LED flashlight

The more current you force through a p-n junction, the more light will come out. With veryvery much current, it will flash very brightly, for a short duration.

w.

Thay all use the same batteries, so it's as simple as a series resistor? But then why does the higher output cost more?

Also, there are flashlights which use multiple LED - you can count them in the housing - why would the manufacturers do that, paying for extra parts, rather than using a single LED with high current? And some of those multiple LED units cost less than the single ones I checked.

Your reply seems too simplistic to be satisfactory -

-- Rich

High power parts generally cost more than low power parts.

Because high power parts cost more than low power parts and it is often cheaper to use several low power parts than one high power part.

Also, when 1 LED out of a 10 LED array goes bad no one notices but if there is only 1 LED it is pretty obvious if it fails.

That's the main reason the LED traffic lights are arrays.

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Jim Pennino

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Is that what you think, jimp? Multiple LEDs so a failure won't be noticed? Give us a break!

As Helmut said, you'll get different efficiencies depending how hard you drive the LED.

Another thing to remember is "In theory, there is no difference between theory and practice. In practice, there is." Yes, a PN junction is a PN junction. Traps, radiative recombination centers, non-radiative recombination centers, light extraction efficiency, and any number of other things affecting the ideality of LEDs and are why LED manufacturers employ engineers and scientists to make their product better.

Bob Pownall

As usual all the "science" people here have their heads up their butts! God help us all if any of you work in any jobs beyond McDonalds.,

A single "bulb" means nothing. it depends what is inside that "bulb". In the old days a tungsten light bulb typically only had one filament inside the "bulb", or if there were two (hi and lo beams for example) only one was turned on at a time to prevent thermal issues.

But LEDs are tiny chips you can put a lot of them inside a "bulb". Now the plastic domes you typically see usually only have one LED (unless they are made to change color) but those flashlight "bulbs" are quite different.

For one thing, they aren't single LEDs or even LEDS producing visible light. What you can't see down in that bulb is an array of LEDs surrounding a fluorescent dot. It is the Dot that gives all the light! So really it's about so much MORE than simple LEDs in a battery circuit, although they DO make those too. I've got one with about 100 leds in it. Nice.

It's not trivial to manufacture a high power high efficiency LED. If it was we'd all be using them for lighting our houses. Technical problems limit how much of the electricity they convert to light and how long the LED will last at a given power output.

There is also a law of physics, specifically the conservation of energy, that limits how much light an LED can produce. You can't get more energy out (in the form of light) than the energy you put in!

This has interesting implications for proposed products such as this prize winning design..

As an exercise work out how heavy the weight would need to be or how high you would need to lift it to make the light work as claimed :-)

I'll bite: Let's assume a generous 100 lm/W for the LEDs. So we have 6 W of power going into the LEDs. Over four hours, that's

86 kJ.

The thing is four feet high and there's a bit of height lost to pedestal and top cover, so let's also assume the weight drops

1 m in those four hours. I'll lazily neglect all other possible losses. So the mass will have to be m = E/gh = 86000/10 = 8600 kg.

Yep, looks like a real winner. Good luck turning that thing over.

Jeroen Belleman

There are several factors that contribute to the LEDs cost. Your base assertion that a p-n junction is a p-n junction is entirely too simplistic. A simple p-n junction composed of a single p layer with a single n+ layer and ohmic contacts will not generate a significant amount of light due to self absorption in the thick p-layer and the fact that the recombination is not well confined resulting in low efficiency of conversion from electricity to photons. These types of LEDs are easy to make by diffusion and thus not very costly, but not a lot of light.

A high output led requires a heterojunction design and most high output leds utilise a double heterojuction design. What that means is a typical high output LED will start with a P type electrode connected to a P GaN layer connected to a P AlGaN layer connected to a N InGaN layer connected to another N AlGaN layer connected to a GaN buffer layer on top of some substrate (usually sapphire, but Cree uses Silicon Carbide). The n electrode is connected to the GaN buffer layer. The thicknesses of the layers have to be precisely controlled to get the proper wavelength output and the proper electrical characteristics which demands a very complicated epitaxial growth process. Additionally, all the different layers require careful processing to eliminate the stress between the different lattice structures to prevent defects.

The result is an epitaxial growth process that can have very low yields and wide variations in performance across the wafer substrate. When the wafer is cut into the die pieces used for the individual LEDs, the die pieces are checked for output and binned according to how bright they are. The highest performing dies are consequently a small part of a production run which leads to the more simple equation that low supply and high demand = high price.

Hopefully that gives you some idea of how prices can vary so much for LEDs and why high output LEDs cost more.

Yep, that is exactly why LED traffic lights are made with arrays.

A failed traffic light is a public safety issue and must be fixed ASAP.

It costs a fair amount of money to send out a crew and truck to change the lamp in a traffic light and it disrupts traffic, so minimizing replacement is an agency goal.

If you ever get out of your bunker where you surround yourself with IPCC reports and into the real world, look closely at the LED traffic lights and you will often see failed LED's; the lamp assembly will be replaced when enough have failed to make it worth the effort.

BTW, my source of information on this is registered traffic engineers; what's yours?

--
Jim Pennino

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Perhaps you haven't heard of 3-way bulbs.

..and even if the LED was 100% efficient it would still be too heavy.

While that is generally true, in this case we're talking about a few watts, not an oven. The housings are all the same. Maybe one uses #20 gauge wire, and another #18, but that isn't going to explain a $3 price difference.

-- Rich

Thanks, that's my idea of a substantial reply.

-- Rich

Point totally missed.

It is the cost of making high power LED's versus low power LED's.

Making a high power LED is a lot more complex and expensive than making the filament of an incandescent lamp bigger and the production yield for the high power devices is lower.

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Jim Pennino

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What? You want be to give you all a complete education for free? You have any idea what the word "typically" implies? Yes, and stop light bulbs have two filaments which if your tail lights are on (at night) and you stop, they BOTH come on. But it is not a "typical" Edison bulb.

So what does this have to do with high powered LED flashlights being fluorescent while all you guys are talking about PN junctions and their currents?

Are you too stupid to understand what YOU wrote? You used the word "typically" referring to to single filament bulbs, not double filament bulbs. And you wrote " if there were two (hi and lo beams for example) only one was turned on at a time to prevent thermal issues. " Which is not generally true, and wouldn't even be true if you had inserted the word "typically".

Paul Cardinale