Eli5; Why do halogen lights get burning hot but LEDS don’t get hot at all?
In: Technology
Halogen lights are actually designed to run hot, hotter than normal incandescent bulbs. Filament bulbs in general produce light by heating the tungsten wire to white heat but that means there is waste heat which makes them inefficient. There is also a limit to how bright they can be before the wire burns out because the metal evaporates. Halogen bulbs have a small amount of iodine or bromine inside which deposits evaporated tungsten back on the filament so they can be brighter and not burn out too soon. For that chemistry to work the inside of the “glass” (fused quartz not ordinary glass) has to be quite hot which is why the bulb is usually fairly small and burns you.
It’s about how efficient you can turn electricity to light. Halogen light bulbs work by heating up a filament inside. What’s different from regular light bulbs is that halogens ones can recycle the vapors of the tungsten filament inside – the filament gets “fixed.” However it works, it still works by making a metal very hot.
So, what about LED? LED uses electrons, or more specifically, photon emitting electrons, to produce light. How can it force electrons to only produce visible light (well, more than light bulbs, at least)? Well, here’s the answer.
There’s two semiconductors. When a current flows through it, electrons goes from one to another, releasing energy as photon during the process.
Now, why does it release electrons? So, semiconductors are usually made of silicon and another material (more in that later). Now, silicon has 4 electrons on its most outside shell. It bonds with each other using covalent bond, by sharing electrons and getting 8 electrons (the number of electrons that is most stable for that shell). Now, add an atom that has 5 electrons on its outer most shell. When it bonds with the silicon, you have 1 electron that is not used in the bond. Ok, we have a semiconductor that has 1 free electron (of course, it’s not only one but go with it). Next, we make the same conductor but this time the atom you add has only 3 electrons. Now, it needs 1 more electron to make 8 (the stable amount).
So, we have one semiconductor with a free electron and another semiconductor that needs 1 more. That free electron moves to the other semiconductor to fill the hole. Yeah, but why does it release a photon? Energy difference.
Only electrons with a certain energy level can be in a shell (well, orbital to be exact). Different shell have different energy levels. That means, if the electrons need to have an energy level of 1 to be in that shell, the electron needs to have an energy level of 1, not 1.1. The free electron has more energy than the hole requires. So what does it do? The electron releases energy, in the form of light. By the way, the energy difference between the semiconductors determine the color of the LED. If you want a blue LED, you’ll need a energy difference bigger than a red one.
Head on over to r/flashlight and check out the holes they’ve burned in things with LEDs yo.
Different method of operation and different efficiency. Halogen lights are just producing light by making a wire get very, very hot to the point it gets white hot, same as old-style incandescent bulbs–so most of their output is actually heat. LEDs produce light via semiconductor interactions in a diode junction, and are far more efficient, so most of what they produce is light rather than heat. That doesn’t mean they don’t produce heat at all–depending on how your LED lamp is constructed it may have an aluminium heatsink behind the LEDs to soak up the generated heat and allow them to last longer, or the newer “filament” style rely on the gas in the bulb to carry the heat away.
OP, it’s a common misconception that LED light sources run without producing heat. It’s simply not true. Yes, they do run cooler compared to a halogen light source, but a LED does run hot.
Halogen bulbs are designed as the worst conductors possible. The tungsten wire in the bulbs acts as this poor conductor, so instead of electricity running “smoothly” through it like in a copper wire, the tungsten filament doesn’t allow much of the current to slow through and instead sucks it up. The energy that it takes into itself is then expressed as heat, and that heat produces light the same reason the hot coil in a radiator produces light. LEDs work via a semiconductor, using electrons to produce light instead of heat. However, where as heat is the goal of a halogen bulb, heat is a biproduct of LEDs. LEDs still produce heat, although significantly less than other bulbs. In my high school computer science class, we used a basic kit that came with an Arduino board, bread board, wires, LEDs, etc. it was know that if you wired your LEDs the wrong way (not using proper resistors) you could get your LEDs hot if you left them on long enough and eventually the plastic of the LED would melt from the inside
Halogen lights are ordinary incandescent lights, with the wire heating up from resisting the electricity flowing through it, and heating up enough to glow in visible light. The “halogen” means they’re filled with mostly a noble gas plus a little of a halogen compound, and the latter pick sup tungsten that evaporated onto the inside of the container and deposits it back on the filament.
So halogen lights get hot like ordinary oldstyle incandescents did.
LED lights, on the other hand, make light by, basically, running the photoelectric effect backwards, not by heating up to red-hot or yellow-hot to make temperature-based light. They have diodes with an n-type and a p-type semiconductor bordering each other; one has extra electrons flowing through it, and the other has missing electrons, so positively-charged “holes” flow through it the other way under the same current.
At the border, electrons coming one way fall into holes coming the other way, releasing a photon each time. They picked types so that the photon is is the visible-light range. Using three LEDs that give off red, blue, and green light can make an LED light that looks like it’s giving off blended white light. They still give off heat, because they’ve got flowing current, but they don’t base their light-making method off heating up from the current, so they don’t have to make high heat the way incandescent lights do.
–Dave, enlightening, I hope
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