LEDs - how it works, polarity, resistor calculation

 LEDs are one of the most popular electronic components used in almost any circuit. The phrase “blinking the LEDs” is often used to refer to the first task when checking the viability of a circuit. In this article, we will learn how LEDs work, make a brief overview of their types, and also deal with such practical issues as determining the polarity and calculating the resistor.

Contents hide ]

  • 1 LED device
  • 2  
    • 2.1 LED colors
  • 3 How LEDs work
  • 4 Types of LEDs, classification
  • 5 LED polarity
  • 6 Resistance calculation for LED

LED device

LEDs are semiconductor devices with an electron-hole junction, which creates optical radiation when an electric current is passed through it in the forward direction.

The light emitted by the LED lies in a narrow range of the spectrum. In other words, its crystal initially emits a specific color (if we are talking about LEDs in the visible range) - in contrast to a lamp emitting a wider spectrum, where the desired color can be obtained only by using an external light filter. The emission range of an LED largely depends on the chemical composition of the semiconductors used.

 

An LED consists of several parts: 

  • an anode through which a positive half-wave is supplied to the crystal; 
  • a cathode through which a negative half-wave is applied to the crystal; 
  • reflector; 
  • semiconductor crystal; 
  • diffuser.  

These elements are in any LED, regardless of its model.  

The LED is a low voltage device. For indicator types, the supply voltage should be 2-4 V at a current of up to 50 mA. Diodes for lighting consume the same voltage, but their current is higher - it reaches 1 Ampere. In the module, the total voltage of the diodes turns out to be 12 or 24 V.  

The LED must be connected observing the polarity, otherwise it will fail.  

LED colors

LEDs come in a variety of colors. There are several ways to get the desired shade.  

The first is the coating of the lens with a phosphor. Almost any color can be obtained this way, but most often this technology is used to create white LEDs.  

RGB technology. The hue is obtained by using three red, green and blue LEDs in one crystal. The intensity of each of them changes, and the desired glow is obtained.  

The use of impurities and various semiconductors. Materials with the required band gap are selected, and from them the LED crystal is made.   

How LEDs work

Any LED has a pn junction. The glow arises from the recombination of electrons and holes in the electron-hole transition. A Pn junction is created when two semiconductors of different types of electrical conductivity are connected. The n-type material is doped with electrons, and the p-type material is doped with holes.  

When voltage is applied, electrons and holes in the pn junction begin to move and take up space. When charge carriers approach an electron-hole transition, electrons are placed in a p-type material. As a result of the transition of electrons from one energy level to another, photons are released. 

Not every pn junction can emit light. To transmit light, two conditions must be met: 

  • the band gap should be close to the energy of a light quantum; 
  • the semiconductor crystal must have a minimum of defects.  

It will not be possible to implement this in a structure with one pn-junction. For this reason, multilayer structures of several semiconductors are created, which are called heterostructures.  

To create LEDs, direct-gap conductors with a permitted direct optical zone-to-zone transition are used. The most common materials of the A3B5 group (gallium arsenide, indium phosphide), A2B4 (cadmium telluride, zinc selenide).  

The color of a light emitting diode depends on the bandgap in which the electrons and holes recombine. The wider the band gap and the higher the energy of the quanta, the closer to blue the emitted light. By changing the composition, it is possible to achieve luminescence in a wide optical range - from ultraviolet to mid-infrared radiation.  


Infrared, red and yellow LEDs are made on the basis of gallium phosphide, green, blue and violet - on the basis of gallium nitrides.  

Types of LEDs, classification

Indicator and lighting LEDs are distinguished according to their intended purpose. The former are used for stylization, decorative lighting - for example, building decoration, advertising banners, garlands. Lighting fixtures are used to create bright lighting in a room.  

By the type of execution, they are distinguished: 

  • Dip leds. They are crystals enclosed in a cylindrical lens. Refers to indicator LEDs. There are monochrome and multi-color devices. They are rarely used due to their shortcomings: large size, small angle of illumination (up to 120 degrees), decrease in radiation brightness by 70% during long-term operation, weak light flux. 
    Dip leds
    Dip leds

     

  • Spider led. These LEDs are similar to the previous ones, but have 4 outputs. In such diodes, the heat dissipation is optimized, and the reliability of the components is increased. They are actively used in automotive engineering.  
  • Smd - LEDs for surface mounting. May refer to both indicator and lighting LEDs. 
    Smd
    Smd

     

