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  1. #1
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    Dual XM-L build. What's the best way to do this?

    Noob here who wants to build a dual beam lite using two XM-Ls.
    What is the best way to wire and drive them?
    Series or parallel?
    Single or two drivers?
    Would like to be able to control them separately (switch either one off/on) but this isn't a deal-breaker.

    I have a couple of these linear drivers laying around that are nice and small:
    https://illuminationsupply.com/8xamc...a2624baa697c44
    I like that they can be driven at low voltage so battery pack can be lighter but don't know how they would work in this configuration.

    Anyway, open to ideas
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  2. #2
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    lighter battery pack?? only if your cycling round the block!
    how long do you ride for, you may end up going 5parallel or 6 parallel
    parallel is good, no balancing required
    i would use both drivers with individual switches to control them
    then you have the option of just 1 light or 2
    ..........cheaper drivers at fasttech.com
    ...Scun.thorpe, UK

  3. #3
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    Quote Originally Posted by HEY HEY ITS HENDO View Post
    lighter battery pack?? only if your cycling round the block!
    how long do you ride for, you may end up going 5parallel or 6 parallel
    parallel is good, no balancing required
    i would use both drivers with individual switches to control them
    then you have the option of just 1 light or 2
    ..........cheaper drivers at fasttech.com
    Lighter because I only need a single cell, voltage wise. Every cell after that is more run time.
    If my driver requires, say 14.8V, I need four cells to add any run time.
    I have that driver pushing a single XM-L with a 1S3P that should get over 3 hrs on high.
    The negative of just running each side with it's own driver is the battery pack, if I understand it correctly. How do I run one pack to supply both sides if the LED/drivers are essentially running in parallel? My driver has a max capacity of 6V, and I have to run higher than that in order to supply both sides. What happens when I shut one off?
    Last edited by roxtar; 01-21-2013 at 05:02 PM.
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  4. #4
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    I have that driver pushing a single XM-L with a 1S3P that should get over 3 hrs on high.
    no! ....once the battery pack voltage sags/drops beneath the LED forward voltage + any losses, your light will go out
    Battery manufacturers claimed capacity is from charged 4.2v down to 2.75v,
    running at 3.0A The XM-L forward voltage is 3.35v
    So, you will only get 90 minutes runtime on high, exactly half of what you expected!
    http://www.cree.com/XLampXML.pdf

    your 3,4,5, or 6 parallel battery can/will supply both drivers wired in parallel
    any higher voltage eg 2s battery...... "POOF"
    ...Scun.thorpe, UK

  5. #5
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    I doubt it would just go out HHIH, most linear drivers progressively dim once battery voltage goes below Vf+overhead. Running multiple cells in parallel reduces voltage sag as there is less current draw per cell, so I doubt 3A off a 3P pack with decent (Panasonic/ Sanyo/ etc) cells would be much problem.

    As for the OP, you could run each LED with it's own driver and switch, but I'd personally find that unnecessarily complicated (low to high at start of DH = switch switch on 2 switches) and you might also get some weird issues on med or low with the PWM frequencies of the 2 drivers interacting. As sticking with 1S batteries is a pretty simple approach I would either do:

    1) Master-slave mod, involves some funky soldering. Basic idea is to have 1 driver per LED, but control both with only one of them. You have to disable the MCU on one driver (slave) and wire the PWM output pin to the one on the other driver (master). Pretty simple when you get your head round it and plenty of examples on BLF.

    2) Stacking 7135 chips and parallel LEDs. Even funkier soldering - here the aim is to buy a bag full of AMC7135 chips from Fasttech and then stack them on top of the existing chips. Very fiddly and fraught with peril, but relatively straightforward way of getting the current you want.

    If you don't mind multiple series batteries, you can run 2 series LEDs off one 7135 linear driver using a Poorman's setup, which I don't especially understand. Plenty of guides around though.

    Or you can just plumb for a buck driver off a Chinese website and use a 3S battery.

    Either way, if you want to get anywhere close to 2h run time with 2 XM-Ls driven hard, you'll need a 4 cell at minimum if you stump for really good batteries (Panasonic NCR18650Bs) or a 6 cell if you go for lower capacity cells.

  6. #6
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    Quote Originally Posted by HEY HEY ITS HENDO View Post
    no! ....once the battery pack voltage sags/drops beneath the LED forward voltage + any losses, your light will go out
    Battery manufacturers claimed capacity is from charged 4.2v down to 2.75v,
    running at 3.0A The XM-L forward voltage is 3.35v
    So, you will only get 90 minutes runtime on high, exactly half of what you expected!
    http://www.cree.com/XLampXML.pdf
    I just did a night ride that went over 2 hrs on high with this 1S3P (7.8Ah)pack:
    Li-ion 18650 Battery: 3.7 V 7800mah (28.86wh) battery module with PCB .

