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  1. #1
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    New question here. Driver check - 3 x XML + maxFlex = 7.4v battery?

    Building up one of my old Altair housings for a mate and have a maxFlex (v5) spare. If I run 3 XML's will a 7.4v battery be the way to go?
    Any idea on output if ran at 1200mA? I'm guessing around 1400L before losses..
    Cheers
    Steve

  2. #2
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    maxflex is a boost converter, input voltage must be less than output voltage. 2 li-ion batteries will give you up to 8.4V. 3 x XM-L @ 1.2A = ~9.18V. Input will always be under output & you are good to go Theoretical cree specified lumen output depending on which bin - T6 a bit under 1400, U2 a bit over.

    Just saw your other post. If you have to buy the LEDs anyway, why not give it a bit more welly and run XM-L2s - same power, more lumens. Lightmalls have them in a 1A tint, or if you prefer a warmer tint, Illumination Supply have a 3C tint on a copper board for $8.20 but no free shipping

  3. #3
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    You might not get the low current levels on fresh batteries.
    DIY LED Bike Lights:
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  4. #4
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    what does the manual say about minimum V difference between Vin and Vf? As znomit pointed out, at low current levels Vf might be too low for the boost circuit to work. It's sort of like buck drivers needing 1V or so overhead to stay in regulation, except with boost drivers they can't go direct drive like out of regulation buck drivers, they just don't work. (I think, usual caveats apply)

  5. #5
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    Quote Originally Posted by mattthemuppet View Post
    ...except with boost drivers they can't go direct drive like out of regulation buck drivers, they just don't work. (I think, usual caveats apply)
    Good thing you put in those caveats.

    Boost drivers do go direct drive if Vin is equal to or greater than Vf. That's why znomit mentioned one might not get the lowest levels when the battery is fresh. At low currents the Vf of the LEDs is low (3 series XMLs at 100mA is ~8Vf) and less than peak Vin (8.4V).
    Last edited by Vancbiker; 08-21-2013 at 02:51 PM. Reason: added values for accuracy

  6. #6
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    ah ha! See, caveats save the day!

    Makes sense though, now I've had a coffee and can think a little more clearly - it's like buck drivers but the other way round. Buck drivers can't do currents that require higher voltage than the battery can provide when they go out of regulation, so it makes sense that boost drivers can't do lower currents as the voltage is too high.

  7. #7
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    Take away at least 0.5V drop on Schottky diode and you're left with 7.9V input. Even the lowest level won't be direct drive with batteries fresh off the charger. But why bother with XM-Ls when you could get better beam with G2 or E2 at that drive current?

  8. #8
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    Sorry if I have led you astray & good thing there are smarter people than me on this forum. I missed three important words that are in the operating manual but not the technical info section "Input voltage must be less than LED Vf when fully dimmed".

    What the operating manual for the Maxflex6 goes on to say is "If the input voltage exceeds the output voltage (at the dialed in drive current), MaxFlex6 will no longer regulate and the input voltage will go through the series inductor and schottky diode directly to the load. This will cause the output current to a LED to rise rapidly since LEDs have a very steep Current vs Voltage curve (Vf)."

    If battery voltage = 8.4V & the schottky on the driver drops a further say 0.4V = output of 8V. There is a direct relationship between current & voltage. As the Vf at 0.1A = ~8V, doesn't that mean that even if the maxflex drops out of regulation, the current still can't exceed ~0.1A ? Of course it is always possible I am missing something again.

    I wouldn't have thought an XP-E2 was a great idea. The max drive current for an XP-E2 according to the cree datasheet, is 1A, although no doubt someone will have found a way to run them at higher current without problems. More fundamentally though it is less efficient, it puts out less light at 1A than an XM-L & it does so at a higher voltage. Less efficient = less run time.
    Last edited by find_bruce; 08-21-2013 at 10:00 PM. Reason: spelling

  9. #9
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    In this instance the only effect one would see is that the lowest level on the flex drive would probably not work until the battery dropped a little bit. That level is 50mA. Hard to tell what the typical Vf will be at that current since Crees chart does not go that low. By the time ~100mA is going through the LEDs, the Vin and Vf will be very near equal. IMO not a problem. Even if the initial low current gets up to a couple hundred mA it won't be for long. There is an initial fairly steep voltage drop on li-ion batteries before they level out in the 3.7V range.

    I think Toaster was meaning a better beam, not neccessarily brighter/more efficient. XPG and XPE make a tighter beam than an XML for a given optic or reflector angle and size. For me the best XML beam I have seen for a bikelight uses 40mm reflectors. Workable as a barlight but pretty big for the helmet.

  10. #10
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    Driver check - 3 x XML + maxFlex = 7.4v battery?

    All good points, don't think I have built enough different lights to express an opinion on beams so I will leave that for others

  11. #11
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    Regarding the cree charts, I've always found they actually run at a lower voltage.
    DIY LED Bike Lights:
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  12. #12
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    Thanks for the replies guys..very useful. I think I'll stick with 3 x MC-E's and a 15v battery as I know that works well. It's for a bar light so the MC-E works brilliantly.
    Was just thinking I could save a bit of cash and just buy some XM-L's. No biggy :-)
    Cheers
    Steve

  13. #13
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    Quote Originally Posted by deesta View Post
    just buy some XM-L's. No biggy :-)
    3 XML no problem
    http://www.cree.com/~/media/Files/Cr...ampXML_EZW.pdf
    DIY LED Bike Lights:
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