, current flows through the semiconductor and light is produced.

The color of the emitted light depends primarily on the semiconductor material and its band-gap characteristics.

LED Symbol

             ↗  ↗
             │  │
Anode        │  │        Cathode
  A          │  │           K
  │          │  │           │
───|>|────────────────────────

The diode symbol with two arrows pointing outward represents a light-emitting diode.

The arrows indicate that light is being emitted from the device.

How an LED Produces Light

When an LED is forward biased, electrons and holes recombine within the semiconductor material.

During this process, energy is released in the form of photons.

The energy of these photons determines the wavelength and therefore the apparent color of the emitted light.

Electrical Energy
       │
       ▼
 Semiconductor
       │
       ▼
Electron-hole recombination
       │
       ▼
     Photons
       │
       ▼
      Light

LED Polarity

LEDs are polarized devices and must normally be connected in the correct direction.

For many through-hole LEDs:

• Longer lead → anode
• Shorter lead → cathode
• Flat edge → often identifies the cathode

These conventions are common but should not replace checking the datasheet when polarity is uncertain.

LED Forward Voltage

An LED requires a certain forward voltage before significant current flows. The forward voltage depends on the LED's semiconductor material, color, current, temperature, and construction.

Typical forward-voltage ranges vary considerably, so a single value should not be assumed for every LED.

These are approximate ranges only. The manufacturer's datasheet should be used for actual circuit design.

LED Current

An LED is primarily controlled by current rather than by simply applying a fixed voltage.

Once the LED enters forward conduction, a relatively small increase in voltage can produce a significant increase in current.

Excessive current can permanently damage the LED.

A suitable current-limiting method is therefore normally required.

Current-Limiting Resistor

For a simple LED circuit powered from a voltage source, a resistor can be connected in series with the LED.

+V ─── Resistor ───|>|─── GND
                   LED

A basic resistor calculation is:

R = (VS − VF) / IF

where:

• VS = supply voltage
• VF = LED forward voltage
• IF = desired LED current

LED Resistor Example

Suppose a red LED has:

• Supply voltage = 12 V
• Forward voltage = 2.0 V
• Desired current = 10 mA

The resistor is approximately:

R = (12 − 2.0) / 0.010

R = 10 / 0.010

R = 1000 Ω

A 1 kΩ resistor would therefore be a suitable starting point for this example, subject to checking the LED's actual specifications.

Resistor Power Dissipation

The resistor also dissipates power.

A simple calculation is:

P = I²R

For a 1 kΩ resistor carrying 10 mA:

P = (0.010)² × 1000
P = 0.1 W

A resistor with an appropriate power rating should be selected with suitable margin.

LED Brightness

LED brightness depends on several factors, including:

• Forward current
• LED efficiency
• Semiconductor material
• Optical design
• Lens geometry
• Viewing angle
• Temperature

Increasing current can increase light output, but the LED must remain within its specified operating limits.

For many modern LEDs, the desired brightness can often be achieved at currents significantly below the maximum rated current.

LED Colors

Different semiconductor materials produce different wavelengths of light.

Common LED colors include:

• Red
• Orange
• Yellow
• Green
• Blue
• Violet
• White
• Infrared

White LEDs are commonly based on a blue LED combined with a phosphor conversion layer, although other technologies also exist.

Visible and Invisible LEDs

Not all LEDs produce visible light.

Infrared LEDs emit wavelengths outside normal human vision and are commonly used in:

• Remote controls
• Optical sensors
• Security systems
• Optical communication
• Proximity sensors

Some specialized LEDs can also operate in other wavelength regions.

Types of LEDs

Indicator LEDs

Small LEDs designed primarily to indicate the state of equipment or a circuit.

High-Brightness LEDs

Designed to produce greater optical output than conventional indicator devices.

High-Power LEDs

Designed for significantly higher electrical power and optical output. They require careful thermal management.

RGB LEDs

Contain red, green, and blue LED elements that can be controlled to produce different colors.

Infrared LEDs

Designed to emit infrared radiation for sensing and communication.

UV LEDs

Emit ultraviolet radiation and are used in specialized applications such as curing, inspection, fluorescence, and sterilization equipment designed for appropriate wavelengths and conditions.

RGB LEDs

An RGB LED contains three LED elements:

• Red
• Green
• Blue

By controlling the current through each element, many different perceived colors can be produced.

RGB LEDs may be available with common-anode or common-cathode connections, as well as individually addressable integrated-control versions.

Always check the datasheet for the pin configuration.

LED Displays

Multiple LEDs can be arranged to form displays.

Examples include:

• Seven-segment displays
• Bar-graph displays
• Dot-matrix displays
• Numeric indicators
• Large LED signage

Seven-Segment Displays

A seven-segment display uses seven LED segments to represent numbers and some letters.

The segments are commonly designated:

       a
      ───
   f │   │ b
     │ g │
      ───
   e │   │ c
     │   │
      ───
       d

Displays are commonly available in common-anode and common-cathode configurations.

LED Lighting

High-power LEDs are widely used for illumination.

Applications include:

• Household lamps
• Flashlights
• Automotive lighting
• Street lighting
• Architectural lighting
• Display backlighting

Unlike a small indicator LED, a high-power LED can generate substantial heat and normally requires appropriate thermal management.

