Signal Lamp
Light Emitting Diode, or Issuing Diode of Light
Characteristic
LED is a diode semiconductor (junction P-N) that when energized emits visible light for that LED (Issuing Diode of Light).
The light is not monochrome (as in a laser), but it consists of a ghastly band relatively it narrows and it is produced by the energy interacções of the electrão. The process of light emission for the application of a source eléctrica of energy eletroluminescência is called. In any junction P-N polarized directamente, inside of the structure, close to the junction, they happen recombinations of gaps and electrões. That recombination demands that the energy possessed by that electrão, that until then was free, be liberated, what happens in the form of heat or light photons.
In the silicon and in the germanium, that you/they are the basic elements of the diodes and transistors, among other components electrónicos, most of the energy is liberated in the form of heat, being insignificant the emitted (due to opacity of the material) light, and the components that work with larger current capacity gets to need of irradiadores of heat (dissipadores) to help in the maintenance of that temperature in a tolerable landing.
Already in other materials, as the gálio (GaAs) arseneto or the gálio (GaP) fosfeto, the number of light fotões emitted is enough to constitute quite efficient light sources.
The simplified form of a junction P-N of a led demonstrates his/her eletroluminescência process. The material dopante of an area of the semiconductor contains atoms with an electron the less in the valency band in relation to the material semiconductor. In the connection, the íons of that material dopante ("taker" íons) remove electrons of valency of the semiconductor, leaving "gaps", therefore (or holes), the semiconductor becomes of the type P. In the other area of the semiconductor, the material dopante contains atoms with an electron the more than the pure semiconductor in his/her valency strip. Therefore, in the connection that electron is available under the form of free electron, forming the semiconductor of the type N.
The semiconductors can also be of the type compensated, that is, they possess both dopantes (P and N). In this case, the dopante in larger concentration will determine what type belongs the semiconductor. For instance, they are existed more dopantes than they would take to P than of the type N, the semiconductor will be of the type P. That will implicate, however, in the reduction of the Mobility of the Bearers.
The Mobility of the Bearers is the easiness with that loads n and p (electrons and holes) cross the crystalline structure of the material without colliding with the vibration of the structure. As larger the mobility of the bearers, minor will be the loss of energy, therefore lower it will be the resistividade.
In the area of contact of the areas, electrons and gaps if recombinam, creating a fine layer practically exempt of load bearers, the call potential barrier, where we just have the íons "donors" of the area N and the "taker" íons of the area P, that for they present not load bearers "isolate" the other gaps of the material P of the other electrons free from the material N.
A free electron or a gap can only cross the potential barrier by the application of external (direct polarization of the junction) energy. Here it is necessary to emphasize a physical fact of the semiconductor: in those materials, the electrons can only assume certain levels of energy (discreet levels), being the valency bands and of transport the one of larger energy levels for the electrons occupy.
The area understood among the top of the one of valency and the inferior part of the one of transport is what is called "forbidden band." If the material semiconductor is pure, he/she won't have electrons in that band (then to be called "forbidden"). The recombination among electrons and gaps, that it happens after having won the potential barrier, can happen in the valency band or in the forbidden. The possibility of that recombination to happen in the forbidden band if it owes to the creation of electronic states of energy in that area for the introduction of other sludges in the material.
As the recombination happens more easily in the closer level of energy of the transport band, it can be chosen the sludges appropriately for the making of LEDs, in way they exhibit her/it appropriate bands for the emission of the color of wanted (specific wavelength) light.
Operation
The emitted light is not monochrome, but the colored band is relatively narrow. The color, therefore, dependent of the crystal and of the dopagem impurity with that the component is manufactured.
The led that uses the gálio arseneto emits infra-red radiations. Taking drugs with match, the emission can be red or it yellows, in agreement with the concentration. Being used gálio fosfeto with dopagem of nitrogen, the emitted light can be green or it yellows.
Nowadays, with the use of other materials, it is gotten to manufacture leds that emit Light Blue, violet and even ultra-violet.
They also exist the white leds, but those are usually issuing leds of blue color, covered with a layer of match of the same type used in the fluorescent lamps, that absorbs the blue light and it emits the White Light.
