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Thread: What are the acceptable temperatures for a flashlight body without early LED failure?

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    Default What are the acceptable temperatures for a flashlight body without early LED failure?

    I have seen a lot of qualitative mentions of the flashlight getting to hot at certain driving currents, but I would like to know actual temperatures that would indicate too much power for a flashlight body. What is acceptable? Assuming the board is properly attached with heat compound, etc?

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    Flashaholic Epsilon's Avatar
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    Default Re: What are the acceptable temperatures for a flashlight body without early LED fail

    Well, at first you will need be able to hold it in you hand without burning it . 45deg C (45*1.8 + 32 = 113deg F) is holdable, but uncomfortable in longer periods of time.

    If the part you hold doens't get hot, then it depends on the components. In general, I allways keep around 80 degC as an absolute maximum for electronics. This allways mean that there are hotspots of higher temperatures.

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    Flashaholic* AnAppleSnail's Avatar
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    Default Re: What are the acceptable temperatures for a flashlight body without early LED fail

    Quote Originally Posted by Jayls5 View Post
    I have seen a lot of qualitative mentions of the flashlight getting to hot at certain driving currents, but I would like to know actual temperatures that would indicate too much power for a flashlight body. What is acceptable? Assuming the board is properly attached with heat compound, etc?
    Assume 4 degrees C per watt for the LED thermal resistance, then 5 to 10 for the LED to heatsink thermal path. If it's a dropin-style light you've likely got another 4 deg C/W hiding in there. In all, a not-optimized light can have 12-20 C per watt hotter at the LED than on the flashlight surface. If the flashlight is 'really warm' in bare hands, the LED is mostly okay if it's driven under 5W. But over 5W, with gloves on, out of your hands, or uncomfortably warm, the LED is probably close to 150C. At elevated temperatures the LED performance degrades quickly.

    If the light goes angry blue, it's far too hot.
    My biggest light-hog is my camera.

  4. #4

    Default Re: What are the acceptable temperatures for a flashlight body without early LED fail

    Quote Originally Posted by AnAppleSnail View Post
    Assume 4 degrees C per watt for the LED thermal resistance, then 5 to 10 for the LED to heatsink thermal path. If it's a dropin-style light you've likely got another 4 deg C/W hiding in there. In all, a not-optimized light can have 12-20 C per watt hotter at the LED than on the flashlight surface. If the flashlight is 'really warm' in bare hands, the LED is mostly okay if it's driven under 5W. But over 5W, with gloves on, out of your hands, or uncomfortably warm, the LED is probably close to 150C. At elevated temperatures the LED performance degrades quickly.

    If the light goes angry blue, it's far too hot.
    Aha. So I should start paying a little more attention to that thermal efficiency measurement in the diode specifications, and then mount it directly to the heat sink if possible to bypass those crappy "star" boards.

    After crunching a few numbers and assuming your ratios, it doesn't seem I have to worry too much about a 2-3 amp XML unless it's the crappiest of crappy heat sinks/boards. Ball parking about 7 watts or so, it could be a crappy 12 degree C/watt inefficiency before I ran around the ~150 degree diode range (starting at room temp). Unless I botched my mathz.

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