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Presentation by Evren EKMEKÇİ

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 ] ight emitting diode (LED) is essentia y a PN junction
opto-semiconductor that emits a monochromatic (sing e co or) ight
when operated in a forward biased direction.
LEDs convert e ectrica energy into ight energy. They are
frequent y used as "pi ot" ights in e ectronic app iances to indicate
whether the circuit is c osed or not.

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The most important part of a light emitting diode (LED) is the
semi-conductor chip located in the center of the bulb as shown at the
right. The chip has two regions separated by a junction. The p region
is dominated by positive electric charges, and the n region is
dominated by negative electric charges. The junction acts as a barrier
to the flow of electrons between the p and the n regions. Only when
sufficient voltage is applied to the semi-conductor chip, can the
current flow, and the electrons cross the junction into the p region.




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hen sufficient vo tage is app ied to the

chip across the eads of the LED, e ectrons can
move easi y in on y one direction across the junction
between the p and n regions.

In the p region there are many more

positive than negative charges.

hen a vo tage is app ied and the current

starts to f ow, e ectrons in the n region have
sufficient energy to move across the junction into
the p region.

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Each time an e ectron recombines
with a positive charge, e ectric potentia
energy is converted into e ectromagnetic
energy.

For each recombination of a negative

and a positive charge, a quantum of
e ectromagnetic energy is emitted in the
form of a photon of ight with a frequency
characteristic of the semi-conductor materia
(usua y a combination of the chemica
e ements ga ium, arsenic and phosphorus)..

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Never connect an LED direct y to a

battery or power supp y! It wi be destroyed
a most instant y because too much current wi
pass through and burn it out.

LEDs must have a resistor in series to

imit the current to a safe va ue, for quick
testing purposes a 1k resistor is suitab e for
most LEDs if your supp y vo tage is 12V or ess.

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The energy (E) of the ight emitted by an LED is re ated to the

e ectric charge (q) of an e ectron and the vo tage (V) required to ight the
LED by the expression: E = qV Jou es.

This expression simp y says that the vo tage is proportiona to

the e ectric energy, and is a genera statement which app ies to any
circuit, as we as to LED's. The constant q is the e ectric charge of a
sing e e ectron, -1.6 x 10-19 Cou omb.

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uppose you measured the vo tage across the eads of an LED,

and you wished to find the corresponding energy required to ight the
LED. Let us say that you have a red LED, and the vo tage measured
between the eads of is 1.71 Vo ts. o the Energy required to ight the
LED is

E = qV or E = -1.6 x 10-19 (1.71) Jou e,

since a Cou omb-Vo t is a Jou e. Mu tip ication of these numbers then gives

E = 2.74 x 10-19 Jou e.


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% ensor ]pp ications

% Mobi e ]pp ications
% ign ]pp ications
% ]utomative Uses
% LED igna s
% I uminations
% Indicators

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{ Medica Instrumentation
{ Bar Code Readers
{ Co or & Money ensors
{ Encoders
{ Optica witches
{ Fiber Optic Communication

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{ Mobi e Phone
{ PD]'s
{ Digita Cameras
{ Lap Tops
{ Genera Back ighting

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{ Fu Co or Video
{ Monochrome Message Boards
{ Traffic/VM
{ Transportation - Passenger Information

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{ Interior Lighting - Instrument Pane s & witches, Courtesy Lighting
{ Exterior Lighting - CHML, Rear top/Turn/Tai
{ Truck/Bus Lighting - Retrofits, New Turn/Tai /Marker Lights

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{ Traffic
{ Rai
{ ]viation
{ Tower Lights
{ Runway Lights
{ Emergency/Po ice Vehic e Lighting

LEDs offer enormous benefits over traditiona incandescent amps

inc uding:

{ Energy savings (up to 85% ess power than incandescent)

{ Reduction in maintenance costs
{ Increased visibi ity in day ight and adverse weather conditions

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×]rchitectura Lighting
×ignage (Channe Letters)
×Machine Vision
×Retai Disp ays
×Emergency Lighting (Exit igns)
×Neon Rep acement
×Bu b Rep acements
×F ash ights
×Outdoor ]ccent Lighting - Pathway, Marker Lights

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LEDs not on y consume far ess e ectricity than traditiona
forms of i umination, resu ting in reduced energy costs, but require
ess maintenance and repair. tudies have shown that the use of LEDs
in i umination app ications can offer:
×Greater visua appea
×Reduced energy costs
×Increased attention capture
×avings in maintenance and ighting rep acements
]s white LED techno ogy continues to improve, the use of
LEDs for genera i umination app ications wi become more preva ent in
the industry.