    Quote Originally Posted by HEY HEY ITS HENDO View Post
    your 3,4,5, or 6 parallel battery can/will supply both drivers wired in parallel
    any higher voltage eg 2s battery...... "POOF"
    Won't wiring them in parallel split the voltage?
    That only leaves, what? <2V/side?
    Democracy is two wolves and a sheep voting on what's for dinner.
    Liberty is a well armed sheep, contesting the vote.

  7. #7
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    no, parallel affects current, series affects voltage.

    2 XM-L in series = 3.3V x 2 = 6.6V, current stays the same

    2 XM-L in parallel = 3.3V, current halves (so 4A driver output = 2A LED current)

  8. #8
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    BTW, I'm not married to this setup. Would I be better off using a different system; say a B3Flex running them both?
    I'm looking for the best (not cheapest) option for building this light.
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    It's hard to beat a B3Flex running the 2 XMLs in series off a 3s or 4s battery pack.

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    Is your only reason for running 2 drivers due to the voltage limitations of the 7135 driver?

    One L-flex @ $25 isn't that much more than 2 of the $8 7135's, and it is the same 17mm footprint if size is a concern.

    You could run both XM-L in series at 3A, ~6.7vf - on a 2S1P pack made of good 3100 cells - and theoretically get around 1hr runtime on high. You'd also gain the ability to custom tune your low and mid power settings to gain efficiency. The same setup run @ 2A instead of 3A would gain another 30mins of runtime, and you're still only using a 2cell pack.

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    A B3Flex or LFlex might be a better option because of the possibility for thermal protection. I'm working on a dual XML light and chose the LFlex because it only has components on one side and draws a quarter the current in sleep mode.

  12. #12
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    Unless you have different optics on the 2 XMLs, It's always more efficient to run both of them at a lower drive current than one of them at a higher drive current.
    Last edited by MtbMacgyver; 01-22-2013 at 09:16 AM.

  13. #13
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    OK, now this is the kind of feedback I was hoping for.
    Spending $25 for a driver is no big deal. A 2S2P or even 2S3P is acceptable weight if it works best.
    I'm just looking for the best way to power a dual XM-L light at max power (full 3A to both) for, say 2 hrs.

    You tell me what I need to do it right.
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  14. #14
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    agree with all of the above - if you're going for best, then skip the 7135 drivers entirely. The newer ones do have some degree of (non-configurable) over temp and over discharge protection, but the biggest advantage of the flex drivers for me is the UI. Not just being able to choose the levels but also how you switch between them.

    for the highest efficiency, a b3flex and 3S pack is the way to go. An lflex is an option, but at higher current draws you'll be dumping a lot of heat into the driver. I've actually been thinking of a twin XM-L using a 2S1P pack and a b3flex as a helmet light (hence the 2 cell) - at moderate currents I should get 2/3 of the way through my pack before the light goes out of regulation. Wouldn't recommend it for high power draw though as the voltage sag would pull it out of regulation much sooner.

  15. #15
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    Quote Originally Posted by mattthemuppet View Post
    I've actually been thinking of a twin XM-L using a 2S1P pack and a b3flex as a helmet light (hence the 2 cell) - at moderate currents I should get 2/3 of the way through my pack before the light goes out of regulation. Wouldn't recommend it for high power draw though as the voltage sag would pull it out of regulation much sooner.
    What are you considering running as your max current in this application?

  16. #16
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    Quote Originally Posted by adrenalnjunky View Post
    What are you considering running as your max current in this application?
    probably 1.5A, perhaps 2A as and when I make a new twin XM-L2 bar light. XM-L2 NWs should give me a bit more light than the XM-L CWs in my helmet light at the mo' with the same current. I've looked at a load of HKJ's battery discharge graphs and I should be able to get ~2.5Ah down to 3.4V from some NCR18650Bs at that current, which is very roughly the point at which the driver should go out of regulation (Vin below ~7V). I've never fully discharged my current 2.5Ah pack on a bike ride, so that should be plenty.

  17. #17
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    Quote Originally Posted by mattthemuppet View Post
    the biggest advantage of the flex drivers for me is the UI. Not just being able to choose the levels but also how you switch between them.

    for the highest efficiency, a b3flex and 3S pack is the way to go. An lflex is an option, but at higher current draws you'll be dumping a lot of heat into the driver.
    OK, don't understand a lot of that
    Remember, me=noob

    soooo,

    Which of these scenarios would get me what I'm looking for; both LEDs running at 3A for 2+ hrs?
    What battery pack config would I need to achieve this?
    Democracy is two wolves and a sheep voting on what's for dinner.
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  18. #18
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    With 2 XM-L in series:
    3S2P using at least 2600mah cells, using a b3flex would get u in the 2.5hr ballpark

    2S3P using 2600's with an Lflex would get about the same 2.5 hrs.

