What is a discrete Light Emitting Diode (LED)?
Light emitting diodes are semiconductor devices that emit light when conducting current. LEDs in this family are single or array configurations for status indication on printed circuit boards. They are differentiated by package/case, wavelength, viewing angle, forward voltage, current rating, millicandela rating, color, and mounting type. Configurations include common anode or cathode and bidirectional. Additional LEDs may be found in the white LED and Chip On Board (COB) LED sections.
An engineer’s perspective
Click to expand
The LED is one of the first non-linear electronic devices you are likely to encounter. One of the most common applications is a microcontroller’s “Hello World!” blinking LED where you adjusted the on and off time. As you recall, the experiment included a caution to use a current limiting resistor to prevent damage to the microcontroller and LED. This circuit is so common that DigiKey has provided a conversion calculator to help select the optimal resistor.
Why is an LED susceptible to damage without the current limiting resistor?
The LED acts as a constant voltage device when forward biased. In this application, the LED maintains a constant voltage across its terminals up to the point of self-destruction. The LED is entirely dependent on the external circuitry to limit the current. For simple circuits, the series current limiting resistor is adequate. Complex current limiting power supplies may be required such as LED lighting featuring multiple series strings of LEDs. This is necessary to maintain consistent intensity and color (color temperature) across all LEDs. The complex power supply also increases system efficiency as there is no power lost to the series resistor.
Tech Tip: There are rare exceptions that include a current limiting mechanism within the LED body. An example is the Dialight 5219183F which includes a current limiting resistor.
Must-know facts about LEDs
-
History: LEDs provide a rich field of study for the historical technologist. LEDs became common in the late 1960 to 1970s. However, they were small, and always red with a forward voltage of about 1.9 VDC. The availability of different colors reflects the advances in semiconductor. Orange and green with their higher forward bias quickly followed the red. The development of the blue LED with its higher forward bias voltage was a major breakthrough. In fact, the blue LED was the groundbreaking technology that gave us the white LEDs. Recall that a white LED contains a blue “pumping diode” to excite phosphors that make the white light.
-
Multiplexing: Many LEDs have specifications for both continuous and for pulsed applications. The pulsed specification is important for multiplexed applications where multiple LEDs are grouped together and driven one at a time in rapid succession. This pulsed current creates an illusion, as the human eye’s flicker fusion sees a constantly lit display. The pulsed current is explored in depth in this article that shows how to construct a high brightness binary Arduino clock.
- Heatsink: The LED is an efficient low power device. Heat dissipation isn’t something we usually consider at the introductory level. Yet, as the power of the LED increases, heat dissipation becomes a primary consideration for long life. DigiKey has a line of related white LEDs and Chip On Board (COB) LEDs mounted on heatsinks.
- Moisture sensitive: Manufacturability is an important consideration for electronics. Surface mount LEDs present a unique challenge as they tend to absorb moisture. Unless proper precautions are taken, these Moisture sensitivity level (MSL) components will crack or explode during the soldering process.
Looking forward to continuing the conversation on this forum.
Best wishes,
Aaron Dahlen
LCDR USCG (Ret.), MSEE
DigiKey Applications Engineer
Introduction to DigiKey’s Product Selection Guide
This page is one of many in DigiKey’s Product Selection Guide (PSG). This particular page is focused on LED indication - discrete. It provides a description of the individual parameters used to characterize the part. This information will allow you to better understand and navigate DigiKey’s parametric search engine. Click here for a case study showing how to use the search tools.
How are LEDs characterized?
-
Color: Indicates the color(s) emitted from the LED.
-
Configuration: Indicates the internal electrical configuration of components.
-
Lens Color: Indicates the color of a device’s lens as it appears under incident ambient lighting. May differ from the color of light emitted by a device.
-
Lens Transparency: Indicates whether a device’s lens is designed to provide specular (clear) or diffuse optical transmission.
-
Millicandela Rating: Characterizes device brightness at the indicated test current or nominal input voltage.
-
Lens Style: Indicates the shape of the lens.
-
Lens Size: Indicates the physical dimensions of the lens.
-
Voltage - Forward (Vf) (Typ): This rating indicates the nominal amount of current you would expect the device to draw during normal operation, at the manufacturer defined test conditions.
-
Current - Test: Indicates the amount of current flow through a device used by a manufacturer as a test condition for characterization of one or more other device parameters.
-
Viewing Angle: Indicates the angle of light emitted from the LED(s).
-
Mounting Type: Indicates how the device is attached.
-
Wavelength - Dominant: The wavelength of a pure monochromatic light source that most closely matches the perceived color of the light emitted by a device.
-
Wavelength - Peak: Characterizes the highest point in a device’s output spectrum when operated at rated test conditions.
-
Features: These are different capabilities or properties of the device.
-
Package / Case: Indicates the type of protective case used on an electronic component to allow easy handling, installation, and protection. This selection was determined to be the closest industry standard applicable to the supplier device packaging. Typically it is best to check the actual dimensions from the datasheet rather than depend on this terminology when designing your circuit.
-
Supplier Device Package: This is what the manufacturer calls the package of this device. They are manufacturer specific. It is typically best to use the actual dimensions from the datasheets rather than to go by this terminology when designing your circuit.
-
Size / Dimension: The overall length, and width of the part.
-
Height (Max): The overall height of the part.
Examples of light emitting diodes
- The Lite-On Inc. brand LTL-307EE is a popular through-hole red LED with a 5 mm, T-1 3/4 lens. Datasheet
- The QT Brightek (QTB) brand QBLP600-RGB-2897 is an ultra-bright red-green-blue LED (common anode) in a 0606 surface mount package. Datasheet
- The ROHM Semiconductor brand SMLK18WBNCW1 is a high-power white surface mount LED featuring InGaN construction packed with a resin containing phosphor. Datasheet
Schematic symbol
The schematic symbol for the LED is shown below.
Schematic symbols courtesy of Scheme-it.
Related media links
Follow up
If you have any general questions on the specifications for selecting a LED, please reply using the button below. If you have questions on a specific LED, please post your questions in the optoelectronics category.
Return to DigiKey’s Product Selection Guide.