Question about LED lightbulbs...

Discussion in 'General Electronics Chat' started by TheLaw, Jul 18, 2011.

  1. TheLaw

    Thread Starter Member

    Sep 2, 2010
    Hi again,

    I Have a question or two on LED lighting, like the new bulbs that can replace your incandescents or CFLs.

    My electronics theory is all over the place lately, so maybe I'm just not thinking right...but...

    How is using LEDs more efficient? Doesn't a lot of power have to get wasted when the voltage is converted to DC and stepped down to a low running voltage? You'll notice that about half of the bulbs surface area is a heatsink.

    How do they manage to get good efficiency, and make it a "green" product?

  2. steveb

    Senior Member

    Jul 3, 2008

    LEDs are very efficient in terms of percentage of light power relative to total power. You are correct about inefficiencies with voltage conversions (20% loss), but the net effect, which include good electrical to optical convertion in the LEDs themselves, is that LEDs are comparable to CFLs and better than incandescents, in terms of efficiency.

    The heat sink issue is definietly an interesting aspect of the technology, in terms of design engineering, and even aesthetics. LEDs require the excess heat to be conducted away, hence the need for a heat sink. However, incandescent bulbs radiate most of the heat away as infrared light. So, don't let the need for the heat sink fool you into thinking that LEDs are inefficient. Incandescents are less efficient due to the fact that a relatively small percentage of the emitted radiation is visible light.
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    Also take into consideration the lifespan of the lighting device. LED's will last many many times longer by far, thus less resources in the long run.
  4. Adjuster

    Late Member

    Dec 26, 2010
    High-power LEDs require heat-sinking because they have to be kept relatively cool. If they get too hot, both efficiency and reliability will suffer. That's a very different situation than for incandescent lamps, which have to be hot in order to function.

    Fluorescent lamps (including CFLs) generally have specific optimum working temperatures, but with the kind of power densities encountered in practice lamps can generally be designed to reach a suitable equilibrium temperature without special cooling.

    The problem with LEDs is not so much the absolute power dissipated as the power density, in relation to the need to limit the temperature rise to an acceptable value. Typically for higher power LEDs this requires a thermal resistance from chip to ambient of fewer °C per W than you would get from just a small encapsulation, so an additional heat radiator is needed.

    As for electrical efficiency, a modern switched-mode converter can power LEDs from AC mains with efficiencies of possibly 90%. Don't forget that fluorescents require either inductive ballasts or switch-mode control circuits, which also cause power losses.
  5. Wendy


    Mar 24, 2008
    I think I'll dispute the conversion efficiencies, but as light bulbs compare to CFLs, CFLs compare to LEDs. The total conversion of electricity to light is much greater. We still have a long ways to go, but if they can get the costs down LEDs will take over eventually. I suspect something better than LEDs will come along eventually, but LEDs are state of the art.

    One of the problems is LEDs running off of mains is a taboo subject for All About Circuits. The text book mentions how it is done in passing, but you can't discuss it here. The problem is basic electrical safety, anything that has a wire connected to AC power is dangerous for a beginner. This includes LEDs, power supplies, electrolysis machines, and several other crazy ideas that have come up in the years I've been here.

    It is basically a catch 22, it is pretty simple, so an advanced user knows how to do it safely, but if you have to ask how you probably shouldn't be doing it.
  6. TheLaw

    Thread Starter Member

    Sep 2, 2010
    I don't want to make one...atleast not right now.

    I was thinking about buying some LEDs lights for my house, but I think I'll wait on that. Maybe if I can get a 3 pack for $10, I'll buy them. :D My CFLs are doing fine...

    So the electricity passing through the LED is physically making them hot? It's not really "excess heat" per say. Though I would have to assume there is some heat being produced by other components. I saw a teardown for one a while ago...It was more complicated than I expected. By memory, I think it was some sort of SMPS, judging by the coils and chips.

    Well thanks. I was just curious...
  7. TheLaw

    Thread Starter Member

    Sep 2, 2010

    Atleast in this bulb, the big heatsink is reserved to cooling the LEDs only. The transistors and ICs for the SMPS use their own heatsink.

    Interesting. Atleast that brand used some nice quality caps. Would stink to have your caps die and everything else be fine.

    I'd like to know how the actual AC->DC stepdown is being done here. I don't see a transformer...I think you still need a transformer regardless. Even if it is tiny. The X and Y capacitors are typically BEFORE the transformers because they are line there doesn't seem to be a trafo after that...hmmph. Anyone?
  8. kubeek


    Sep 20, 2005
    I guess the transformer is the toroid thingie covered in heatshrink tubing on the far end of the board.
    It doesn´t even have to be a transformer, a simple inductor for a buck converter is enough as the LEDs are isolated and the whole thing is under a cover.
  9. Wendy


    Mar 24, 2008
    LEDs drop a set amount of voltage. If they are white it will be around 3.6V Vf. Modern high power LEDs tend to draw either 0.7A or 1.4A, and devices exist that draw even more current (the R&D isn't over yet).

    So at 0.7A it will be 3.6V X 0.7A, or 2.5W. Add the fact that there will be 10 or more of these suckers and you can see where the numbers are going.

    Every read the AAC book on LEDs?

    If you want to discuss how to connect them to a 36VAC source from a transformer that would be allowed on AAC. The key is a transformer, it is the only device that provides proper isolation from the AC mains. Needless to say, the cheap commercial stuff doesn't have it.
  10. KJ6EAD

    Senior Member

    Apr 30, 2011
    Last edited: Jul 19, 2011