I cant remember what im doing. Help! (need driver help).- Mtbr.com
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  1. #1
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    I cant remember what im doing. Help! (need driver help).

    I used to modify leds for fun. Made a few bike lights that I still have, but its been years now. I forgot quite a bit about how driver boards regulate amperage and voltage in relation to how the leds run. I was working with single 18650s, single leds, and a single driver. I got the itch again, but want to setup a dual emitter with a bigger battery pack.


    I want XML2's, driven at 3 amps each max, with a low setting. Instead of just soldering together what works, I want to understand whats going on first!

    4 18650's should have the voltage and amperage I need for the draw. 2 emitters pulling 3 amps each max... uhh, needs a 6 amp driver? For some reason I dont remember this being correct.

    Im seeing 3 amp drivers, so Im guessing I wire the emitters in parallel and dump more voltage across all of them. 10w each, so I need a 3 amp 20w driver, correct? b3flex is rated 3000ma and 25w max. Wire batteries 2s2p and im good?

    Please let me know if im missing something. Thanks!

  2. #2
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    Couple of things off the top of my head...

    When you wire 2 emitters in parallel, each emitter will see 1/2 of the total current (assuming reasonably closely matched emitters). When you wire two emitter in series each emitter sees the same amount of current, but the voltage requirements of the string doubles (again assuming closely matched emitters).

    The B3flex works most efficiently when the input voltage is above, by at least a couple of volts of requirements of the emitter(s). So with 2 XML2s running @ 3A the total voltage requirement would be around 7 volts.

    18650s wired in a 2S2P configuration will give you 8.4V fully charged, but under load they will typically sag a fair bit... enough that they will fall below the voltage the B3flex needs to stay in regulation, so you really need a higher voltage configuration. I would recommend going 4S1P in this case, and ideally running 3 x XML2s.

    Hope this helps.

    Karl

  3. #3
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    That does help, thanks! I mixed up series/parallel. Series makes sense. Curious why 3 instead of 2? Run lower amperage across 3 instead of higher across 2? Two emitters should provide the light output I need. If 3 amps makes these things excessively cook I wouldnt be opposed to using 3.

  4. #4
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    The B3flex is a "buck" driver, and generally for efficiency it is best to a power a buck driver as close to the voltage needed by the LEDs as possible with a little bit more for headroom so that the driver can stay in regulation.

    With 2 x XML2s you need 7 volts, but a 4S battery puts out 16.8 volts when fully charged which is quite a bit larger a gap than is ideal. b3Flex Driver Technical Information explains this much better than I can.

    If you ran 3 LEDs you'd get better efficiency with a 4S battery, or you could run 2 LEDs with a 3S battery... just depends on what kind of light you're trying to build really.

  5. #5
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    I run the Lflex with a 2S2P battery on a 2S dual XML light. It works quite well. It will go to 3.5A also. It's a bit less efficient than the b3flex when the pack is fully charged, but since it is a linear regulator it gains in efficiency as the battery and Vf of the LEDs get closer. Another possible side benefit to the Lflex is the smaller size.
    GoPro adapters for bike lights http://www.pacifier.com/~kevinb/index.html

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