The LED: During and operating principle circuit schematic with explanation
Operating principle
The word LED stands for Light Emitting Diode (Light Emitting Diode in French). The LED symbol similar to that of the diode but two outgoing arrows was added thereto to represent the emitted light radiation. Symbol of the LED.
Electroluminescence
Physical semiconductor teaches that the electrons in the crystalline solids are at specific energy levels. These levels very close to each other, are grouped into "bands of energy." An electron from the valence band can pass into the conduction band provided acquire additional energy at least equal to Delta E. C ' is the photoelectric effect.
An electron from the conduction band can pass into a valence band. In this case it releases energy at least equal to Delta E.
This energy can be:
Dissipated as heat (phonons)
emitted as light (photons).
It is the electro-luminescence (visible or not).
PN junction
This electroluminescence phenomenon will be obtained on condition of creating a large amount of electrons in the conduction band. By carrier injection is obtained by polarizing in the forward direction, a PN junction semiconductor.
The same result could have been achieved by irradiating the crystal with a light source of high energy (photoluminescence) or electron beam ( cathodoluminescence). According to the manufacturing, the light can be emitted either laterally or perpendicularly through the thin layer N or P.
Optical Characteristics
Of the emission peak wavelength
This value indicates the wavelength (lambda p), in nano-meter, which is transmitted to the larger portion of the radiation (wavelength). The value is given for a intensity current (I F).
Spectrum or spectral width at half intensity
The a LED emission spectrum is relatively narrow.
For example, for a wavelength at maximum intensity equal to 520 nm, the wavelength at half intensity may be in the range of 505 nm to 535 nm (an spectral width of 30 nanometers). There are currently several types of LEDs each giving different spectra. This is achieved by the variety of semiconductor used to make the PN junctions. Examples in the following table to obtain certain wavelengths:
| Material | Radiance | Wave length |
|---|---|---|
| InAs | infrared | 315 nm or 3.15 microns |
| InP | infrared | 910 nm |
| GaAsP 4 | red | 660 nm |
| GaAsP 82 | yellow | 590 nm |
| GaP | green | 560 nm |
Color Matching, wavelengths and photon energy
| Color | Wavelength (nm) | Energy photon (eV) |
|---|---|---|
| Ultraviolet | <390 | > 3.18 |
| Purple | 390-455 | 2.72 to 3.18 |
| Blue | 455-490 | 2.53 to 2.72 |
| Cyan | 490-515 | 2.41 to 2.53 |
| Green | 515-570 | 2.18 to 2.41 |
| Yellow | 570-600 | 2.06 to 2.18 |
| Orange | 600-625 | 1.98 to 2.06 |
| Red | 625-720 | 1.72 to 1.98 |
| Infrared | > 720 | <1.72 |
Radiation pattern
The luminous flux is not uniform around the LED. The spatial distribution of the emitted power depends on the shape of the LED:
émissivee shape of the part (dot, line ...)
with or without focusing lens,
diffusing or not.
This distribution is defined by the radiation pattern representing the angular distribution of the transmitted relative intensity.
Example:
Half intensity beam angle
Manufacturers often specify the angle at which the light intensity was reduced by half.
In the diagram above, the red dot indicates a 10 degree angle and Green developed a 50 ° angle for a relative intensity of emitted 50%.
Light intensity
The light intensity (measured in candelas) is the amount of light emitted in a certain direction to 1 meter away. In the optical characteristics of the LEDs we also express micro-candela (mcd) and note I V.
Electrical Specifications
Operating point and direct voltage
LED behaves electrically like a diode. To make it must be biased. The characteristic I F (V F) shows that the conduction voltage of the LED (forward voltage) is about 1.5 Volts to 2 V. The current I E is about E-2V / R. In practice, the manufacturer recommends 10 to 20 mA. The current through the LED determines the light intensity emitted. Note: some diodes construction voltages of the order of 3 Volts and more.
Reverse Voltage (V R)
In some cases, you may need to reverse bias the LED.
