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  1. #1
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    Idea! over-powering lights

    *Edited for correct current levels on the XP-G, thanks znomit!

    Hello, I was wondering what the consensus is on upping the current through the LED star by 100 to 300ma more current than the max rating is. For example the Cree XP-G is listed at 1500ma max current, but would it be possible to put 1.6a or up to 1.8a through the light without really drastically reducing its lifespan? Is it worth it in light output?

    Though debated, for computers in overclocking, the notion is that if you can sufficiently cool your CPU, you can overclock it without any drastic affect on the lifespan of the chip. Some would never overclock, but it seems that by providing enough cooling, you can squeeze a little more performance out of a processor without harming it.

    Is that possible or even worth it with LED's? I'm suggesting to build a case that has more than ample cooling so that the light would stay under 50C while running up to 300ma more current than it is designed for. My understanding is that light output dips appreciably after 40-50C temperature, so presumably nothing is to be gained by overamping the LED and causing temperatures to rise.

    Are lights brighter at 30C than at 50C enough for it to make sense to instead invest the effort in making a case that keeps the light cool enough for 30C and the increased light output or is 40-50C a good operating temperature, and if I can keep the light at that temperature and feed it more current there would be more light output on that route?

    Would very much appreciate some answers to these questions, sorry that I rambled so long! I'm into computer building so I am interested to know if what applies for computer processors and overclocking also applies to LED's and overcurrenting.

    Or would most people just say, if you want more light, add another LED? I'm probably going for a dual flood on the handlebars and a tight beam for helmet mount. Would most just say, make it 3 up front?

    Cheers,
    tailight
    Last edited by DIYtailight; 05-23-2010 at 04:07 PM.

  2. #2
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    XPG is rated at 1.5A
    See the cree datasheets..
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

  3. #3
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    Ahhh! Duh... I hadn't looked at the data sheets yet, just went by the LEDsupply listing: http://ledsupply.com/creexpg-w417.php

    Any reason why they limit their star to 1a when the diodes can handle 1.5a? What is the theoretical lumen output at 1.5a?

    Thanks for the heads up!

  4. #4
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    Quote Originally Posted by DIYtailight
    Ahhh! Duh... I hadn't looked at the data sheets yet, just went by the LEDsupply listing: http://ledsupply.com/creexpg-w417.php

    Any reason why they limit their star to 1a when the diodes can handle 1.5a? What is the theoretical lumen output at 1.5a?

    Thanks for the heads up!
    Original spec was 1A.

    http://www.candlepowerforums.com/vb/...&postcount=354
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

  5. #5
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    The LED isn't going to blow up at any particular current as long as you keep it cool. Whether it's worth it depends mostly on what you're trying to accomplish. But, here are a few things to consider.

    Unlike with a computer CPU where you get 20% more performance if you boost the clock rate by 20%, you do not get 20% more light from an LED with 20% more current. That's due to the effects of temperature on light output and since you can't change the thermal resistance of the LED package itself you can only do so much with case cooling. Unless you do things like supercool with nitrogen.

    Also unlike a computer where you have access to virtually unlimited power, for battery powered lights, lumens per watt of power is often more important the absolute light output. Since LED emitters aren't really all that expensive, most of the time it makes sense to use more LEDs and run them in a more efficient current range to get the amount of light you need.

    The lights I use on regular basis consist of a total of 6 XPGs. I run them at 500ma drive current unless I need to boost them up for something really technical. I picked 6 LEDs because it was a good balance between cost of LEDs, the 1200 lumens of desired light, 120 lumen / watt efficiency which results in a 6.5 hour runtime.
    Last edited by MtbMacgyver; 05-23-2010 at 06:13 PM.

  6. #6
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    So I guess the best thing to do is to just build a well cooled case, then adjust the current based on temperature measurements and runtime tests, if more light is needed, extra LED's can be added. That also gives the flexibility like in your setup, where you get nice runtime from high efficiency, but if you need the really high output, you can boost it at the expense of a much shorter runtime. I'm planning on using a hipflex, so I have the option of adjusting the current easily. Thanks for the answers!

    One question remains though, is it worth it to pursue a light that runs at 30C or a relatively cool temperature, or what is the accepted operating temperature of these lights?

  7. #7
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    Quote Originally Posted by DIYtailight
    One question remains though, is it worth it to pursue a light that runs at 30C or a relatively cool temperature, or what is the accepted operating temperature of these lights?
    Yes, it makes sense to design a light that runs cool. Again, that'll optimize efficiency. My priority tends to be on efficiency and weight. A lot of folks seem to optimize for size, but if you're focused on efficiency and weight, it doesn't necessarily make sense to go for the smallest size. That's because larger optics are typically more efficient. Also, weight is a function of the amount of material in the case. A larger case made out of thin aluminum will still be lightweight and will have lots of surface area for cool operation and high efficiency.

    On my lights, I have the temperature warning in the driver set at 50 C and I've never seen it trip even running at 1A sitting still. When running at 500ma and airflow from riding, I can't even feel any warmth from the light. The actual case temp will be a function of the ambient temperature.

  8. #8
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    Wonderful info, thanks mtb!

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