Technology
Light-Emitting Diode
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Blue, green, and red LEDs; these can be combined to produce any color, including white. Infrared and ultraviolet (UVA) LEDs are also available.
A light-emitting diode, usually called an LED, is a
semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction, as in the common LED circuit. This effect is a form of electroluminescence.
An LED is usually a small area light source, often with optics added to the chip to shape its radiation pattern. LEDs are often used as small indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. LEDs can also be used as a regular household light source. Besides lighting, interesting applications include sterilization of water and disinfection of devices.
Discovery
The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round. However, no practical use was made of the discovery for several decades.
Independently, Oleg Vladimirovich Losev published "Luminous carborundum [silicon carbide] detector and detection with crystals" in the Russian journal Telegrafiya i Telefoniya bez Provodov (Wireless Telegraphy and Telephony). Losev's work languished for decades. The first practical LED was invented by Nick Holonyak, Jr., in 1962 while he was at General Electric Company. The first LEDs became commercially available in late 1960s, and were red. They were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, maintaining the efficiency and the reliability to an acceptable level, therefore LEDs became bright enough to be used for illumination.
Most LEDs were made in the very common 5 mm T1³⁄₄ and 3 mm T1 packages, but with higher power, it has become increasingly necessary to get rid of the heat in order to keep good reliability, so the packages have become more complex and adapted for heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs (see, for example, Philips Lumileds).
Considerations in use
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. Some LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
Because the voltage versus current characteristics of the LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.
Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LED's operating range. Thus the power can be considered to be essentially proportional to the current. In order to keep power nearly constant with variations in supply and LED characteristics, the power supply should be a “current source”, that is, it should supply an almost constant current. If high efficiency is not required (e.g., in most indicator applications), an approximation to a current source is made by connecting the LED in series with a current limiting resistor to a regulated voltage source.
Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up the LED, or the LED may be tested in series with a resistor on a sufficiently low voltage supply to avoid the reverse breakdown. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel.
LEDs can be purchased with built in series resistors. These can save PCB space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.
Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.
Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity, however, more than two electrodes (leads) are required.
LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.
Generally, for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 mA, a commonly-found maximum value).
LEDs also behave as photocells, and will generate a current depending on the ambient light. They are not efficient as photocells, and will only produce a few microamperes (µA), but will produce a surprising electrical potential—as much as 2 or 3 V. This is enough to operate an amplifier or a CMOS logic gate. This effect can be used to make an inexpensive light sensor, for example to decide when to turn on the LED illuminator.
Advantages of using LEDs
LED schematic symbol
- LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
- LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
- The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
- When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
- LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
- LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
- LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours
- LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
- LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states “less than 100ns.” LEDs used in communications devices can have even faster response times.
- LEDs can be very small and are easily populated onto printed circuit boards.
- LEDs do not contain mercury, unlike compact fluorescent lamps.
Disadvantages of using LEDs
LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps. In December 2007, scientists at Glasgow University claimed to have found a way to make Light Emitting Diodes brighter and use less power than energy efficient light bulbs currently on the market by imprinting holes into billions of LEDs in a new and cost effective method using a process known as nanoimprint lithography.
LED Applications
Lighting
- Grow lights composed of LEDs are more efficient, both because LEDs produce more lumens per watt than other alternatives, and also because they can be tuned to the specific wavelengths plants can make the most use.
- Light bulbs
- Lanterns
- Streetlights
- Large scale video displays
- Architectural lighting
- Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination.
- Motorcycle and Bicycle lights
- Flashlights, including some mechanically powered models.
- Emergency vehicle lighting
- Backlighting for LCD televisions and displays. The availability of LEDs in specific colors (RGB) enables a full-spectrum light source which expands the color gamut by as much as 45%.
- Stage lights using banks of LED's as replacement for incandescent bulbs. LED's produce less heat so LED stage lighting is cheaper to operate and reduces the risk of fire considerably.
