A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for general lighting purpose.
Light-emitting diodes are used in applications as diverse as aviation lighting, automotive lighting, advertising, general lighting, and traffic signals. LEDs have allowed new text, video displays, and sensors to be developed, while their high switching rates are also useful in advanced communications technology.
Normally LEDs are designed to operate with no more than 20–50 milliwatts (mW) of electrical power.
HOW DOES LED EMMIT LIGHT
When a light-emitting diode is switched on, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.
WHAT DETERMINES THE COLOUR OF LED
The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.
The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction.
CONNECTING LED
A LED is a two-terminal electrical component, to identify them
Positive terminal can be identified as the longer leg and shorter leg is the negative terminal. I have marked it on a LED.
OR
If you look inside LED you can see two small lead, fat one looks like a flag which is the cathode (negative) and other one looks almost straight which is the anode (positive)
SYMBOL OF LED
LIFETIME AND FAILURE
LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. LED’s have a relatively longer life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be even longer, but remember that the heat and current can extend or shorten this time significantly. The most common problem of LED failure is the gradual lowering of light output and loss of efficiency.
LED POWER CONSUMPTION
1. Low-current: typically rated for 2 mA at around 2 (approximately 4 mW consumption).
2. Standard: 20 mA LEDs (ranging from approximately 40 mW to 90 mW) .
3. 1.9 to 2.1 V for red, orange and yellow.
4. 3.0 to 3.4 V for green and blue.
5. 2.9 to 4.2 V for violet, pink, purple and white.
6. Ultra-high-output: 20 mA at approximately 2 V or 4–5 V, designed for viewing in direct sunlight.
7. 5 V and 12 V LEDs are ordinary miniature LEDs that incorporate a suitable series resistor for direct connection to a 5 V or 12 V supply.
TYPES OF LED
1. HIGH-POWER
High-power LEDs (HPLEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs.
2. RGB LEDs
RGB LEDs (red, green, and blue) in general using a four-wire connection with one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tiny electronic control unit.
3. WHITE LIGHT
There are two primary ways of producing white light-emitting diodes (WLEDs), LEDs that generate high-intensity white light. One is to use individual LEDs that emit three primary colors red, green, and blue—and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light, much in the same way a fluorescent light bulb works.
ADVANTAGES
The main advantage of LED’s are low energy consumption, low maintenance and small size. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, faster switching and emit less heat.
LEDs emit more light per watt than incandescent light bulbs. The efficiency of LED lighting fixtures is not affected by shape and size, unlike fluorescent light bulbs or tubes.
LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.
LEDs can be very small and are easily attached to printed circuit boards.
LEDs light up very quickly. LED can achieve full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.
LEDs can very easily be dimmed either by pulse-width modulation or lowering the forward current. This pulse-width modulation is why LED lights viewed on camera, particularly headlights on cars, appear to be flashing or flickering.
LEDs mostly fail by dimming over time, rather than the abrupt failure like incandescent bulbs.
LEDs can have a relatively longer life.
LEDs, being solid-state components, are difficult to damage with external shock
DISADVANTAGES
LED performance largely depends on the temperature of the operating environment. Over-driving an LED in high temperatures may result in overheating the LED package, eventually leading to device failure. An adequate heat sink is needed to maintain long life. This is especially important in automotive, medical, and military uses where devices must operate over a wide range of temperatures.
Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity.
SAFETY AND HEALTH
The vast majority of devices containing LEDs are safe under all conditions of normal use
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