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Old 08-27-2001, 11:19 AM
Jinx Jinx is offline
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OK, how many people does it take to change - an LED? Do these light emitting diodes ever stop emitting? Also, what exactly is the source of the light?
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  #2  
Old 08-27-2001, 11:29 AM
Evnglion Evnglion is offline
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well, I am not sure about the source, but i do know that they are supopsed to last, theoretically, forever. I know a lot of car companies are starting to encorporate them into their brake light design so that they will never have to be replaced.
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Old 08-27-2001, 11:43 AM
Spritle Spritle is offline
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LEDs are low voltage things. They don't generate as much destructive heat as lightbulbs. They also don't have filaments as traditional bulbs have. They can last a right long while.

That said, they can blow. This occurs mostly if a spike voltage crosses the diode. If it blows, it can be replaced.
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Old 08-27-2001, 12:01 PM
Patty O'Furniture Patty O'Furniture is offline
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With careful circuit design (limiting current to safe levels), LEDs can last tens of thousands, possibly hundreds of thousands of hours.

http://hyperphysics.phy-astr.gsu.edu...ic/led.html#c2
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Old 08-27-2001, 12:17 PM
ModernRonin2 ModernRonin2 is offline
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Yes, but it takes a long, long time...

MTBF (mean time between failures - basically, on average how long a component lasts) for most LEDs is something like 100,000 hours. That's, like, 11 years of continuous use.

If I'm remembering right, the main reason LEDs burn out is because doping atoms migrate from one region of the diode to the other. Basically, LEDs are composed of two slightly different kinds of silicon sandwiched together. They both have special kinds of impurities mixed into them to give them certain electrical properties - like, for instance, glowing when you run a current through them. Some of these impurities can slowly migrate through the silicon sandwich and contaminate the other parts. Like the moisture in a tuna sandwich making the bread soggy. When enough contamination happens, the LED doesn't work any more.

But it takes a long, long time. Much longer than a conventional lightbulb. Ever hear of an average light-bulb lasting 11 years?


-Ben
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Old 08-27-2001, 12:38 PM
Chronos Chronos is offline
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Wasn't there a case of a single light bulb remaining on contuinuously for 90 years, or some such? "On" time doesn't put too much wear on an incandecent light bulb; it's the process of being turned on that usually does it. LEDs do have a considerable advantage here, though, since they don't seem to be worn at all by being turned on.
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Old 08-27-2001, 01:49 PM
panamajack panamajack is offline
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Everything you ever wanted to know about LEDs but were afraid to be bored to death

As to the source of the light, I'll attempt a simple explanation :

At the very simplest level, light is emitted by the diode when a current passes through it. Electrons release light when they interact with certain portions of the material inside the diode. This is different from a light bulb, in which something is heated up so hot that it begins to throw off photons. This is, as Spritle indicated, the reason LEDs don't 'burn out'1 like light bulbs.

and a slightly more complicated explanation:

Light consists of packets of energy (quanta, pl. of quantum, hence quantum physics) called photons. You can think of them like familiar particles2, but they also have a frequency like waves. Now, light in this sense doesn't mean just the light that you see, but can be of basically any frequency. A more general term is electromagnetic radiation, which can be anywhere in the electromagnetic spectrum (range of frequencies). The specific term for the frequencies of light you can see is the visible spectrum of light. The frequency of a photon is dependent on its energy, so referring to a photon's energy is exactly the same as saying it is of a particular frequency3.

Electrons are also bounded by the rules of quantum physics (note: everything is, in fact, but for larger things the effect is very small). An electron that's part of or near a material has its energy quantized, meaning it can only have discrete values. The distinct energy values are called bands.

Now when an electron in this position gains or loses energy, we say it 'jumps' bands, since it has to have some particular quanta of energy gained or lost. An electron that jumps to a lower band actually loses its energy by releasing, you guessed it, a photon. If the energy lost corresponds to the visible spectrum (remember, specific energy is specific frequency for light), then you will see the photon being released by the electron which jumped bands.

Inside a diode, the semiconductor structure is such that most of the electrons in different parts are in different bands. When a current (flow of electrons) moves through the device, electrons will move to the other parts and will jump bands, losing energy and thus giving off a photon. In an LED, of course, the design is such that the photons emitted will be in the visible spectrum, so you can see the light. The light in most LEDs is in a fairly narrow part of the spectrum, so a red LED is actually only producing red light.

and the most complicated, therefore containing the most potential for error :

A semiconductor is said to have a 'valence band' and a 'conduction band', with a significant gap in energy between the two. The intrinsic material (Si in most diodes), is 'doped' with other elements to increase the number of electrons in the conduction band (n-type) or holes4 in the valence band (p-type). A diode has p-type material on one side, and n-type material on the other. At the junction of the two, the electrons and holes recombine to produce a 'neutral' region in the center5.

When current flows, electrons and holes flow from one region to the other, and thus more recombination occurs. Photons may be emitted from a diode during electron-hole recombination. This is most likely to occur in direct band-to-band recombination in a direct band material6 , so the energy of the photon corresponds to the bandgap energy (lambda = hc/Eg).

Since the bandgap energy is what determines the emitted wavelength, more exotic materials than Si or simple GaAs are needed to produce visible light. The bandwidth is actually visually fairly narrow, but not perfectly so, since energetic holes or electrons can also recombine to produce light close to these frequencies. Typical bandwidth is 30-40 nm. Laser diodes produce more coherent light using some techniques I don't know really well, but it's the same basic process of electron-hole recombination.



Incidentally, the 'opposite' of an LED is a solar cell, in which incoming light causes a current to flow.


_____________

1 This is suggestive of losing fuel for fire, though light bulbs go out more often due to the sudden increase in current that leads to a sharp increase in heat when you switch them on. The weak points that fail are created while the light bulb is on, so 'burn out' is accurate enough. I now see Chronos already said this, but I don't want to renumber my footnotes.

2 Don't jump all over me for using 'particles' followed by 'but' here -- I'm using 'familiar particles' to mean the classical sort for someone who is unfamiliar with the wave-particle duality thing.

3 The frequencies and energies are directly proportional, which is why X-rays will kill you pretty quickly, but living near power lines won't. Okay, unless they fall on you or something.

4 Holes are the absence of electrons in the crystal structure. They are considered to be electrical particles just like 'free' electrons, however, so try not to get confused.

5 It's sometimes thought that this 'neutral region' is entirely static. Of course it's not, it's just that the average of everything going on comes out neutral.

6 I'm not going to define 'direct band' here (hasn't this post gone long enough already?) other than it has the most sharply defined bandgap for the material; also, recombination does not always emit photons since the energy can be given to another charge carrier in the band and eventually lost as heat (this is known as Auger recombination and is more likely to occur in heavily doped materials), or if Shockley-Read-Hall recombination occurs in which a 'trap' exists (a band within the 'forbidden zone' between the conduction and valence band).
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Old 08-27-2001, 01:59 PM
k.os k.os is offline
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Light bulb keeps burning after 100 years

Here is a link to a story about that light bulb,Chronos. I remembered it from a special on TLC or Discovery. Was that the bulb you're talking about? It's 4 watts and has been on since 1901, though they don't give the date. It's actually pretty cool! [url="http://www.centennialbulb.org/photos.htm"]Here[url] is a link the centennial bulb webcam.
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  #9  
Old 08-27-2001, 02:27 PM
yabob yabob is offline
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Quote:
Light bulb keeps burning after 100 years
<reference obscurity_level=moderate>

Gee, was it named Byron?

</reference>
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