The LED

LED stands for Light Emitting Diode (light emitting diode). The first LED was developed by Nick Holonyak Jr. (born 1928) in 1962.

LED circuit symbol
The device uses the optical properties of some semiconductor materials to produce photons from the recombination of electron-hole pairs. The electrons and holes are injected into an area of recombination through the two regions of the diode doped with impurities of different types, namely, n-type and p for the electrons for the gaps. The color of the emitted radiation is defined by the distance in energy between the energy levels of electrons and holes, and typically corresponds to the value of the forbidden band of the semiconductor in question.

The LEDs are a special type of pn junction diodes, formed by a thin layer of doped semiconductor material. When subjected to a voltage to reduce the potential barrier of the junction, the electrons of the conduction band of the semiconductor is recombine with holes in the valence band releasing enough energy to produce photons. Because of the reduced thickness of the chip a reasonable number of these photons can leave and be emitted as light. The LEDs are formed from GaAs (gallium arsenide), GaP (Gallium Phosphide), GaAsP (gallium arsenide phosphide), SiC (silicon carbide) and GaInN (gallium nitride and indium). The exact choice of semiconductors determines the emission wavelength peak of the photons, the efficiency in electro-optical conversion, and then the light intensity at the output.

Although it is little known, LEDs are “reversible machines”, in fact if their junction is directly exposed to strong light source or sunlight, a voltage appears at the terminals, depending on the intensity of the radiation and the color of the LED concerned (the maximum for the Blue). This feature is normally used in the construction of sensors, for pointing systems (solar trackers) of small photovoltaic systems or concentrator.

The first LEDs were only available in red. They were used as indicators in electronic circuits, in the seven-segment display and in optocouplers. Subsequently were developed LED that emitted yellow light and green and were built devices that incorporate two LEDs, one red and one green, generally, in the same container, allowing you to view four states (off, green, red, red + green = yellow) with the same

In the 90’s were built with LED efficiency increasingly high and in a range of colors increasing until the realization of blue LED was not possible to develop a device that, by integrating three LEDs (one red, one green and one blue) , could generate any color.

The LEDs in recent years have spread in all applications where need to have high reliability, long life and high efficiency. Some of the main ones are:

• in infrared remote controls
• status indicators (pilot lights)
• backlit LCD display
• in traffic lights and in the “stop” of cars
• message boards
• lighting

The LEDs are increasingly used in lighting replacement of some traditional light sources. Their use in home, then to replace incandescent lamps, halogen and compact fluorescent lamps (commonly called energy saving), it is now possible with remarkable results achieved thanks to the innovative techniques developed in the field. Through the new studies, in fact, the yield quantity of light / consumption was calculated for a minimum of 3 to 1. Basically the limit of LEDs for this type of application is the amount of light emitted (luminous flux in lumens) than in latest models for professional use is around 120 lm but in cheaper models reaches only 20 lumens. An incandescent bulb 60 W emits a luminous flux of 550 lumens. Furthermore, LEDs are brighter than those still cold light with color rendering relatively low.
Their use becomes far more interesting in the professional where the yield of 40-60 lm / W makes them an attractive source. As a comparison just think that an incandescent lamp has yields of about 20 lm / W, while a halogen of 25 lm / W and a fluorescent linear up to 104 lm / W. Other limit their functional lighting is that their emission characteristics and durability are strongly conditioned by the characteristics and power dissipation. Therefore it becomes difficult to identify direct relationships between the various variables, among which also comes into play a further parameter, which is the emission angle of the light beam, which can vary in a range of about 4 degrees to over 120.

LED high brightness SMT

From the application point of view LEDs are now widely used when the lighting system must have the following characteristics:

• miniaturization
• saturated colors
• dynamic effects (color variation RGB)
• long life and robustness
• enhancement of shapes and volumes

In conclusion the advantages of LEDs by the lighting point of view are:

• duration of operation (high emission LEDs arrive at approximately 50,000 hours)
• absence of maintenance costs
• high efficiency (compared to incandescent and halogen)
• clean light because it lacks components IR and UV
• ease of realization of efficient optics plastic
• flexibility of installation of the light
• saturated colors
• possibility of a strong spot effect (almost point-like source)
• safe operation because very low voltage (normally between 3 and 24 Vdc)
• cold-start (up to -40 ° C) without problems
• insensitivity to moisture and vibration
• absence of mercury

As for the absorptions, these are minor compared to those in normal LEDs with high brightness, according to the following table:

The commercial strength of these devices is based on their potential to achieve high brightness (four times greater than that of fluorescent lamps and tungsten filament), low price, high efficiency and reliability (the duration of an LED is one-two orders magnitude higher than that of traditional light sources, especially in conditions of mechanical stress); moreover, they do not require complex power supply circuits, have high switching speed and their construction technology is compatible with that of silicon integrated circuits.