LED Drivers

LEDs used for significant lighting power are commonly driven using a dedicated current-regulating circuit rather than a simple resistor.

An LED driver may provide:

• Constant current
• Current regulation
• Dimming
• Thermal protection
• Open-load protection
• Short-circuit protection

Constant-current operation is particularly important for high-power LEDs.

LED Dimming

LED brightness can be controlled in several ways.

Analog Current Control

The LED current can be changed continuously within its operating range.

PWM Dimming

Pulse-width modulation rapidly switches the LED on and off. The perceived brightness is controlled by the duty cycle.

High brightness:
████████████████────

Lower brightness:
████────████────████

PWM is widely used because it can provide predictable brightness control while keeping the LED current at a suitable operating value during the ON period.

LEDs in Optical Communication

LEDs can be used as light sources for transmitting information.

The light output can be modulated according to an electrical signal.

Applications include:

• Infrared remote controls
• Short-range optical links
• Optical sensors
• Data transmission

LEDs in Optocouplers

An LED is commonly used as the light source inside an optocoupler.

The LED transmits light to an isolated photodetector, allowing information to cross an electrical isolation barrier without a direct conductive connection.

Optocouplers are widely used in:

• Power-supply feedback
• Microcontroller isolation
• Industrial control
• Interface circuits

LED Polarity on a PCB

When installing an LED on a PCB, verify the polarity markings.

The board may identify the cathode using:

• Flat edge marking
• Line or bar marking
• Specific PCB footprint shape
• Component documentation

Never rely solely on the physical shape when the PCB marking or datasheet is available.

Testing an LED with a Multimeter

Many digital multimeters include a diode-test function that can be used to perform a basic LED test.

Test Procedure

1. Set the meter to diode-test mode.
2. Connect the red probe to the LED anode.
3. Connect the black probe to the cathode.
4. Observe the forward-voltage reading.

Some LEDs will emit a small amount of light during the test.

If the LED does not illuminate, this does not necessarily mean it is faulty. The multimeter's test current may be insufficient, especially for some high-power LEDs.

Testing an LED for a Short Circuit

An LED that has failed short-circuit may show conduction in both directions.

Test the component in both polarities using diode-test mode.

A low reading in both directions can indicate a shorted device, although in-circuit testing may be affected by other components.

Testing an LED for an Open Circuit

An open LED may show no forward conduction during a multimeter diode test.

If the meter indicates an open circuit in both directions, test the LED outside the circuit to rule out the influence of surrounding components.

Common LED Faults

LED Thermal Management

Heat is one of the most important considerations for high-power LEDs.

Excessive junction temperature can reduce LED efficiency, shorten operating life, and eventually damage the device.

Thermal management may involve:

• Aluminium heatsinks
• Thermal interface materials
• PCB copper area
• Metal-core PCBs
• Forced airflow

LED Efficiency

LED efficiency depends on the semiconductor technology, operating current, temperature, optical design, and driver.

Electrical input power can be estimated as:

P ≈ VF × IF

Not all of this electrical power becomes visible light. A portion becomes heat and other forms of optical or electrical loss.

LEDs and Temperature

LED characteristics change with temperature.

Temperature can affect:

• Forward voltage
• Light output
• Efficiency
• Wavelength
• Operating lifetime

High-power LED designs should therefore consider the complete thermal path from the semiconductor junction to the surrounding environment.

LED Replacement

When replacing an LED, physical appearance alone is not enough.

Check:

• Forward voltage
• Forward current
• Color or wavelength
• Brightness
• Viewing angle
• Package
• Polarity
• Thermal characteristics

For a simple indicator circuit, a visually similar LED may be sufficient. For precision displays or high-power lighting, the optical and thermal specifications become much more important.

LED vs Conventional Diode

Common LED Selection Mistakes

• Connecting an LED directly to a voltage source without current limiting
• Ignoring LED polarity
• Exceeding the maximum forward current
• Ignoring thermal requirements
• Using the wrong forward voltage when calculating a resistor
• Replacing an LED without checking its package
• Ignoring wavelength or color requirements
• Using an ordinary indicator LED for a high-power lighting application

Quick LED Selection Checklist

• ✔ Identify the LED type
• ✔ Check forward voltage
• ✔ Determine operating current
• ✔ Provide current limiting or a suitable LED driver
• ✔ Check optical output
• ✔ Check color or wavelength
• ✔ Check viewing angle
• ✔ Check package
• ✔ Check thermal requirements
• ✔ Verify polarity
• ✔ Confirm the manufacturer's datasheet

Important Safety Notes

• Never connect an LED directly to a voltage source unless the circuit provides appropriate current regulation.
• High-power LEDs can become extremely hot during operation.
• Allow adequate thermal management for high-power devices.
• UV LEDs can produce radiation that may be hazardous to eyes and skin.
• Use appropriate protection when working with high-intensity or UV LEDs.

Key Points

• LEDs are semiconductor diodes that emit light when forward biased.
• LEDs are polarized and normally require current limiting.
• Forward voltage varies with LED technology, current, and temperature.
• Different semiconductor materials produce different wavelengths and colors.
• High-power LEDs require proper thermal management.
• LEDs are used for indicators, displays, lighting, sensing, signalling, and optical communication.
• Always check forward voltage, current, polarity, package, and thermal requirements before installing an LED.