    There's not a good solution for a parallel XM-L situation, cause you'd have to have a 6A current driver.

  19. #19
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    Quote Originally Posted by adrenalnjunky View Post
    With 2 XM-L in series:
    3S2P using at least 2600mah cells, using a b3flex would get u in the 2.5hr ballpark

    2S3P using 2600's with an Lflex would get about the same 2.5 hrs.
    Of those two, which would you recommend and why?

    Quote Originally Posted by adrenalnjunky View Post
    There's not a good solution for a parallel XM-L situation, cause you'd have to have a 6A current driver.
    So, I assume I'd be wiring the LEDs in series for both the above systems, right?
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  20. #20
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    Yes - both examples I posted are in series.

    bflex is slightly more efficient - but costs more. Runs cooler - buck drivers don't have the heating issues that linear drivers can.

    lflex is cheaper and smaller - if you have a good thermal path to your heatsink, a series XM-L setup on 2S should be ok.

    I don't know what your housing/heatsink/space requirements are.

    Personally - in the dual XM-L housing setup I'm about to build - I'm going L-flex and 2S 4400ma battery. Mix of high and mid power settings should get me close to 2 hours. I don't really need more than that for any of my rides, and if i did, I'd only have to buy 2 batteries to extend runtime incrementally.

  21. #21
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    The b3flex will give you more versatility than the lflex since it'll happily accept a wider range of input voltages. You would have the option to plug in any pack between 3S and 5S cells. With a 2 XML setup, I use a 4S1P pack of NCR18650A cells (2900mah) and get a runtime right at 2 hours with a lightweight 4 cell pack. If I want longer, then I can use 3S2P pack made with the same cells and get a runtime of 3 hours. With NCR18650Bs, you can can squeeze another 15 minutes out of a 4-cell pack.

  22. #22
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    Cool, just ordered the b3flex.
    I'm sure I'll need some help with wiring it up when it gets here.

    Thanks all for your help.
    Democracy is two wolves and a sheep voting on what's for dinner.
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  23. #23
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    Quote Originally Posted by roxtar View Post
    OK, don't understand a lot of that
    Remember, me=noob
    ah, sorry, that's easy to forget.

    Big downside of simple drivers is that you often have to cycle through all the modes to get to the one you want. So to go from Low > High you have to go through Med, to go from Med > Low you have to go through High. Takes time and can be annoying.

    The *flex drivers allow you to set 3 different ways of going through modes. 5 mode and trimode is the same as above (have to cycle through each level to get to the next), just with 5 and 3 levels. Threemode has 3 levels, but once in Med, each click shuttles between Med and High (like a 2 mode driver) but a press gets you back to Low. Much more user friendly out on the trail.

  24. #24
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    Quote Originally Posted by mattthemuppet View Post
    I've actually been thinking of a twin XM-L using a 2S1P pack and a b3flex as a helmet light (hence the 2 cell) - at moderate currents I should get 2/3 of the way through my pack before the light goes out of regulation. Wouldn't recommend it for high power draw though as the voltage sag would pull it out of regulation much sooner.
    Would this be a 7.4V battery Matt? I'm thinking of doing something similar, 2xXM-L - lFlex - 7.4v battery...would it work? Would need to drive above 2A to get the output I'm looking for...

  25. #25
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    Quote Originally Posted by mattthemuppet View Post
    ah, sorry, that's easy to forget.

    Big downside of simple drivers is that you often have to cycle through all the modes to get to the one you want. So to go from Low > High you have to go through Med, to go from Med > Low you have to go through High. Takes time and can be annoying.

    The *flex drivers allow you to set 3 different ways of going through modes. 5 mode and trimode is the same as above (have to cycle through each level to get to the next), just with 5 and 3 levels. Threemode has 3 levels, but once in Med, each click shuttles between Med and High (like a 2 mode driver) but a press gets you back to Low. Much more user friendly out on the trail.
    The TaskLED drivers have so many user-configurable options, that I eventually end up disabling. Sometimes I feel like I'm throwing money away, until I use a light with a cheap driver and realize how annoying cycling through 3 or 5 modes to get where I want. Also - only having to find somewhere for a low current momentary switch is usually way easier than a big battery-interrupt switch.

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