Here present the LED. and this article to knowledge about LED.
LED is light emitting diode. LED are a simple invention with huge potential to change the lighting industry for the better.
What is LED?
A light emitting diode is a semiconductor device that emits visible light when an electrical current passes through it. It is essentially the opposite of a photovoltaic cell(a device that converts visible light into electrical current.)
Light emitting PN-junction diode.
– Visible or infrared light.
• 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.
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.
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.
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.
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.
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 LEDs exist.
– Used in high-brightness flashers (e.g., bicycle flashers).
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
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