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× Househo d app iances

× VCR/ DVD/ tereo and other audio and video devices
× Toys/Games
× Instrumentation
× ecurity Equipment
× witches

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{ ]na og LED Drive Circuits

{ Digita LED Drive Circuits

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LEDs are made from ga ium-based
crysta s that contain one or more additiona
materia s such as phosphorous to produce a
distinct co or. Different LED chip
techno ogies emit ight in specific regions
of the visib e ight spectrum and produce
different intensity eve s.
LEDs are avai ab e in red, orange, amber, ye ow, green, b ue and
white. B ue and white LEDs are much more expensive than the other
co ours. The co our of an LED is determined by the semiconductor
materia , not by the co ouring of the 'package' (the p astic body). LEDs of
a co ours are avai ab e in unco oured packages which may be diffused
(mi ky) or c ear (often described as 'water c ear'). The co oured packages
are a so avai ab e as diffused (the standard type) or transparent.

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The most popu ar type of tri-co our LED has a red and a green
LED combined in one package with three eads. They are ca ed tri-
co our because mixed red and green ight appears to be ye ow and this
is produced when both the red and green LEDs are on.

The diagram shows the construction of a tri - co our LED. Note

the different engths of the three eads. The centre ead (k) is the
common cathode for both LEDs, the outer eads (a1 and a2) are the
anodes to the LEDs a owing each one to be it separate y, or both
together to give the third co our.

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] bi-co our LED has two LEDs wired in

'inverse para e ' (one forwards, one
backwards) combined in one package with two
eads. On y one of the LEDs can be it at one
time and they are ess usefu than the tri-
co our LEDs described above.

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LED performance is based on a few primary characteristics:

{ o or
{ hite ight
{ Intensity
{ Eye safety information
{ Visibi ity
{ Operating Life
{ Vo tage/Design urrent

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Peak wave ength is a function of the LED chip materia .

] though process variations are ±10 NM, the 565 to 600 NM
wave ength spectra region is where the sensitivity eve of the
human eye is highest. Therefore, it is easier to perceive co or
variations in ye ow and amber LEDs than other co ors.

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hen ight from a parts of the visib e spectrum over ap one

another, the additive mixture of co ors appears white. However,
the eye does not require a mixture of a the co ors of the
spectrum to perceive white ight. Primary co ors from the upper,
midd e, and ower parts of the spectrum (red, green, and b ue),
when combined, appear white.

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LED ight output varies with the type of chip, encapsu ation,
efficiency of individua wafer ots and other variab es. evera LED
manufacturers use terms such as "super-bright," and "u tra-bright´ to
describe LED intensity. uch termino ogy is entire y subjective, as
there is no industry standard for LED brightness.

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The need to p ace eye safety abe ing on LED products is

dependent upon the product design and the app ication. On y a few
LEDs produce sufficient intensity to require eye safety abe ing.
However, for eye safety, do not stare into the ight beam of any LED
at c ose range

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Luminous intensity (Iv) does not represent the tota ight

output from an LED. Both the uminous intensity and the spatia
radiation pattern (viewing ang e) must be taken into account. If two
LEDs have the same uminous intensity va ue, the amp with the arger
viewing ang e wi have the higher tota ight output.


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Because LEDs are so id-state devices they are not subject to

catastrophic fai ure when operated within design parameters. DDP®
LEDs are designed to operate upwards of 100,000 hours at 25°
ambient temperature. Operating ife is characterized by the
degradation of LED intensity over time. hen the LED degrades to ha f
of its origina intensity after 100,000 hours it is at the end of its usefu
ife a though the LED wi continue to operate as output diminishes.
Un ike standard incandescent bu bs, DDP® LEDs resist shock and
vibration and can be cyc ed on and off without excessive degradation.

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LEDs are current-driven devices, not vo tage driven. ] though

drive current and ight output are direct y re ated, exceeding the
maximum current rating wi produce excessive heat within the LED chip
due to excessive power dissipation. The resu t wi be reduced ight
output and reduced operating ife.
LEDs that are designed to operate at a specific vo tage contain a
bui t-in current- imiting resistor. ]dditiona circuitry may inc ude a
protection diode for ] operation or fu -bridge rectifier for bipo ar
operation. The operating current for a particu ar vo tage is designed to
maintain LED re iabi ity over its operating ife.

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Bargraph 7-segment tarburst Dot matrix


 


{ http://www.marktechopto.com/
{ http://acept. a.asu.edu/courses/phs110/expmts/exp13a.htm
{ http://www.kpsec.freeuk.com/components/ ed.htm
{ http://www.fiber-optics.info/artic es/LEDs.htm
{ http://www.the ed ight.com/technica 1.htm
{ http://hyperphysics.phy-astr.gsu.edu/hbase/e ectronic/ eds.htm
{ www.kpsec.freeuk.com/components/ ed.htm


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