The LEDs are particularly interesting for their characteristics of high luminous efficiency AU / A and reliability. The first high-efficiency LEDs were investigated by engineer Alberto Barbieri at the laboratories of the University of Cardiff (UK) in 1995, noting excellent characteristics for devices in AlGaInP / GaAs contact with transparent indium and tin (ITO). The evolution of the materials is therefore the key to obtain the light sources of the future which have all the characteristics to replace almost all those currently used. In mobile phones are present in smaller format on the market, for the lighting of the keys, on some models of cars and mopeds of new production, are replacing incandescent lamps, for the lights of “location” and “stop” .

In many cases, the LEDs are fed in continuous current with a resistor Rs in series to limit the current to the value of work, which can vary from 5-6 mA to 20 mA when it is required a lot of light. This value also depends on the wavelength of the light emitted by the LED. The necessity of the use of the same is also justified by the need to ensure a long life to the device. If there was no such resistance, a small increase in current use would decrease the value of the differential resistance of the “light emitting diode” as provided by the graph voltage / current.Such variations, if sufficient identity, could trigger a negative differential as the progressive decrease of the resistance of the LED would cause (by feeding a constant voltage as a rule) the increase of the current and an increasing Joule heating which would lead quickly to device to burn. The value of the series resistance Rs is calculated using Ohm’s law knowing the current job If required, the supply voltage Vs and the potential difference of the LED to the current working date, Vf. The potential difference Vf of the LED can be estimated from that date for a current of 20 mA in the datasheet of the product. The LEDs must be made to work only with direct voltage and must not be subjected to reverse voltages greater than a few volts that may damage them. Unlike incandescent bulbs are very sensitive to voltage variations: just 10% less because not light and 10% longer to burn. In general, when you do not have the datasheet specific, it can be considered for the usual LED 5 mm diameter a voltage Vf equal to about 2 V and a current job If prudential of 10-15 mA, up to 20 mA. Higher values of current are generally supported, but do not provide a long-lasting operation.For LEDs flash type, for which as has been said, the current can vary between 20 and 40 mA, the minimum and maximum values of the resistance will be 250 and 500 (standard values 270 Ohm and 470 Ohm). If an LED is fed in alternating must be protected from reverse voltage by means of a simple circuit. The simplest method is to use a diode connected in a configuration in which electronic jargon is called “anti-parallel” to the LED, that is, in parallel to the LED but with reversed polarity (to limit the reverse voltage).

Solitamente il terminale più lungo di un diodo led (diametro package 3 mm, 5 mm o superiori) è chiamato anodo (+) e il terminale più corto catodo.

Per polarizzare correttamente un diodo LED possiamo usufruire inoltre di una caratteristica particolare del package: se si guarda infatti il led dall’alto, si può notare come la parte laterale del package non sia regolare ma squadrata da un lato: questa “squadratura” identifica il catodo (-). Nel caso dei led 3 mm, si rende necessario l’uso di un tester in quanto tale “segno” (se presente) non è quasi visibile.

Se si utilizza un tester, dopo aver selezionato la scala di resistenza con fattore 1 (X1), se si pone il puntale positivo sull’anodo e il puntale negativo sul catodo, il tester segnerà un valore di resistenza dell’ordine di qualche centinaio di ohm, nel caso il tester fosse un modello analogico con pila di alimentazione a 3 volt, se il led è efficiente, essendo polarizzato direttamente, il piccolo flusso di corrente che lo attraversa lo farà accendere, invertendo i puntali, invece, il tester non dovrà segnare alcuna continuità.

Esiste anche un metodo “visivo” ma quasi mai usato dai tecnici per il riconoscimento del catodo o dell’anodo di un led: guardando all’interno del package trasparente si possono vedere due parti metalliche separate di diversa grandezza collegate ai terminali, la parte più grande è sempre collegata al catodo, e di conseguenza la più piccola all’anodo (+).

Conventional LEDs are composed of various inorganic materials which produce the following colors:

• AlGaAs – red and infrared
• GaAlP – Green
• GaAsP – red, orange-red, orange, and yellow
• GaN – green and blue
• GaP – red, yellow and green
• ZnSe – blue
• InGaN – blue-green, blue
• InGaAlP – orange-red, orange, yellow and green
• SiC as a substrate – blue
• Diamond (C) – ultraviolet
• Silicon (Si) as substrate – blue (under development)
• Sapphire (Al2O3) as substrate – blue

Furthermore, the voltage drop of the LED is related to the color of the emitted light, as shown in the following table: