Direct Driving an XPG?- Mtbr.com
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  1. #1
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    Direct Driving an XPG?

    Wondering if you gurus think this might work. I have a bunch of the Harbor Freight "free" 9 led flashlights that I'm thinking of "upgrading" to a single XPG with Regina. Would I be able to direct drive the XPG from 3 AAA NiMh rechargable batteries? I think I can press fit a 3/4" copper pipe cap in the end, so that I can use the original battery holder and switch. I would just have to rig up a "pad" to connect the positive of battery holder to positive of LED, negative would go through the flashlight body to the cap with a short wire soldered to negative.

    Do you think it might work?

    Thanks

  2. #2
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    I have not done this with an XP-G, but have with a Luxeon 3 a few years ago. It works because the 3S AAA battery setup cannot deliver more amperage than the LED can tolerate.

  3. #3
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    Maybe throw a resistor in there....

    I took a quick look at the data sheets for the XP-G and a "typical" NiMH AAA cell. The turn-on voltage of an XP-G is lower than that for a Luxeon or XR-E. At 3.6V (the nominal voltage for your three cells), the XP-G will pass 1.5A, which is overdriving it a bit. When the cells are fully charged, the XP-G will be at 4.5V, so the current would be considerably higher, probably ruining the XP-G. Like Vancbiker said, the AAA cells will have some internal resistance that will limit the forward current, but a "typical" cell will have an internal resistance of only 0.1 Ohm when fully charged. For a very rough approximation, three AAAs in series will limit the forward current to (4.5-3.6V)/(0.3 Ohm) = 3 Amps. A resistor will eat up some of your run time, but will save your XP-G (and be easier on your batteries, too; they don't like high currents in the long run).

    This is all just looking at the data sheets, but in theory, theory and practice are the same...

  4. #4
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    In practice i've ran XPG of a freshly charged 3000mAh Ultrafire and burned the LED. At 4.2V and 2+A it just didn't survive although it was well heatsinked. So do use a resistor if you want to avoid the magic smoke.

  5. #5
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    That's the problem with direct drive. The final drive current will depend on the resistances in the circuit that are hard to measure or predict. Even putting a meter in the circuit will dramatically change the actual current. Often, direct drive will work because there is enough resistance in the wiring to limit the maximum current. But you won't have much wiring in this case so that's going to tend to push up the current. In your case, it's going to depend heavily on the batteries. There is a lot of variance in internal resistance of nimh cells from brand to brand and specific battery model. So it may work with some batteries and burn up with others.

  6. #6
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    Quote Originally Posted by Vancbiker
    I have not done this with an XP-G, but have with a Luxeon 3 a few years ago. It works because the 3S AAA battery setup cannot deliver more amperage than the LED can tolerate.
    I missed the OP's intended usage of NiMH cells. I use alkaline AAAs in my converted light. Whether or not that makes a difference, I don't know.

  7. #7
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    Can anyone comment on the use of rechargables vs alkaline batteries. I was planning to use the NiMh over the alkaline as the nominal voltave of NiMh would be approx 3.6 vs 4.5 for the alkaline. Is the internal resistance of the typical alkaline battery higher than that of a typical NiMh? My HF cheapie 9 led light runs off 3 alkaline AAA just fine with no resistors. The light is wired 9P, I believe these 5mm leds want 3.5v and 50ma or so, does that mean that the 3S AAA pack is only putting out about 450ma?

    Thanks

  8. #8
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    Quote Originally Posted by ndrordr
    Can anyone comment on the use of rechargables vs alkaline batteries. I was planning to use the NiMh over the alkaline as the nominal voltave of NiMh would be approx 3.6 vs 4.5 for the alkaline. Is the internal resistance of the typical alkaline battery higher than that of a typical NiMh? My HF cheapie 9 led light runs off 3 alkaline AAA just fine with no resistors. The light is wired 9P, I believe these 5mm leds want 3.5v and 50ma or so, does that mean that the 3S AAA pack is only putting out about 450ma?

    Thanks

    The internal resistance of alkaline batteries is much higher than nimh batteries.

  9. #9
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    Bottom line is that a nominal 3.6V is waaaay to high for an XP-G without any additional resisitors. Sure there are some variables and other constraints (such as wiring) but the current vs. voltage is exponential and shoots up extremely quickly. E.g., the chart below tops out at 3.53V @ 2.6A. Add in some inadequate heat sinking and no thermal protection, you end up with a cooked LED in no time. You can get away with on an XR-E R2 though with higher Vf values at similar current levels but obviously lower lumen output. Alternatively, I think 2 series XP-G on a dd 6V would work great if you want to double up.


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  10. #10
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    Quote Originally Posted by Homebrew
    Bottom line is that a nominal 3.6V is waaaay to high for an XP-G without any additional resisitors.
    So in the 3 AAA alkaline setup, does the voltage drop due to the inability of the cells to provide more current? I have never put a meter on the 3 AAA light I converted, so don't have any numbers to toss out. I think the Vf of the LED is ~3.6V. New alkalines in the pack should be ~4.5V. Going by the numbers it should go up in smoke. Reality shows that it can work. I suspect that just changing to 3 AAs would smoke it as the deliverable current would be much higher (I'm not going to try it).

    On a slightly different note, the Vf of an LED appears to affect current flow in a circuit with some form of driver. I did a swap of a Luxeon 1 to an XPG in a Pelican headlamp. It has a 3 mode plus flasher "driver" in it and is powered by 3 AAs. The lower Vf of the XPG allowed an additional ~100mA current from the "driver". This still equated to only ~450mA so plenty safe (and like 3 times more light).

  11. #11
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    So in the 3 AAA alkaline setup, does the voltage drop due to the inability of the cells to provide more current?
    Yes, voltage drops significantly. Alkalines have a high internal resistance relative to Li or Ni-xx cells. The internal resistance naturally keeps the current to a safe value.

    On a slightly different note, the Vf of an LED appears to affect current flow in a circuit with some form of driver. ...is powered by 3 AAs.
    3AA lights are almost always direct drive or resistor regulated. This explains the increase in current with a lower Vf LED. With a true constant current driver, the current should not have changed.

  12. #12
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    Resistor Value?

    So can anyone recommend an appropriate resistor value?

    Several of the online calculators suggest a 2-3W capable 1 Ohm resistor. Seems a bit big to me, not to mention I can't find one at RS.

    Any help is appreciated.

  13. #13
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    1 ohm does seem a bit too much resistance. Try using two 1 ohm, 1/2W resistors in parallel (equal to 0.5 ohm). These should be available at the Shack.

    When you get everything wired up, briefly turn it on and measure the voltage across the resistor. You can then calculate the current (I=V/R) flowing through the LED, and also the power dissipated in the resistor (P=V^2 / R) to make sure you're not going to exceed the resistor power rating.

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