LEDs
(20 267)The working principle of LEDs
The p-n junction is the basic element for semiconductor diodes. It is characterised by virtually zero resistance when it is forward biased - provided that a small value of the voltage called the diffusion voltage is exceeded. In the case of the reverse bias of the p-n junction, it presents an almost infinitely great resistance, and the leakage current that flows through the reverse biased junction is called the reverse current or the drift current. The described property of the p-n junction is referred to as asymmetrical current conduction. Among all types of these components, one of the most common is light emitting diode, also known asLED.
Characteristics of LEDs
A light-emitting diode (LED) is a semiconductor optoelectronic light-emitting device that is widely used in electronics. An LED contains a pn junction, which is the contact of two semiconductors with different types of conductivity (p-cathode, n-anode). LEDs emit infrared, visible, or UV light when charged with an electric current as a result of electroluminescence. This phenomenon is based on the recombination of electrons with electron holes, resulting in the release of energy in the form of photons (electroluminescence). The colour of the light depends on the semiconductor material. It can be, among others, gallium arsenide, gallium phosphide, or gallium arsenophosphide. This type of LED is characterised by high efficiency, long life, and high luminance. Many of them are also distinguished by their low power consumption, although COB (Chip on Board) LEDs are an exception in this respect. Applications include signalling e.g., the presence of power or the operation of a device, special lighting for shops or street lighting, and many others. The selection should be based on the main parameters of the LEDs - luminous efficacy, power, or maximum current. The key is the rated current. It is also worth considering the colour of the LED or the mounting method.
The use of LEDs. LEDs are an increasingly common light source that is used, among others, for signalling, e.g., in the form of control lamps, in various types of displays, but also as light sources that replace traditional light bulbs, halogen lamps, or standard automotive lighting, including the so-called xenons. This is mainly due to their high durability and low power consumption, but also high efficiency, low supply voltage, low energy losses and small dimensions with high luminance values.
Construction of Light Emitting Diode (LED).
Light emitting diodes, as the name suggests, are based on the phenomenon of luminescence. Contrary to ordinary light bulbs that produce light with a wire filament heated to a high temperature, in the case of LEDs, the electric energy is directly converted into light energy (electromagnetic radiation quanta). LEDs can emit, in addition to visible light of different colours, infrared or ultraviolet radiation. Specific LEDs emit light in a narrow wavelength range, which means that they light up in one defined colour. The colour of the diode's light depends on the type of semiconductor, and more specifically on the junction semiconductor material.
Given the fact that white light is the sum of all visible wavelengths, making a white LED requires special solutions. One of them is placing three LEDs, red, green, and blue, in one package, which, with properly selected light intensity of each of them, allows us to obtain white colour. The disadvantage of this solution is the high price, while the advantage of this type of LEDs is its high light efficiency. Another solution uses ultraviolet light emitting LEDs and a phosphor that emits white light when excited by the UV LED. This solution is cheaper, but the LEDs using the described phenomenon have lower efficiency. The problem in this case is also the complete elimination of harmful UV radiation. The last method is to combine a blue LED and yellow phosphor. The phosphor is excited by UV LED and emits light of a different wavelength (yellow light). The phosphor ratio determines the perceived colour. In this way, warm or cold light can be obtained, depending on the type of phosphor used.
Choosing a LED
LEDs come in many different housings and are available in surface and through hole mounting versions. The former is widely used mainly in small-size electronics, but not exclusively. Examples include indicator lights, which are mounted on microcontroller development boards, notification lights in smartphones, and indicator lights in various household appliances and even industrial equipment.
LED connection vs forward voltage and power supply.
When connecting LEDs in a circuit, several aspects need to be considered. One of the most important is to limit the current that will flow through the LED, which is crucial to avoid overheating and burning it out. This can be done using a resistor connected in series, the resistance value of which should be chosen according to the supply voltage and the current to flow through the diode. This can be calculated using Ohm's law, which states that the current flowing through a conductor is proportional to the voltage between its ends and inversely proportional to the resistance of the conductor. This can be expressed using the following mathematical formula: R = U / I, the supply voltage minus the voltage drop across the LED, given in volts [V], and I - the selected current in amperes [A]. R is the required value of the resistance of the resistor in ohms [Ω]. The higher the value of the current flowing through the LED, the more intensively it will shine. This relationship is linear over a large current range, but the current rating of the LED should not be exceeded. This can damage it permanently due to excessive heat build-up. If the user intends to use a larger number of LEDs connected in series, it is worth familiarising oneself with the subject of powering such circuits with a constant current source, e.g., in the form of a suitable power supply. Thus, the same current flows through all the LEDs; this will not change even in the event of a short circuit on one or more of them. This solution is used, among others, in more and more popular LED strips, used as lighting or interior decoration.
LED power
Depending on the application and expectations, it is often necessary to choose the LED power (in watts [W]) and its brightness (in lumens [lm] or candelas [cd]). LEDs used e.g. as controls will not need high power and brightness, but in the case of the most powerful LEDs for lighting applications, e.g. for flashlights or room lighting, higher parameters are required.
Light-emitting diodes (LEDs)
Of course, when choosing a light-emitting diode, it is also worth paying attention to its housing, dimensions, or beam angle, which can be very narrow, e.g. 10 °, but also very wide, reaching even 175 °. Sometimes the colour of the LED lens can also be important. In this case, it is best to pay attention to the wavelength emitted.
LEDs in TME's offer
TME offers LEDs in a wide range of types and with a wide range of parameters. In the search filters, you can choose the type of LED, light colour, operating voltage, or maximum supply current (from 1 mA to even 6000 mA). TME offers both individual elements and ready-made light sources in the form of tapes, strips, and modules. When choosing, it is also worth checking the mounting variant - THT or SMD.
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