L      

                     Light Emitting Diode

                    Light-emitting diodes, or LEDs, are widely used as a standard source of light in electrical equipment. It has a wide array of applications ranging from your mobile phone to large advertising billboards. They find applications in devices for showing what the time is and for displaying different types of data. In this post, the main focus would be on learning a lot about LEDs, such as its operations and functions.

What is  light emitting diode:

                   A light releasing diode is an electric component that emits light when the electric current flows through it. It is a light source based on semiconductors. When current passes through the LED, the electrons recombine with holes emitting light in the process. It is a specific type of diode having similar characteristics as the p-n junction diode. This means that an LED allows the flow of current in its forward direction while it blocks the flow in the reverse direction. Light-emitting diodes are built using a weak layer of heavily doped semiconductor material. Based on the semiconductor material used and the amount of doping, an LED will emit a colored light at a particular spectral wavelength when forward biased.

LED symbol:

           Below is the demonstration of the LED symbol. The symbol is similar to that of the p-n junction diode. The difference between these two symbols is that the two arrows indicate that the diode is emitting the light.

Light emitting diode circuit:

            The LEDs also have a specific drop in voltage forward in cases where it is used typical circuits like a conventional diode. The drop in voltage depends on the current of the LED, the color of the produced light, etc. There are different values in the drop of voltage that would vary from 1.5V to 2.5 V current for 10 to 50 mA current.

Working Principle of LED:

                 The holes lie in the valence band, while the free electrons are in the conduction band. When there is a forward bias in the p-n junction, the electron which is a part of the n-type semiconductor material would overrun the p-n junction and join with the holes in the p-type semiconductor material. Therefore, regarding the holes, the free electrons would be at the higher energy bands.

                 When this movement of free electron and hole takes place, there is a change in the energy level as the voltage drops from the conduction band to the valance band. There is a release of energy due to the motion of the electron. In standard diodes, the release of energy in the manner of heat. But in LED the release of energy in the form of photons would emit light energy. The entire process is known as electroluminescence, and the diodes are known as a light-emitting diode.

                In LED, energy discharged in light form hinges on the forbidden energy gap. One could manipulate the wavelength of the light produced. Therefore, from its wavelength, the light color and its visibility or cannot be controlled. The color and wavelength of the light emitted can be determined by doping it with several impurities.

Uses of LED:

              LEDs find applications in various fields, including optical communication, alarm and security systems, remote-controlled operations, robotics, etc. It finds usage in many such areas because of its long-lasting capability, low power requirements, swift response time, and fast switching capabilities. Below are a few standards LED Uses:

• Used for TV back-lighting
• Uses in displays
• Used in automotive
• LEDs used in the dimming of lights

Types of LED:

Below is the list of different Types of LED that are designed using semiconductors:

• Miniature LEDs
• High-Power LEDs
• Flash LED
• Bi and Tri-Colour
• Red Green Blue LEDs
• Alphanumeric LED
• Lighting LED

Advantages and Disadvantages of LED:  

Advantage:

• Energy efficient – LED’s are now capable of outputting 135 lumens/watt

• Long Lifetime – 50,000 hours or more if properly engineered

• Rugged – LED’s are also called “Solid State Lighting (SSL) as they are made of solid material with no filament or tube or bulb to break

• No warm-up period – LED’s light instantly – in nanoseconds

• Not affected by cold temperatures – LED’s “like” low temperatures and will startup even in subzero weather

• Directional – With LED’s you can direct the light where you want it, thus no light is wasted

• Excellent Color Rendering – LED’s do not wash out colors like other light sources such as fluorescents, making them perfect for displays and retail applications

   

  Environmentally friendly – LED’s contain no mercury or other hazardous substances

• Controllable – LED’s can be controlled for brightness and color

Disadvantage

 :

• Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems. 
• Light quality: Most cool-white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly badly by typical phosphor-based cool-white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs. 
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate. 
• Blue pollution: Because cool-white LEDs (i.e., LEDs with high color temperature) emit proportionally more blue light than conventional outdoor light sources such as high-pressure sodium lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages the use of white light sources with correlated color temperature above 3,000 K.
• Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies. 
• High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. 
• Area light source: LEDs do not approximate a “point source” of light, but rather a Lambertian distribution. So LEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less