A Light Emitting Diode (LED) is a semiconductor diode capable of emitting light when a voltage is applied to it in the forward direction. In fact, it is a diode that converts electrical energy into light. Depending on the material from which the LED is made, it can emit light of different wavelengths (different colors) and have different electrical characteristics.
LEDs are used in many areas of our life as a means of displaying visual information. For example, in the form of single emitters or in the form of structures of several LEDs - seven-segment indicators, LED matrices, clusters, and so on. Also, in recent years, LEDs have been actively occupying the segment of lighting devices. They are used in car headlights, lanterns, lamps and chandeliers.
LED designation in the diagram
In electrical diagrams, an LED is indicated by a diode symbol with two arrows. The arrows are directed away from the diode, symbolizing the light emission. Do not confuse it with a photodiode with arrows pointing towards it.
On domestic circuits, the letter designation of a single LED is HL.
LED pins and marking
A standard single-color LED has two leads, an anode and a cathode. You can visually determine which of the leads is the anode. For lead wire LEDs, the anode is usually longer than the cathode.
SMD LEDs have the same pins, but on the back side there is usually a marking in the form of a triangle or similar to the letter T.
If it is not possible to visually determine where which conclusions are, you can ring the LED. This requires a power supply or adapter capable of supplying a voltage of about 5 volts. We connect any output of the LED to the minus of the source, and connect the other to the plus terminal of the source through a resistance of 200 - 300 Ohm. If the LED is connected correctly, it will light up. Otherwise, we swap the conclusions in places and repeat the procedure.
You can do without a resistor if you do not connect the positive terminal of the power supply, but quickly "strike" it at the output of the LED. But in general, it is impossible to apply a large voltage to the LED without limiting the current - it may fail!
LED voltage
An LED emits light when a voltage is applied to it in the forward direction: positive to the anode and negative to the cathode.
The minimum voltage at which an LED starts to glow depends on its material. The table below shows the LED voltages at 20 mA test current and the colors they emit. I took this data from the Vishay LED catalog, various datasheets and Wikipedia.
The highest voltage is required for blue and white LEDs, and the lowest voltage is required for infrared and red.
The radiation of an infrared LED is not visible to the human eye, therefore such LEDs are not used as indicators. They are used in various sensors, camcorder backlights. By the way, if the infrared LED is powered up and looked at through the camera of a mobile phone, then its glow will be clearly visible.
The table shown gives approximate LED voltage values. This is usually enough to turn it on. The exact forward voltage of a particular LED can be found in its datasheet in the Electrical Characteristics section. It indicates the nominal value of the forward voltage at a given LED current. For example, let's look at the datasheet for a red SMD LED from Kingbright.
Current-voltage characteristic of the LED
The current-voltage characteristic of an LED shows the relationship between the applied voltage and the LED current. The figure below shows a direct branch of the characteristic from the same datasheet.
If the LED is connected to a power source (to the anode +, to the cathode -) and gradually increase the voltage across it from zero, then the LED current will change according to this graph. It shows that after passing the "bend" point, the current through the LED will sharply increase with small voltage changes. This is exactly the reason why an LED cannot be connected to any power source without a resistor, unlike an incandescent light bulb.
The higher the current, the brighter the LED glows. However, it is naturally impossible to increase the LED current to infinity. With a high current, the LED will overheat and burn out. By the way, if you immediately apply a high voltage to the LED, it can even slap like a weak firecracker!
Other characteristics of the LED
What other characteristics of the LED are of interest from the point of view of practical use?
Maximum power dissipation, maximum values of direct and impulse forward currents and maximum reverse voltage. These characteristics indicate the limiting values of voltages and currents, which should not be exceeded. They are described in the datasheet in the Absolute Maximum Ratings section.
If you apply a voltage to the LED in the opposite direction, the LED will not light up, and in general it may fail. The fact is that with a reverse voltage, a breakdown may occur, as a result of which the reverse current of the LED will increase sharply. And if the power allocated to the LED (reverse current * for reverse voltage) exceeds the permissible value, it will burn out. In some datasheets, the reverse branch of the current-voltage characteristic is additionally given, from which you can see at what voltage the breakdown occurs.
Radiation intensity (luminous intensity)
Roughly speaking, this is a characteristic that determines the brightness of the LED at a given test current (usually 20 mA). It is designated - Iv, and is measured in microcandles (mcd). The brighter the LED, the higher the Iv value. The scientific definition of luminous intensity is on Wikipedia.
Also of interest is the graph of the dependence of the relative radiation intensity of the LED on the forward current. For some LEDs, for example, as the current increases, the radiation intensity grows less and less. The figure shows several examples.
Spectral characteristic
It determines in which wavelength range the LED emits, roughly speaking, the color of the radiation. Typically, the peak value of the wavelength and a graph of the LED intensity versus wavelength are given. I rarely look at this data. I know, for example, that the LED is red and that's enough for me.
Climatic characteristics
They determine the operating temperature range of the LED and the dependence of the LED parameters (forward current and radiation intensity) on temperature. If you plan to use the LED at high or low temperatures, you should pay attention to these characteristics.
How does an LED work?
The material of the article is designed for beginner electronics engineers, and therefore I deliberately do not touch on the physics of the LED. The realization that an LED emits photons as a result of the recombination of charge carriers in the pn junction region does not provide any useful information for the practical use of LEDs. And not only for use, but also for understanding in principle.
However, if you want to delve into this topic, then I give a direction where to dig - V.V. Pasynkov, L.K. Chirkin. "Semiconductor devices" or Zi.S "Physics of semiconductor devices". These are university textbooks - everything is grown up there.
About connecting LEDs in the next article ...
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