Why can't we...?

[Cyclops942]

Why can\'t we...?

Okay, I haven't seen this question asked yet, and I've been reading this board since November, so I'll go ahead and show my ignorance. I know there's got to be a reason, because somebody would have done it by now otherwise.

Instead of making arrays of multiple tiny LEDs if more brightness is needed, why don't we just make an LED that's, say, 3 inches (approx. 7.62 cm) in diameter?

I'd like a fairly technical answer, please. Replies to the effect of "It wouldn't work, bozo!" aren't real helpful.

 

[Badbeams3]

Re: Why can\'t we...?

I SECOND THIS QUESTION. Does mankind posses the ability to do this or is it not practical.

 

[hmmwv]

Re: Why can\'t we...?

I'm guessing here - but if you look inside the LED, there's a small grain of salt looking structure with a wire bond on it that actually produces the light (let's ignore the chemistry for a while and treat it as a circuit element) - if it were larger, then it would have more surface area, and the resistance across the "grain" would be lower, causing the current to go up, and the i2r heating to go up exponentially, causing the "grain" to get real hot and possibly melt.

Without getting itno electrons, holes, photons, and the like, that's the best explanation I could offer. I think the grain size, voltage, and current work out nicely right now such that the few miliwatts of heat produced can be heat-sunk (?? is that a word??) down the leads and out of the chip. Heat is a MAJOR factor in LED failure.

 

[Cyclops942]

Re: Why can\'t we...?

Nice guess. Can anyone confirm, contradict, or amplify this?

 

[busbar]

Re: Why can\'t we...?

These two might blister your forehead. . .
http://www.optotech.com/greatwht.htm
http://www.equipped.org/pp/pic435.htm


--busbar

 

[Brock]

Re: Why can\'t we...?

I understand the light is actually emmited in a really small place, but why can't they make an LED with say 10 of those tiny points in the same 5mm case? Basically 10 led's in one case. I have seen multiple color in one LED, say red and green, so couldn't they just do the same thing with all white? My guess as to why they don't do it is becuase we are the only people who want LED's that bright.

Brock

 

[RonM]

Re: Why can\'t we...?

At some point the question becomes, "why?"

LEDs provide nice white light at very low current draw. This lets us have flashlights with nice beams that save on batteries. Once we start using too many LEDs, or the proposed BIG ONE, then our current draw increases. Despite what I (we) want to believe, halogen lights are actually more energy efficient than LEDs. Except for new, more efficient, LED technologies there isn't that much sense in making high output LED flashlights. That is in terms of energy use. In terms of reliability and life, they can't be beat. Sorry if this is heresy.

RonM

 

[Cyclops942]

Re: Why can\'t we...?

Ron,

I don't know why anyone else would want to do it, but I've got the two reasons you mention (reliability and life), plus the relatively useless one of THE "NEATO" FACTOR. TNF doesn't rank real high in most honest cost/benefit analyses, but it sure makes a lot of impulse sales.

Let's take a trusty, tried-and-true 3- or 4-cell Mag, and, instead of an incandescent bulb, let's put in a darn-near-indestructible LED. Makes a much better security device now, doesn't it? We don't have to worry about breaking that tiny little overheated wire when we're breaking some thug's head. (Although, I have to admit, a much more likely occurrence is me dropping said flashlight on the sidewalk. Come to think of it, I'm glad that's more likely.)

Let's talk automobile applications. Especially on larger vehicles with longer depreciation schedules (ambulance, dump truck, semi trailer, bus, or any other commercial vehicle), the higher initial cost of the LED fixtures can be more easily justified by the practically nonexistent maintenance costs. The durability and long burn-life of the LED again speaks well for this choice.

Ditto on exterior applications, such as traffic lights (stoplights), flashing warning lights, and even emergency-vehicle warning lights.

(my $.02 worth, at least for today)

 

[Unicorn]

Re: Why can\'t we...?

I have seen larger LEDs at Radio shack. Not the 3 inches Cyclops942 mentioned, but only about a 1/2 inch in diameter. I have no idea how bright, or the current draw.

I'v also been wondering about replacing most of the lights on automobiles with LEDs. Everything but the headlights, backup, and turn/cornering lights. LEDs are being used on some emergency vehicles, but only as deck and dash lights. The halagen and strobe lights are still cheaper, and also much brighter when mounted in the bar lights. they would also be good as marker or clearance lights on semis I would think.

 

[Cyclops942]

Re: Why can\'t we...?

In Louisville, KY, the TARC buses are starting to use LED fixtures for the taillights. I've been seeing semi trailers with LED fixtures for a while now, too. Also, I've seen more and more traffic lights (stoplights) using them lately, and I've seen them used for WALK/DON'T WALK signs for years.

 

[RonM]

Re: Why can\'t we...?

No doubt that when reliability and long life are the driving factors our wonderful LEDs are the bulbs of choice. Think about how much the highway dept must be saving by not having to change bulbs on street signs (plus the extra safety of never having a street sign go out). Ever notice how many leds are in those baby's. Makes the Trek7 green look like child's play. High initial cost, low ongoing cost.

You can bet that I will continue to buy LED flashlights. Always been a sucker for the "neato factor". Just have realistic expectations of their output per mW.

Let your light shine.
RonM

 

[**DONOTDELETE**]

Re: Why can\'t we...?

There's a short article about LEDs in this month's issue of Scientific American. It briefly reviewed some of current uses for LEDs (completely failing to mention their use in flashlights!), and mentioned the following:

A lot of European car manufacturers are now using LEDs for brake lights, and (IIRC) starting to use them for turn signals.

10 percent of all traffic lights in the US use LEDs.

They also mentioned that red LEDs in the lab have now achieved 55 % efficiency (55 % of the electrons are converted to photons).

There's a nice chart showing the change in efficiency for various LEDs over time (ending in 2000), vs various other light technologies such as halogen, incandescant and fluorescent. If I recall correctly, as of 2000 white LEDs were 20 (slightly better than incandescant), yellow 35, green 40, and red 45 (don't remember the units for sure, perhaps lumens/watt?). Green was more efficient than halogen, but much less efficient than fluorescent, which was around 80. Blue LEDs were around 15 or so. I'm not sure whether those figures represent commercial or (more likely) lab results.

They predicted that in 5-10 years LEDs would have replaced many/most white light applications around the house.

 

[phyhsuts]

Re: Why can\'t we...?

LEDs can be made in large sizes. Indeed they are made on wafers of 4"-6" diameters! Granted, they are made as individual little diodes and are then cut to seperate them. The thing that is harder to do is to find a packaging that allows the dissipation of the higher power a larger diode will have. Even that is not a unsurmountable problem. The real problem up to now is, will there be sufficient demand to recover the cost of development in a reasonable time frame?? HP has LEDs with a rated current of 150mA NOW.
It is also true that efficacy of LEDs are still far behind that of Halogen-cycle lamps , not to mention fluorescent lamps! It will be a while before they can seriously be used as general lighting lamps. BUT for applications where shock resistance is of primary importance, they are the best.