  • Cob (Chip-On-Board) - the crystal is installed directly on the board. The advantages of this solution include protection against oxidation, small dimensions, efficient heat dissipation and uniform illumination throughout the entire area. LEDs of this brand are the most innovative. Used for lighting. More than 9 LEDs can be installed on one substrate. From above, the LED matrix is ​​covered with a phosphor. They are actively used in the automotive industry to create headlights and turn signals, in the development of televisions and computer screens.   
    Cob
    Cob
  • Fiber - development in 2015. Can be used in the manufacture of clothing.  
    Fiber
    Fiber
  • Filament is also an innovative product. They are characterized by high energy efficiency. Used to create lighting lamps. An important advantage is the ability to mount directly on a glass substrate. Thanks to this application, it is possible to spread the light 360 degrees. The structure consists of sapphire glass with a diameter of up to 1.5 mm and specially grown crystals, which are connected in series. The number of crystals is usually limited to 28 pieces. The LEDs are placed in a bulb covered with a phosphor. Filament LEDs can sometimes be classified as COB products. 
    Filament
    Filament

     

  • Oled. Organic thin-film LEDs. Used to build organic displays. They consist of an anode, a foil or glass substrate, a cathode, a polymer layer, a conductive layer of organic materials. The advantages include small size, uniform illumination over the entire area, wide beam angle, low cost, long service life, low power consumption.  
    Oled
    Oled

  • A separate group includes LEDs emitting in the ultraviolet and infrared ranges. They can be with outputs, and in the form of smd execution. Used in remote controls, germicidal and quartz lamps, sterilizers for aquariums.  

LEDs can be:

  • blinking - used to attract attention;
  • multicolor flashing;
  • tricolor - in one case there are several unconnected crystals that work both separately and all together;
  • RGB;
  • monochrome.

LEDs are classified by color. For the most accurate color identification in the documentation of the device, its radiation wavelength is indicated.  

White LEDs are classified by color temperature. They come in warm shades (2700 K), neutral (4200 K) and cold (6000 K). 

In terms of power, LEDs are distinguished, consuming units of mW up to tens of watts. The intensity of the light directly depends on the power.  

LED polarity

LED polarity
LED polarity

If turned on incorrectly, the LED may break. Therefore, it is important to be able to determine the polarity of the light source. Polarity is the ability to carry electrical current in one direction.  

Polarity can be mono determined in several ways: 

  • Visually. This is the easiest way. To find the plus and minus of a cylindrical diode with a glass bulb, you need to look inside. The cathode area will be larger than the anode area. If you can't look inside, the polarity is determined by the contacts - the long leg corresponds to the positive electrode. SMD LEDs have polarity markings. They are called a bevel or key that points towards the negative electrode. On small smd, pictograms are applied in the form of a triangle, the letters T or P. An angle or protrusion indicates the direction of the current - which means that this conclusion is a minus. Also, some LEDs may have a label that indicates polarity. It can be a dot, a circular strip.  
  • By connecting the power supply. By applying a low voltage, the polarity of the LED can be checked. To do this, you need a current source (battery, accumulator), to the contact of which an LED is applied, and a current-limiting resistor through which the connection is made. The voltage needs to be increased and the LED should light up when turned on correctly.  
  • With the help of testers. The multimeter allows you to check polarity in three ways. The first is resistance testing in position. When the red probe touches the anode and the black cathode, a number other than 1 should light up on the display. Otherwise, the number 1 will light up on the screen. The second method is in the dialing position. When the red probe touches the anode, the LED will light up. Otherwise, he will not react. The third way is by installing an LED in the socket for the transistor. If a cathode is placed in hole C (collector), the LED will light up.  
  • According to the technical documentation. Each LED has its own label, which can be used to find information about the component. The polarity of the electrodes will also be indicated there.  

The choice of polarity detection method depends on the situation and whether the user has the correct instrument.  

Resistance calculation for LED

The diode has a low internal resistance. If you connect it directly to the power supply, the element will burn out. To prevent this from happening, the LED is connected to the circuit through a current-limiting resistor. The calculation is performed according to Ohm's law: R = (U-Uled) / I, where R is the resistance of the current-limiting resistor, U is the power supply of the source; Uled is the nameplate voltage value for the LED, I is the current strength. According to the obtained value, the power of the resistor is selected.  

It is important to correctly calculate the voltage. It depends on the connection diagram of the elements.  

You can skip calculating the resistance if you use a powerful variable or trimmer resistor in the circuit. Current limiting resistors are available in different accuracy classes. There are products for 10%, 5% and 1% - this means that the error varies within the specified range.  

When choosing a current-limiting resistor, you need to pay attention to its power. almost always, if with little heat dissipation, the device will overheat and fail. This will break the electrical circuit.  

When to use a current limiting resistor: 

  • when the issue of the effectiveness of the circuit is not the main one - for example, indication; 
  • laboratory research. 

In other cases, it is better to connect LEDs through a stabilizer - driver, which is especially true in LED lamps. 


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