About LEDsLED Types

LED Packages

LEDs can be broadly classified into the following two types by shape.

1. Through-hole circular type LED

Such LEDs have a dome shape, and are generally referred to as through-hole type LEDs. This is a typical package that has been around since the earliest days and is available in several sizes, but the most common have diameters of 3 mm or 5 mm. The outer resin is available in three types: colorless transparent, colored transparent, and colored diffuse. For example, the diffuse type is used when a wide viewing angle is required, while the colorless transparent type is used when it is desirable for the luminous color to be invisible when the light is off. In addition to the circular type, rectangular and cylindrical types are also available.

Mount Method: Insertion mounting
Lead terminals (legs) are passed through holes (through-holes) in the board and soldered (flow soldering).

2. Surface-mount (chip) type LED (SMD: surface-mount device)

In a surface-mount LED, the light-emitting element is sealed in a surface-mountable package, and is generally called a chip LED. This type of LED includes a mold type and reflector type with a reflector inside the package, and can brightly illuminate a wide area in a space-saving manner.

Mounting method: Surface mounting
Chip terminals are soldered directly to the surface of the board (reflow soldering)

Most chip LEDs are of the top-emitting type, but there are also side-emitting (side-view) types and rear-emitting types mounted on the underside (backside) of the board. The choice of LED type is based on factors such as space-saving mounting and the required illumination direction.
In terms of size, ROHM offers ultra-compact 1006 (1.0 × 0.6 mm) to high-power 4520 (4.5 × 2.0 mm) sizes, depending on the application.

2-color emission, 3-color emission

There are also 2-color and 3-color LEDs that incorporate multiple light-emitting elements inside a single LED package. These types of LED can emit one color at a time or different colors simultaneously to create a variety of color mixtures. 3-color RGB LEDs are also called full-color LEDs and can express any color by combining the three primary colors of light: red, blue, and green.

Infrared emission, ultraviolet emission

In addition to visible light, there is an increasing number of LEDs that emit invisible infrared and ultraviolet light. Since infrared light has no effect on the human body, it is widely used in familiar fields such as remote controls for electrical appliances, short-distance transmission between smartphones and electronic devices, security cameras, and sensor applications.
On the other hand, ultraviolet (UV) light has a high energy and affects the human body, causing sunburn and skin cancer, but it has a wide range of applications, including sterilization and deodorization in the medical and hygiene fields and resin curing in the industrial field.

How to distinguish between anode and cathode

An LED has a plus and minus side, referred to as the anode and cathode, respectively.
The anode (+) is connected to the positive side of the power supply and the cathode (-) to the negative side.
In a through-hole type LED, the longer of the two legs is the anode (+) and the shorter is the cathode (-).

In the case of a chip LED, a triangular (◂), T-shaped, or convex mark printed on the back of the LED indicates the direction of current flow. In LEDs, the current flows from the anode to the cathode, so the pointed end of the indicator mark (the side with the arrowhead) is the cathode.

In reflector types, the side with a notch on the corner is generally the cathode.
However, each manufacturer has its own method of distinguishing polarity, so be sure to check the polarity by referring to the dimensional drawings in the datasheet before use.

An RGB three-color (full-color) LED incorporates three light-emitting elements R (red), G (green), and B (blue) in a single package. There are two types: a six-terminal type with an independent circuit for each color, and a four-terminal type with either anode or cathode electrodes in common. That with a common anode is called an “anode common” type, and that with a common cathode is called a “cathode common” type.