The LED is then off: it emits more light intensity.
But beware, the LED can not withstand excessive reverse voltages as a diode recovery for example. Common values are such that V R max = ± 3V to 5V (reverse phase); Beyond these values in there endomagement or destruction of the component. If necessary we place a normal diode in series with the LED.
Continuous forward current (I F)
The direct current (mA) is generally given for an ambient temperature (TA) of 25 ° C. It is the continuous current that can suporter the diode. As a semiconductor heater (with agravement ifTA> 25 ° C), it is recommended to reduce the amount of current (forward current).
Direct peak current (I FM)
This is the intensity of a maximum forward current pulse can be applied to the LED during a specific period. Between two pulses of this intensity, the component must have time to cool. We must therefore choose a ratio of pulse duration and large enough pause time.
Power and operating temperature
The junction temperature must remain below 125 ° C. But often the LEDs are mounted in plastic housings. In this case, the operating temperature should not exceed 100 ° C. The power that can dispel a common LED (or used as a light) is of the order of 20 to 100 mw. The power LED diodes for local lighting applications and public places are the Agenda Watt see much more when it comes to LED module.
Influence of forward voltage
All LEDs have forward voltage variations as a function of junction temperature changes. The temperature coefficient depends on the type of junction. The InGaAlP LEDs (yellow, orange and red) have a coefficient between -3.0 mV / K at -5.2 mV / K, and InGaN LED (blue, green and white) have a coefficient between -3 6 mV / K and -5.2 mV / K.
Influence current I F on the light intensity
The eye is sensitive to the average light intensity emitted. The light intensity given by the manufacturer is achieved under operating conditions that must be specified. Generally uses a direct current (at RT = 25 ° C).
Other current values are reflected by other light intensities. By exploiting other characteristics I v (IF) then we see that the light intensity increases faster than the current, that is to say, the yield increases to a current I F, high but brief, called.
It is therefore extremely interesting to power the LED in pulsed current instead of direct current.The current peak value then provides the important light intensities. Therefore we can:
increase light intensity emitted average power consumption equal,
lower power consumption while achieving an equal light intensity,
reduce the heating of the junction.
Influence of light intensity on the temperature
The light intensity decreases as the temperature increases. This is a result of the evolution of efficiencies in the semiconductor, and not the result of the variation of the forward voltage depending on the temperature. This temperature change is nonlinear.
Offset chromaticity coordinates
The LED color characteristics are dependent on direct current. Particular attention should be paid when using drivers or driver used with LED RGB. Lighting dimmers should not change the color rendering. The preferred solution is a PWM dimming LED for each of the RGB component, or driven with direct current adapted.
Physical characteristics
Through-hole components
The manufacturers now offer various forms of LEDs; the most common of all being around. It is available in several diameters: 1.35 mm, 3 mm, 5 mm ... 10 mm. We also find LED rectangular, triangular, square and bar. We must identify the anode and cathode connections and follow the instructions for implementation during the implementation of the led on the PCB or welding with son.
SMD components
In this form the boxes are less bulky and can be welded in addition to a data surface. They are suitable for performing display, signaling lights, miniature electronic modules or a LED matrix.
LEDs network
Électroluminescences discrete diodes can be arranged in linear or planar networks.
In the first case, they can replace a conventional analog display (galvanometer).
In the second case, they will be used for all kinds of displays, including graphics and controls will usually multiplexed.
Control of a linear array with analog signal
After displaying by the galvanometer and the digital display, there are now more and more, a mixed display, where the analog values are quantized and displayed in scale by all or nothing.
It is the linear analogue display "bar -graph ".
Shapes
The shapes can be varied:
horizontal line
vertical,
multiple,
simple circular,
multiple.
Examples of network linear LEDs: Block Diagram of a LED network: this configuration, although note the availability of each connection of the LEDs.
Illustration of a circular array LED: The display can meet all the mathematical laws (eg logarithmic) and controllable from coded information of all kinds.
