What is Light-Emitting Diodes (LEDs) and how it works

What is Light-Emitting Diodes (LEDs) and how it works

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On: 13 Nop, 2018

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LED  101—I
* 2 lead semiconductor device.
* Light emitting PN-junction diode.
    - Visible or infrared light.
* Has polarity.
* Recall diodes act as a one way gate to current flow.
 &n...

LED  101—I
* 2 lead semiconductor device.
* Light emitting PN-junction diode.
    - Visible or infrared light.
* Has polarity.
* Recall diodes act as a one way gate to current flow.
    -–A forward-biased PN-junction diode allows current flow from anode to cathode
* An  LED conducts  and  emits  light  when  its  anode  is  made  more  positive  (approx. 1.4V) than its cathode.
    – With reverse polarity, LED stops conducting and emitting light


what is light emitting diodes (leds) and how it works 1.jpg


LED  101—II
* Similar to diodes, LEDs are current-dependent devices.
    LED brightness is controlled by controlling current through LED.
        Too little current through LED -> LED remains OFF
        Small current through LED -> dimly lit LED.
        Large current through LED -> brightly lit LED.
        Too much current through LED -> LED is destroyed.
* A resistor placed in series with LED accomplishes current control.


what is light emitting diodes (leds) and how it works 2.jpg


LED  101—III
* Let Vs be the supply voltage
* Let Vf be the required forward bias voltage for the LED.
* Let I be the desired current flow through LED.
* Then,  the  current  limiting  resistance  R  is  sized  as  follow:
what is light emitting diodes (leds) and how it works 3.jpg


what is light emitting diodes (leds) and how it works 4.jpg

what is light emitting diodes (leds) and how it works 5.jpg


* If  R  is  chosen  smaller  than  the  above  value,  a larger current will flow through the LED.
–LEDs can  handle  only  limited  current  (varies  from 20mA to 100mA).
–If current through LED is larger than the maximum allowed value, than the LED will be damaged.
Visible-Light LED
* Inexpensive and durable.
• Typical usage: as indicator lights.
• Common  colors:  green  (~565nm),  yellow  (~585nm),  orange  (~615nm), and  red (~650nm).
• Maximum forward voltage: 1.8V.
• Typical operating currents: 1 to 3mA.
• Typical brightness levels: 1.0 to 3.0mcd/1mA to 3.0mcd /2mA.
• High-brightness LEDsexist.–Used in high-brightness flashers (e.g., bicycle flashers).


what is light emitting diodes (leds) and how it works 6.jpg


Blinking LED  
• Contain a miniature integrated circuit that causes LED to flash  from 1 to 6 times/second.
• Typical usage: indicator flashers. May also be used as simple oscillators.


what is light emitting diodes (leds) and how it works 7.jpg


Tricolor LED
• Two LEDs placed in parallel facing opposite directions
• One LED is red or orange, the other is green.
• Current flow in one direction turns one  LED  ON  while the  other remains OFF due to reverse bias.
• Current flow in the other direction turns the first LED OFF and  the second LED ON.
• Rapid  switching  of  current  flow  direction  will  alternatively  turn  the  two  LEDs  ON giving yellow light.
• Used as a polarity indicator.
• Maximum voltage rating: 3V
• Operating range: 10 to 20mA

what is light emitting diodes (leds) and how it works 8.jpg


7-Segment LED Display   
• Used for displaying numbers and other characters.
• 7 individual LEDs are used to make up the display.
• When a voltage is applied across one of the LEDs, a portion of the 8 lights up.
• Unlike liquid crystal displays  (LCD), 7-segment  LED displays tend to  be  more rugged, but they also consume more power.

what is light emitting diodes (leds) and how it works 9.jpg


How LED Works
• The light-emitting section of an LED is made by joining n-type and p-type semiconductors together to form a pn junction.
• When  the pn junction  is  forward-biased,  electrons  in  the  n  side  are  excited  across  the pn junction and into the p side, where they combine with holes.
• As the electrons combine with the holes, photons are emitted.
• The pn-junction  section  of  an  LED  is  encased  in  an  epoxy  shell  that  is doped  with  light scattering particles to diffuse light and make the LED appear brighter.
• Often a reflector placed beneath the semiconductor is used to direct the light upward.
what is light emitting diodes (leds) and how it works 10.jpg

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