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Light Emitting Diodes

Posted on : Sun , 03 2014 by : virusi

Theory OF OPERATION :


LED construction :

Light Emitting Diodes or LED´s is basically just a specialized type of PN junction diode, made from a very thin layer of fairly heavily doped semiconductor material.

LED_construction
Fig.1 Led Construction

When the diode is forward biased, electrons(K) from the semiconductors conduction band recombine with holes(A) from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single color) of light. Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a colored light output.

An LED junction does not actually emit that much light so the epoxy resin body is constructed in such a way that the photons of light emitted by the junction are reflected by the reflective cavity(see Fig.1). Light in reflective cavity is focused upwards this is why light appears to be brightest at the top of the LED.

The flat spot on the body of the LED is the cathode (K) and on the other PIN of the LED is the Anode(A).

Unlike normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, the light emitting diode produces a “cold” generation of light which leads to high efficiencies than the normal “light bulb” because most of the generated energy radiates away within the visible spectrum.

Light Emitting Diode Colors :

Depending on the semiconductor compounds LED will emitted different wavelength that will lead to a different color that is emitted by the LED.

Light_Emitting_Diode_Colours
Fig.2 LED colors

Therefore the color of the light emitted by an LED is NOT determined by the coloring of the LED’s plastic body although these are slightly colored to both enhance the light output and to indicate its color when its not being illuminated by an electrical supply. Like conventional PN junction diodes, light emitting diodes are current-dependent devices with its forward voltage drop VF, depending on the semiconductor compound (its light color) and on the forward biased LED current. The point where conduction begins and light is produced is about 1.2V for a standard red LED to about 4V for a white LED. The exact voltage drop will of course depend on the manufacturer because of the different doped materials and wavelengths used.

Light Emitting Diodes V-I Characteristics :

LEDcharacteristcs
Fig. 3 Led Characteristics

Before a light emitting diode can “emit” any form of light it needs a current to flow through it, as it is a current dependant device with their light output intensity being directly proportional to the forward current flowing through the LED. As the LED is to be connected in a forward bias condition across a power supply it should be current limited using a series resistor to protect it from excessive current flow. Never connect an LED directly to a battery or power supply as it will be destroyed almost instantly because too much current will pass through and burn it out. From the Fig 3. We can see that each LED has it own its own forward voltage drop across the PN junction and this parameter which is determined by the semiconductor material used, typically for a forward current of 20mA.

PRACTICAL EXAMPLE :


If you don’t see the example below than you should follow this steps:

– In your browser allow Java SE 7.

– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .

– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).

Let’s take a look at a simple example, where we connect a LED in series with a resistor to a battery. We can manipulate the current that is flowing through the LED by changing the resistor value.

Sorry, you need a Java-enabled browser to see the simulation.

If you want to change resistor or battery value than you should double click on the element and insert the desired value. First Diagram is showing LED V-I characteristic, you can play with the resistance value and see how the I-V characteristic is changing. The second diagram shows the I and V that is flowing through the LED.

Last updated on Mon , 03 2014
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