With the barateamento of the price, his/her high income and his/her great durability, those leds become great substitutes for the common lamps, and they should substitute them the medium or long period.
They also exist the called white leds RGB (more expensive), and that are formed by three "chips", a red (red R), a green (green G) and a blue (blue B). A variation of the leds RGB are leds with an integrated microcontrolador, what allows that is obtained a true show of lights just using a led.
He/she is the physical aspect of some leds and his/her electric symbol.
In general, the leds operate with level of tension from 1,6 to 3,3V, being compatible with the circuits of solid state. It is interesting to notice that the tension is dependent of the length of the emitted wave.
Like this, the infrared leds usually work with less than 1,5V, the reds with 1,7V, the yellows with 1,7V or 2.0V, the green ones among 2.0V and 3.0V, while the leds blue, violet and ultra-violet usually need of more than 3V.
The necessary potency is in the typical strip from 10 to 150 mW, with a time of useful life of 100.000 or more hours.
As the led is a junction device P-N, his/her characteristic of direct polarization is similar to the one of a diode semiconductor.
Being polarized, most of the manufacturers adopts an identification "code" for the determination expresses of the terminals THE (anode) and K (cathode) of the leds.
In the round leds, two codes are common: he/she identifies the terminal K as being that close to a small one chamfers in the lateral of the circular base of his/her involucre ("body"), or for being the shortest terminal of the two. Manufacturers that adopt the two identification forms simultaneously exist.
In the rectangular leds, some manufacturers mark the terminal K with a small "enlargement" of the terminal close to the base of the component, or then they leave that shorter terminal.
But, it can happen of the component not to bring any external reference of identification of the terminals. In that case, if the involucre is semi-transparent, it can identify the cathode (K) as being the terminal that contains the wider internal electrode than the electrode of the other terminal (anode). Besides wider, sometimes the cathode is lower than the anode.
The issuing diodes of light are also used in the construction of the alpha-numeric displays.
There is also leds bi-color, that are constituted by two junctions of different materials in a same involucre, so that an inversion in the polarization changes the color of the emitted light of green for red, and vice-versa.
They still exist bicolor leds with three terminals, being one to work the junction doped with material to produce green light, other to work the junction doped with material to generate the red light, and the third party common to the two junctions.
The common terminal can correspond to the interligação of the anodes of the junctions (bicolor leds in common anode) or of their cathodes (leds bi-color in common cathode).
Although it is usually treated by bicolor (vermelho+verde) led, that led type is in the reality a "three-colored" one, since besides the two independent colors, each one generated in a junction, those two junctions can be polarized simultaneously, resulting in the emission of orange light.
Usually, the leds are used in substitution to the signalling lamps or pilot lamps in the panels of the instruments and several apparel. For fixation in those panels, it is common the use of plastic supports with thread.
As the diode, LED cannot receive tension directly among their terminals, once the current should be limited so that the junction is not damaged. Like this, the use of a resistor limitador in series with Led is common in the circuits that use him/it.
To calculate the value of the resistor the following formula it is used: R = (Vfonte-VLED) / ILED, where Vfonte is the available tension, VLED is the correct tension for LED in subject and ILED is the current that he can support with safety.
Typically, big (of approximately 5 diameter mm, when round) LEDs work with currents of the order from 12 to 30 MA and the small (with approximately 3 diameter mm) ones operate with the half of that value.
Like this:
We adopted I1 = 15 MA and I2 = 8 MA, Vfonte = 12 V, VLED = 2 V:
R1 = (12 - 2)/0,015 = 10/0,015 = 680*
R2 = (12 - 2) /0,008 = 10/0,008 = 1K2*
We approximated the results for the closer commercial values.
LEDs don't support reverse (Vr) tension of significant value, could deteriorate them with only 5V of tension in that sense.
Therefore, when fed by tension C.The., LED be accompany of a diode rectifier in antiparalelo (inverted polarity in relation to LED), with the purpose of driving the semi-cycles us which him - LED - it is in the cut, limiting that reverse tension around 0,7V (tension direct maxim of the diode), a value sufficiently low so that his/her junction doesn't deteriorate.
It can also be adopted a connection in series between the protection diode and LED.
