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Thread: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

  1. #1

    Default SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    http://light-reviews.com/Flashlight-...ardian-review/

    I was going to buy one but I found this. Now I'm confused.

    Is it too much wanting it to have at least 2+ hour flat runtime?

    Also, what's with the sudden 1 minute drop in the beginning?

    Thanks.

  2. #2
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    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    For me the result (1:45h until output is <80%) is not so bad if CR123 are used. I love my M6LT. The good thing is it also runs with two 18650 cells if you have an oddmods battery carrier. It is a great aftermarket part I use for both M6 and M6LT.
    Last edited by WarriorOfLight; 06-19-2012 at 11:13 PM.
    Looking for Orange Surefire G2 Host. If you have one you don't like please PM me.

  3. #3

    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    I think it is safe to assume that the XM-L in the M6LT is driven at max drive current (3A). The XM-L datasheet says that Vf at 3A is 3.35V. That means that the LED draws 10W. For 3S2P 123A cells, I would guess that the approx current draw is around 1.2A. Referencing Silverfox's 123 Shootout, SureFire 123A cells have about 3.1Wh energy at 1A draw. Thus, the total energy is 6*3.1W = 18.6Wh. If we also assume that the M6LT driver is 90% efficient, then the calculated run time is about 18.6Wh/10W*0.9 ~1.7 hours, or about 1h40m. And in fact, that is where the light appears to fall out of full regulation based on the light-reviews graph.

  4. #4

    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Quote Originally Posted by ryank17 View Post
    http://light-reviews.com/Flashlight-...ardian-review/

    I was going to buy one but I found this. Now I'm confused.

    Is it too much wanting it to have at least 2+ hour flat runtime?

    Also, what's with the sudden 1 minute drop in the beginning?

    Thanks.
    Not sure what's to be confused about in that review...

    As one famous engineer once said "Ya canna break the laws o' physics"

    The drop is fairly typical of primaries in high current draw lights. I'm not certain that it doesn't occur even with rechargeables, just more noticeable with primaries.

    But yes...it does appear that this review model fell short of the advertised specs from Surefire ...
    Last edited by lightfooted; 06-23-2012 at 03:36 AM.

  5. #5

    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Any light that drops like a cliff during the first minute or couple of minutes is unacceptable. That's a regulator design FAIL.

  6. #6
    *Flashaholic* wquiles's Avatar
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    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Quote Originally Posted by fnj View Post
    Any light that drops like a cliff during the first minute or couple of minutes is unacceptable. That's a regulator design FAIL.
    As the LED warms up its light output drops, so I don't see it as a "regulator" failing - more than likely it is poor heatsinking and/or poor/no thermal paste, etc..

    The other option is that the cell's can't keep up with the current demand, and quickly sag to their steady state voltage given the current draw. It would be great to drive this head from high current cells or a stiff power supply to test this theory.

    Has anyone seen one of these open to look at how the LED is attached?

    Will
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  7. #7

    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Quote Originally Posted by wquiles View Post
    As the LED warms up its light output drops, so I don't see it as a "regulator" failing - more than likely it is poor heatsinking and/or poor/no thermal paste, etc..

    The other option is that the cell's can't keep up with the current demand, and quickly sag to their steady state voltage given the current draw. It would be great to drive this head from high current cells or a stiff power supply to test this theory.

    Has anyone seen one of these open to look at how the LED is attached?

    Will
    The graph shows a drop to 80%, with almost all of it occurring within 1-2 minutes, and the balance within 20 minutes or so. If it's temperature related, and assuming they can't raise the current in response to rising junction temperature because they don't want to exceed the 3 A LED rating, that would correspond to an increase in T sub J from 25 C to about 120 C (ref: http://www.cree.com/products/pdf/xlampxm-l.pdf). Perhaps that is what is happening. I wouldn't think anyone would design a light that runs that hot in terms of T sub J - that's way above the boiling point of water - but I suppose they might have.

    As for regulation ... Justin Case convincingly calculates total LED + driver power of 11 W, which is only 1.8 W per cell. That's nothing for a CR123A. Figure 2.4 V at 0.75 A. A good CR123A (i.e., Paranasonic, Duracell, or Energizer) will run over an hour before hitting 2.4 V (ref: http://www.powerstream.com/cr123a-tests.htm), and even half-decent CR123As will run 40 minutes before hitting 2.4 V.

    If Surefire doesn't make a regulator that holds the output current at about 3.35 V constant for an input varying from 18 to 12 V (much less 14.4 V, which is 2.4 V per cell), it's because they don't want to, not because they can't. And if they don't make a temperature-compensated regulator to hold the LUMENS absolutely constant, assuming our first paragraph accurately describes the limiting factor, it's because they don't want to lower the initial output from 900 to 720 lumens (a difference which makes no practical effect in the real world, but only in marketing), which would allow a line as flat as a table until cell depletion.

    Bottom line: the heat sinking is probably very poor, because the T sub J is rising 95 C above ambient, and sure as hell the flashlight casing isn't rising more than one quarter that much, because you couldn't hold it.

  8. #8

    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Quote Originally Posted by ryank17 View Post
    http://light-reviews.com/Flashlight-...ardian-review/

    I was going to buy one but I found this. Now I'm confused.

    Is it too much wanting it to have at least 2+ hour flat runtime?

    Also, what's with the sudden 1 minute drop in the beginning?

    Thanks.
    The drop at the beginning is the typical cheat used to get high ANSI lumen: Throttling of output after the 3 minute mark of the required testpoint.

    I guess the days of "Surefire Lumen" are over.

  9. #9
    *Flashaholic* wquiles's Avatar
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    Default Re: SUREFIRE M6LT RUNTIME GRAPH!! IS THIS REAL?

    Quote Originally Posted by IMSabbel View Post
    I guess the days of "Surefire Lumen" are over.
    I "miss" those days
    Please no PM/Visitor Msg's. Email for questions/Paypal: wquiles [at] gmail {dot} com. Please visit my new website.

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