LED Testing: Measurement and Parameters

This article explores the various tests and measurements used to assess the performance of LEDs, including the test setups used.

What is an LED?

Introduction

• LED stands for Light Emitting Diode.

• It’s made from a compound called gallium arsenide phosphide.

• LEDs typically radiate infrared light at a wavelength of 850 nm.

• They have two leads: a longer lead called the “anode” (positive terminal) and a shorter lead called the “cathode” (negative terminal).

• LEDs are commonly used as indicator lights in electronic devices and on circuit boards for debugging.

• The figure below illustrates the appearance of an LED and its schematic symbol:

  

LED Types

• LEDs are broadly categorized into inorganic LEDs and organic LEDs (OLEDs).
• Inorganic LEDs are made using inorganic materials like AlGaAs and GaAsP.
• OLEDs are made by printing a very thin film of organic material onto a glass substrate.
• QLEDs (Quantum dot LEDs) are a more recent development.
• LEDs are used in fiber optic and other light-based communication systems, typically for lower-speed applications (around 30 Mbps).
• They’re a cost-effective light source with very low power consumption.

Measurements in LED Testing

Here are some typical parameters measured during LED testing:

• Sizes: Common sizes include 3 mm, 5 mm, 8 mm, and 10 mm.
• Colors: LEDs come in various colors, such as white, blue, green, orange, yellow, and multi-color options.
• Voltage: A typical forward voltage drop for an LED is in the range of 1.8 to 2.2 V (e.g., for an LED with part number RR991-04).
• Current: A common operating current is around 20 mA.
• Other parameters: Intensity (measured in millicandela or mcd), color frequency, and viewing angle are also important.
• Lifespan: How long the LED is expected to last.
• Quantum efficiency: The ratio of photons emitted to electrons consumed.
• Power output: The total light emitted in terms of power.

LED Testing Video

There are many video resources available online that provide tutorials on how to test LEDs. Here is a link to a Youtube video demonstrating basic LED testing using a digital multimeter.

Test Setup for LED Testing

The following figure-2 depicts a basic test setup used for verifying an LED, usually with a digital multimeter.

The procedure is relatively simple:

1. Set the multimeter to “diode” test mode.
2. Forward Bias Test: Connect the red probe (positive) to the anode (longer lead) and the black probe (negative) to the cathode (shorter lead). In this configuration, the LED should light up, and the multimeter will display a voltage drop of around 1.8 V.
3. Reverse Bias Test: Connect the red probe (positive) to the cathode (shorter lead) and the black probe (negative) to the anode (longer lead). In this configuration, the current is blocked, and the LED will not illuminate.