A light-emitting diode (LED) is a type of p-n junction diode that converts electrical energy into light energy. The p-n junction consists of a p-type semiconductor and the n-type semiconductor. This is mostly useful in making a 7-segment display.
Working Mechanism of Light-Emitting Diode
We know that p-type semiconductor consists of holes (+ charge) as a majority charge carriers. Similarly, an n-type semiconductor consists of free electrons (- charge) as the majority charge carriers. So, when these two types of semiconductors are brought together, the process of recombination occurs between free electrons and holes.
The negative and positive nature of the charge carrier cancels out each other during the recombination process. This chargeless zone is the depletion layer. Only the charges near the junction take part in the recombination process, as they are close to each other. After that, the diffusion stops.
The edge of the p-type semiconductor will become negatively charged as it receives more free electrons. Similarly, the edge of the n-type semiconductor becomes positively charged. This leads to the formation of an electric field whose direction is from positive to negative.
Now, during the process of forward bias, you supply the positive terminal of the voltage source to the p-type semiconductor and the negative terminal to the n-type semiconductor. The positive charge from the power supply repels the holes of the p-type semiconductor towards the depletion layer. Similarly, the negative charge repels the electrons of the n-type semiconductor towards the depletion layer. Similarly, the electric field created by the depletion layer and the external voltage act in two opposite directions. So, they cancel out each other. Thus, the width of the overall depletion layer decreases in size.
Now, the recombination of holes and free electrons begins again. This leads to the flow of electrical energy. Now, due to the recombination, free electrons release extra energy in the form of a photon while moving from the conduction band to the valence band. Due to this photon of light, LED emits lights of different colors.
Different variations in LED
The band of energy where lies the free electrons is known as the conduction band. Similarly, the band of energy where lies the valence electrons is known as the valence band. The conduction band has got higher energy than that of the valence band. Similarly, the difference between the conduction band and the valence band is the energy gap.
During recombination, electrons release photon energy equal to that of the energy gap. The energy gap depends upon the nature of impurities or the amount of impurity used in the doping of the semiconductors.
An energy gap of less than 1.8 eV does not lie in the visible region. Thus, below it can not be used for the color diode in the visible region. However, you can also find IR LED and UV LED in various applications.
The symbol shows a type of equilateral triangle. The broader side is the positive end. That is the anode or the p-type semiconductor. Similarly, the other thinner end of the symbol is the negative end. That is the cathode or the n-type semiconductor. This is the symbol of a normal diode. Now, two arrows facing upward in the symbol indicate the photon of energy releasing from the diode. This converts the normal diode to the light-emitting diode. Also, the direction of the arrow of the LED is exactly opposite to that of the photodiode.
The given figure is a simple two-pin LED such as different colored diodes, white LEDs, infrared diodes, etc. You can see in the figure that the anode is the longer part of the diode. It is the positive terminal that corresponds to the p side of the p-type semiconductor. Similarly, the cathode is the shorter part of the diode. It is the negative terminal that corresponds to the n side of the n-type semiconductor.
RGB (Red-Green-Blue) LEDs are actually three LEDs compacted tightly. One of the four terminals connects to a common point. It can either be an anode or cathode. The remaining other points connect to different points. The longer one is the anode and the shorter one is the cathode.
We know that red green and blue are the three primary colors. The mixture of these colors at different intensities can cause variation in colors. So, different voltages are applied to various terminals of RGB LEDs to produce different colors.
The LED lamp that you use in your house is also similar to the high-power lamp. In this case, the high AC voltage that you supply to the lamp converts into the DC voltage. Since the LEDs are arranged in the series connection, the lamp needs high power which is possible by the alternating current.
(Application of LED)
- IR LEDs:- They are present in the remote system. Some of them include televisions, air conditioners, OT tables, etc. They are also useful in cameras to capture the day and nighttime images. Infrared is invisible to human eyes.
- UV LEDs:- Ultraviolet radiation is also invisible to the human eyes similar to infrared radiation. UV LEDs are useful for disinfection, sterilization, etc.
- Simple LEDs:- These are the most popular LEDs in the world. They are present in almost every circuit board within electronic devices. They generally act as an indicator to indicate the condition of the board. Besides that, they are also useful for students for study purposes.
- High power LEDs:- They disperse at least 1 watt of power. Nowadays, they are highly used in torch lights. If used in an array, you may even be able to use it in the headlights of automobiles, OT lights, etc.
