LED is the acronym for “Light Emitting Diode”. LEDs are semiconductor devices that produce light. These were initially used as indicator lights but are now used extensively for indoor and outdoor lighting. LED Lighting Supply specializes in commercial and industrial LED Lighting.
Below is the electronic symbol of the LED:
Symbol of the light emitting diode showing the cathode, anode and the direction of light emitted.
LED dates back to 1907 when a British physicist discovered that silicon carbide crystals could produce light when subjected to electric currents. In 1962, a GE scientist, Nick Holonyak developed the first visible light LED. This LED produced red light. Later George Craford developed the yellow light LED. The humble LED then cost close to $ 200 for a single LED! Within a few years the cost had fallen to 5 cents. Since then the cost of the LED has been falling while light output has been going up.
How does an LED work?
The P-N junction is the basis of the functioning of the LED. The LED has an anode and a cathode separated by a crystal of semiconductor material. Addition of impurities to the semiconductor material produces P-N junctions within the chip. The entire assembly is within a plastic cover that can also double up as a lens to guide the light emitted by the LED.
A schematic diagram of a Light Emitting Diode. (Picture credit https://commons.wikimedia.org/wiki/File:LED_Device.jpg)
When voltage is applied across the electrodes the current flows from the anode (P side) to the cathode (N side). When an electron meets a hole at the P-N junction it falls in to a lower energy state. The difference in energy of the two states is called the ‘Band gap’ which is a characteristic of the material comprising the P-N junction.
The excess energy of the electron is emitted as a Photon. More is the ‘Band Gap’ higher is the energy difference and shorter is the wavelength of the light emitted.
The following are important for increasing the lighting efficiency of LEDs
- Advances in material sciences to create materials with better band gaps
- Better fabrication techniques for reducing the cost and increasing the efficiency
- Improvement in heat dissipation
- Light extraction from the material comprising the diode. New materials allow more light to be extracted. This improves the lumen per watt characteristics of LEDs.
- Improvements in phosphor technology to increase the efficiency of conversion of light from one wavelength to a wider band of wavelengths.
A single LED is very small and produces a defined amount of light according to its design type. There are low-, mid- and high-power LEDs. Several LEDs need to be combined to produce the desired amount of light.
The picture below shows 9 LED menorah resting on a fingertip indicates the relative size of an LED:
The image below is a graphic representation of a section of an LED light:
A LED product contains the following components
LED cluster: Produces the required amount of light.
Driver Electronics: Changes the household Alternating Current to Direct Current and maintains the right current to power the LED. The electronic ballast converts input voltage to 12V or 24V which is the often occurring input voltage for LED circuit boards. Other Voltages are also possible, even AC driven LEDs. The driver is the brain of the LED bulb.
The two main components of an LED driver are
- The driver integrated circuit and
- The driver circuit the driver circuit is also called the electrical control gear.
Instead of using ICs and driver circuits, the required voltage drop can be achieved by using a resistor. But resistors can lead to unacceptable voltage drops and far higher currents than the LED is designed to handle.
Heat sink: The heat generated by the electronic parts and the LED need to be dissipated. Heat sinks that are not designed well or with excess drive currents can result in overheating. This can result in elevated junction temperatures that in turn will compromise both the life and the light produced by the LED.
For good thermal management three things are important
a) Substrate material: Often a metal core PCB mounts LEDs. Besides providing a substrate for mechanical mounting of LEDs, the metal core spreads the heat over a larger area. This helps to transfer it to the heat sink.
b) Interface materials:Usually a film or grease is used as an interface material. These help in removing heat while electrically separating the passive heat sink from the energized components.
c) Heat sinks:Heat sinks are of two types. Active heat sinks often use fans to circulate air. Passive heat sinks use metal fins to dissipate heat. Other structural features can improve air flow around the metal fins and ensure better heat dissipation. Active heat sinks dissipate more heat but given the advances in passive heat sink design these are not needed in most applications. Only when several LEDs are being used in a confined space an active sink may be needed to control temperatures.
Optics: LED light is directional light. The standard light distribution angle of a LED is 180 degrees. The light is emitted into the upper half-space. For some LEDs the distribution angle is adjustable, there are narrow, wide-beam till batwing optics available. The viewing angle can be altered by lenses. Lenses can be built into the structure of the LED (first optic) or a secondary lens can be used to further control the viewing angle. Either there can be one secondary lens for several LEDs in a bulb or each LED may be given a separate secondary lens for tighter control of light output.
Advantages of LED Technology
Light output: In 2002, light output from LEDs was in the region of 20 lumens per watt. Today LED Commercial lighting devices can produce 130+ Lumens per watt. New LED designs can produce as much as 200+ Lumens per watt. This is greater than the light produced by incandescent bulbs (15 lumens per watt) or fluorescent tubes (80-95 Lumens per watt).
Lower power consumption means that a typical LED light at www.ledlightingsupply.com can pays for itself in a 1 to 2 years.
Life span: LED lights last anywhere between 30,000 to 100,000 hours. Most commercially available LED lights are rated for a 50,000 to 100,000 hour life span. This means that once installed an LED will last anywhere between 10 to 30 years, depending on the running hours per day. The long life span reduces maintenance expenses and makes these bulbs suitable for difficult to reach locations.
Operating characteristics: LED operate and are not sensitive to low temperature. Additionally, they are not affected by on off cycling. This makes them safer and more efficient in cold environments. They are also better for applications requiring frequent switching of and off lights. The fixtures are not affected by vibrations making them the best choice for places like bridges.
Shock resistant: The energized components of the LED are separated from the outer surface with quality insulation. The electrodes are embedded in the bulb matrix and the driver electronics are encased in its shell. A layer of interface material between the LED and the heat sink ensures that no current can leak to the heat sink.
Vibration resistant: Looking at the LED at the beginning of the post you will see that the electrodes are encased in transparent acrylic. There are no suspended filaments as LEDs are resistant to vibrations. Many avid off road driving fans use LED lights in their SUVs because of this feature alone.
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