UNDERSTANDING LIGHT EMITTING DIODES( LEDs)

Hi…. friends . I am again here with some discussion on Light Emitting Diodes (LEDs)
So in this article we will discuss about function and uses of LED . LED are most basic electronic component to be used in almost all the electronic circuit in different forms. If you are going to start with electronics you must have proper knowledge of using LEDs efficiently.

INTRODUCTION ( FUNCTION AND CONNECTION)


LEDs are unidirectional devices like diode and works only in forward bias condition and emits light when a power supply is connected across the terminal of LED. There are two terminal in simple LED ( some special LEDs contain more than two terminals).As shown in figure below these two terminal are termed as anode ( represented with a or +) and cathode (represented with k (not c) or -). These two terminals must be connected correct way round as a to positive and k to negative terminal of supply. You can determine ANODE and CATHODE by checking the LED as the larger lead is anode and shorter lead is cathode and cathode is flat part in main LED body. LEDs normally operate at very low voltages 1.5 to 3v .
LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

                                                              anode                                           cathode

SYMBOL

                     

TESTING LEDs


As we know LEDs operate at very low voltage hence Never connect an LED directly to a battery or power supply!

It will be destroyed almost instantly because too much current will pass through and burn it out.
LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

 TYPES OF LEDs


There are a number different LEDs available in the market some of them are discussed here-

Single colour LEDs
LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.
The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent. 

Tri colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.
The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs


A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.

LEDs with different shape and size

There are also so many LEDs varying in shapes like round shape led , square, rectangular and triangular. LEDs. LEDs also vary in size like 5mm (standard) LED and 3mm LED etc.

LEDs different shape and size

As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less. 


Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED.


LED Displays

LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:

 Connecting LEDs in series and Parallel

In Series


Connecting LEDs in series is a good idea to prolong battery life cause in this case we are driving all the LEDs with the same current as just one. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL.

like A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
VL = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage VS is 9V and the current I must be 15mA = 0.015A,
Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200Ω

so choose R = 220Ω

(the nearest standard value which is greater).

In parallel

Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it.
Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

so friends it was all about LED i hope you found it useful in case of any query, doubt or critical view just leave a comment here.