- RGB LEDs:-They are suitable for decoration in your house, office, concerts, etc. They are also used during the time of festivals such as Tihar.
- LED lamps:-They are used for the lighting purpose during the night time.
- Alphanumeric LEDs:- They are responsible for the display of alphabets and numerical values in the LED display board. A 7-segment display is one of the most popular examples of this kind of LED. You can find this kind of display board in numerous instruments such as biochemistry analyzer, freeze, etc.
From the above table, we can come up with the following conclusions.
- The forward current is 20 mA for the normal color LEDs. It is the required maximum safe current drawn by the LEDs to light up at its full capacity. Since the forward current is too low, resistors are used in the circuit to limit the flow of the current.
- Normal using current is the safe operating current.
- Peak forward current is the current above the forward current. One should never cross this peak value. Else it will burn out.
- Operating temperature is from room temperature to 850 C. If you go beyond this range, there is a high chance of device failure. In such a case, you need to add a heat sink to the system.
- The forward voltage can slightly differ from one color to another. If you supply voltage less than the forward voltage, then it will not turn on.
Testing of LED light
You can check the LED light by using a multimeter. For this, connect the black cable from the common point of the multimeter to the cathode of the LED. Similarly, connect the red cable from the voltage point to the anode. This is the condition for a forward bias. Now set the multimeter to the diode/continuity mode. If the LED is OK, it will turn on and will start to emit light. You will also be able to get some reading in the display of the multimeter. But do remember that you won’t be able to detect light in the case of infrared or UV LEDs.
Since you can not see IR waves with your naked eyes you will need a camera to test the LED. First of all, forward bias the IR LED as stated above. Then turn on the camera, on your mobile phone. Set the camera to video mode. Then observe the IR LED through the camera. You will be able to observe the light if it is working properly. Else you can see the reading in the display to know the condition of the LED.
Another way can be by supplying forward voltage to the diode. For this use the coin lithium battery. This is because the coin battery does not provide a high forward current. You can find this kind of battery in remote controls, watches, etc.
7- Segment Display
A 7-segment display is a type of display that consists of 7 LEDs arranged in a specific order. In the given figure you can see the red lines. Each of these lines indicates one LED. In total, there are 7 lines. Thus it is known as a 7-segment display. At the side of the display, you may even find one or more decimal points that depend upon the manufacturers.
The outer layer indicates the actual border of the individual 7-segment display. In total there are 10 pins. They are numbered from 1 to 10. The alphabets indicate the particular LED. You may even find the display with a slightly different pin configuration.
You can search for the truth table for the display on the internet.
Types of 7-Segment Display
There are two types of 7-segment displays. One of them is the common cathode and the other one is the common anode.
Testing of 7-Segment Display
First of all look at the picture of the 7-segment display shown above. The middle pins (3rd and 8th in our case) can either be a common cathode or common anode. Details may be given on the sides of the display.
Now take a multimeter. Then set the multimeter in the diode/continuity mode. The two middle pins in the opposite directions are continuous. So, you can do the continuity check for those two pins. Then connect the black cable to the common cathode or red cable to the common anode of the 7-segment display. This is to forward bias. Keeping one of the cables fixed at the mid pin, connect the other cable to every other pin one after another. If all the LEDs turn ON, then the display is OK. Else you may have to replace it with a new one.
Advantages of LED Light
- Life span:- It has got a high life expectancy of about 10 – 50 thousand hours.
- It has got high energy efficiency. It can convert at least 75% of electrical energy to light energy. Thus it emits bright light. The remaining portion of energy converts into heat energy.
- It does not consist of any bulb, tubes, or filaments. Thus, there is less chance of the break down of the light.
- LEDs do not produce much heat. As a result, one may easily handle it.
- When you supply electrical energy it will light up instantly. In other words, it does not have any warm-up period.
- It can produce a wide range of colors. This is useful in various types of applications such as in the circuit board.
- It does not consist of any harmful elements in it. So, it is environmentally friendly.
- It is 100% recyclable and according to a latest research the cost of LED light in Australia, US and other countries is anywhere between $5 to $20.
Drawbacks of LED Light
- LED only works at forward biasing conditions, when the minimum threshold value is met. Also, you need to be precise with the amount of supply voltage.
- In case of the application in higher temperature conditions, there can be chances of device failure. In such a case, it is better to use the heat sink to cool down the device.
- The initial cost of LED light is more than other sources of light.
- There might be a risk of blue-light hazard due to the emission of blue light by the blue LED or cool-white LED lamp.