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Thread: Super low XML VF's

  1. #1
    Flashaholic* jasonck08's Avatar
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    Default Super low XML VF's

    Man these LED's have a low VF! I've tested 2 LED's so far XML T6 2T and TD. Both have basically the same vF till 3A, then the 2T takes the lead. 2D had a vF of 3.250v @ 5A.



    My test setup: BK Precision 1744A

    Heat sink: Designed for Intel Q6600 CPU (95W)

    DMM: Calibrated Fluke 179



    1A - 3.000v

    2A - 3.103v

    3A - 3.147v

    4A - 3.177v

    5A - 3.218v



    Cree datasheets state 3.000v @ 1A, 3.241 @ 2A, and 3.337v @ 3A.

  2. #2
    Flashaholic* LilKevin715's Avatar
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    Default Re: Super low XML VF's

    I've encountered similar results as well in terms of low Vf. Of the 3 XM-L Mag's I've built only one of them has been semi-close to the cree data sheets.

    T6 1D @ 3.5A - 3.088v
    T5 3C @ 3.5A - 3.026v
    T4 4000k @ 3.5A - 3.395v (still within reasonable range)

    I know there will always be sample variance... but I don't mind these results at all.
    My Mag Mods: SST-50 , XM-L

  3. #3
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Super low XML VF's

    Cree datasheet, as far as I can discern, shows approximately the following for average XM-L:
    1A - 2.98V
    2A - 3.18V
    3A - 3.33V

    At 25C

    IMO, we are exposed to the most efficient XM-Ls by nature of this hobby. If we had some cheaper bins, I'd bet they'd be closer to the published numbers, if not slightly over (i.e. higher Vf).

  4. #4

    Default Re: Super low XML VF's

    Wow. That is lower then expected. I guess that IMR cells are no longer really suitable for direct drive without blowing up the leds...

  5. #5

    Default Re: Super low XML VF's

    Yes I have blown an XM-L direct driving it with an IMR 18650. It will shoot up to over 10 amps and then nothing, the led stops working. Also direct driving them with high quality cells like my Samsung 2600 mah cells will draw upwards of 5-6 amps, too high for continuous use. I have a couple direct drive lights and I have to use cheapy Trustfire cells to bring the current to around 4 amps.

  6. #6
    Flashaholic Hoop's Avatar
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    Default Re: Super low XML VF's

    The cree datasheets are at 25C Tj temp, so as the Tj temp increases the voltage drops at the same amperage.

  7. #7
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Super low XML VF's

    Yeah, This might be a possible explanation as well. Make sure your LED is thermally connected good to that beefy heatsink.

  8. #8
    Flashaholic
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    Default Re: Super low XML VF's

    These sound like a prime candidate for running on A123 batteries. I tried them on an xp-g linger special from nailbender, but ended up buying IMR's instead. On high the A123's were as bright as the IMR's on med. If these have decent output on the LiFePo4's, I may be able to narrow down my battery stock to only two types, and more specifically, two sizes.

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