LEDs are semiconductor diodes, electronic devices that permit current to flow in only one direction. The diode is formed by bringing two slightly different materials together to form a PN junction. In a PN junction, the P side contains excess positive charge (“holes,” indicating the absence of electrons) while the N side contains excess negative charge (electrons).
When a forward voltage is applied to the semiconducting element forming the PN junction (heretofore referred to as the junction), electrons move from the N area toward the P area and holes move toward the N area. Near the junction, the electrons and holes combine. As this occurs, energy is released in the form of light that is emitted by the LED.
What determines the color of an LED?
The material used in the semiconducting element of an LED determines its color. The two main types of LEDs presently used for lighting systems are aluminum gallium indium phosphide (AlGaInP, sometimes rearranged as AlInGaP) alloys for red, orange and yellow LEDs; and indium gallium nitride (InGaN) alloys for green, blue and white LEDs. Slight changes in the composition of these alloys changes the color of the emitted light.
White LED Technology
Mixed-color white light: One approach is to mix the light from several colored LEDs to create a spectral power distribution that appears white. Similarly, so-called tri-phosphor fluorescent lamps use three phosphors, each emitting a relatively narrow spectrum of blue, green or red light upon receiving ultraviolet radiation from the mercury arc in the lamp tube. By locating red, green and blue LEDs adjacent to one another, and properly mixing the amount of their output (Zhao et al. 2002), the resulting light is white in appearance.
Phosphor-converted white light: Another approach to generating white light is by use of phosphors together with a short-wavelength LED. For example, when one phosphor material used in LEDs is illuminated by blue light, it emits yellow light having a fairly broad spectral power distribution. By incorporating the phosphor in the body of a blue LED with a peak wavelength around 450 to 470 nanometers, some of the blue light will be converted to yellow light by the phosphor. The remaining blue light, when mixed with the yellow light, results in white light.
Advantages and Disadvantages LEDs
Advantages of Mixed-Colour White LEDs
Higher overall luminous efficacy
Good colour rendering properties
Complete flexibility for achieving any desired color property
Advantages of Phosphor-converted white LEDs
Results in a single, compact, white light source
Disadvantagesof Mixed-Colour White LEDs
Difficult to completely mix light
Difficult to maintain colour stability over life and at different operating conditions, including dimming
Disadvantagesof Phosphor-converted white LEDs
Lower overall luminous efficacy
Uniform application of phosphor in manufacturing process is more difficult to control
Limited range of available color properties based on phosphor availability
Application of LEDs
Sensor Applications:Medical Instrumentation, Bar Code Readers, Encoders, Optical Switches, Fiber optic Communication
Mobile Applications: Mobile Phone, PDA’s, Digital Cameras, Laptops, General Backlighting
Sign Applications:Full Colour Video, Monochrome Message Boards, Traffic/VMS, Transformation-Passenger Information
Signal Applications:Traffic, Rail, Aviation, Tower Lights, Runaway Lights, Emergency/Police Vehicle Lighting
Illumination:Architectural Lighting, Signage (Channel Letters), Machine Vision, Retail Displays, Emergency Lighting (Exit Signs), Neon Replacement, Bulb Replacement, Flashlights, Outdoor Accent Lighting –Pathway, Marker Lights
Indication:Household Appliances, VCD/DVD/Stereo and other audio and video devices, Toys/Games, Instrumentation, Security Equipment, Switches
Why is it important to control the current through an LED?
A slight change in voltage can result in very large changes in current which can be seen in following figure. Since the light output of an LED is proportional to its current, this can result in unacceptable variation in light output. If the resulting current exceeds limits recommended by the manufacturer, the long-term performance of the LED can be affected, resulting in shorter useful life.
What is an LED driver?
An LED driver performs a function similar to a ballast for discharge lamps. It controls the current flowing through the LED. Most LED drivers are designed to provide current to a specific device or array. Since LED packages and arrays are not presently standardized, it is very important that a driver is selected that is matched to the specific device or array to be illuminated.
This is a very important product to prevent your LEDs for damaging as the forward voltage of the high powered LED changes with temperature level. Forward Voltage is the number of volts the light-emitting diode requires to conduct electricity and light up. With the increase in the temperature, the forward voltage decreases that result in LEDs to draw more current. This process will continue until LED run out of power and burst with exhaustion. This process is called thermal runaway.
LED drivers serve the purpose of power supply that has electrical characteristics similar to LEDs. This driver will prevent the situation of a thermal runaway as it will offer the extra power to LEDs during the forward voltage situation in the season change.
Types of LED Drivers and How to choose them?
First of all, you need to understand that there are two types of LED drivers available – low voltage DC input power ranging between 5-36VDC and high voltage AC input power varies between 9 0-277VAC.
The high voltage AC drivers are called offline drivers. But, in most cases – low voltage DC input drivers are recommended by experts. They are very useful for small gadgets with lots of dimming and adjusting options. However, if you have large commercial devices running in your home, then you have to use the AC drivers.
Driver current specifications
Secondly, it is advised to check driver current specifications in the context of LED on which you want to use the driver. The higher driver will produce more light and consumes more wattage as well. It is highly important to have correct information about your LED driver’s current and heat sink so that your LED lights won’t get affected from over or less current supply.
You need to also clear about your requirements from the drivers like if you want driver for dimming purposes, then you have to pick a driver with dimming functions.
