All of the LED flashlights, taillights, etc, (in other words, every use besides the indicator lamps the ones from Radio Shack are used for) use an array of small LEDs rather than a single large one. I assume the ready availability of small LEDs has something to do with it but is it possible to make an LED large enough to replace a 100W incandescent lamp?
My understanding of it is that LEDs cast a very narrow ‘cone’ of light compared to incandescent lights. One large LED would create a small very bright area but most of the coverage would be very weak.
I think there are practical limitations to the size of the semiconductor elements; I think I’m right in saying that the light emission takes place at an interface - a boundary between two materials; using bigger blocks of material does create a larger interface area, but also obscures more of it at the same time.
That’s not an intrinsic property of LEDs. It’s just that LED emitters are encased in transparent plastic, and the front surface of the plastic is usually shaped to form a lens, producing a narrow cone of light. If you grind it flat (or make it flat to begin with) you get a non-directional light source.
As for the upper limit, they do go up to about 5W these days, which is (arguably) comparable to a 12-watt halogen bulb. But I think heat dissipation becomes a big problem beyond that point - the LED emitter is very sensitive to temperature changes, and requires a big heat sink to keep it at operating temperature.
And for a vehicle taillight, you don’t want a single tiny red light. You want a large area with moderate brightness. A large array of low-power LEDs is simpler, cheaper and more reliable than a single high-power LED behind a big diffuser (lens).
My vehicle taillight is a single tiny incandescent light, made into a large area with moderate brightness by a red plastic cover. Why couldn’t this work with a single LED? If cheaper, I can see that, but what does “reliablity” have to do with it?
You have to replace that tail light when the bulb burns out. On the other hand, a few broken LEDs out of a couple hundred won’t make a difference. Plus they’re more energy efficient than the equivalent incandescent bulb.
So it’s cheaper to make a bunch of small LEDs than one large one? Replacing the taillight bulb on my car is rare, and I thought LEDs are even less likely to burn out, although I can see the utility of having more than one light source if one does burn out.
As scr4 points out, making large LEDs is difficult. As for whether making an array of small LEDs is cheaper than a single incandescent bulb, I don’t know. Probably not. But the energy savings probably make up for it.
LEDs usually don’t burn out, but they can break or get shaken loose from time to time.
It would work. It’s just not the best solution. As already pointed out, you have a single point of failure (one bulb goes out and you lose the whole taillight). You have all the power concentrated in one spot, so you need a big heat sink there. And it takes more space; an array of small LEDs is essentially a flat panel.
The reasons they didn’t (usually) make arrays of small incandescent bulbs are:
[ul]
[li]Small incandescent bulbs are less efficient than large ones. (Small LEDs are just as efficient as large ones, if not more efficient)[/li][li]Small incandescent bulbs aren’t much cheaper than large ones. (High-power LEDs are much more expensive than lower powered ones.)[/li][li]Incandescent bulbs have a limited lifetime, so you need to make the individual bulbs replaceable. That means lots of tiny little sockets. It’s much simpler with LEDs - you just solder a bunch of them onto the circuit board.[/li][li]The more bulbs you have, the more frequently you have to replace a burnt out bulb. Think about how much work it is to keep a Christmas tree light working all the time. [/li][/ul]
There is actually a problem with large semiconductor devices, known as current crowding.
One way of looking at a large LED is a bunch of small LEDs all connected in parallel. They draw equal current if the doping is uniform AND if the temperature is equal across the die. The doping is never perfectly uniform, and so one point on the die will draw more current, and run a little hotter, which will make it draw even more current.
On a small die, the temperature difference across the die is small enough that this does not become a huge problem. Make the die large enough, though, and you can no longer operate at full power, because one place on the die will overheat.
To get around the problem, you have to create lots of isolated diodes on the die, and connect them with ballasting resistors. But using totally separate packages allows you to dissipate heat better, and if one is bad, you only discard that one, not the whole large die.
They do make large LEDs. Most of the new LED flashlights use single 1 or 3 watt LEDs rather than arrays of smaller ones.
I have a 3 watt, 3 D cell Mag-Lite that’s brighter than the conventional bulbed Mag-lite it replaced. When I bought it a week ago, most of the flashlights in the flashlight section at Lowe’s were single LED units, with only a few older ones being arrays. Now that Mag-Lite is also selling LED flashlights, I don’t think they had any incandescent flashlights at all.
Here are the LED Mag-Lites:
This has all happened in the last six months or so. I’m not sure what happened, but suddenly high powered Luxeon and Nichia LEDs are plentiful and the prices are plummeting.