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Thread: Help! Electricity Question - Battery and LED: Cree MC-E w/ 5V Power Bank

  1. #1

    Lightbulb Help! Electricity Question - Battery and LED: Cree MC-E w/ 5V Power Bank

    I'm intending to make a white blade using Cree MC-E using 4 LEDs wired in parallel, though this will be my first build. Will be buying a "cheap" empty of about 12" long.

    First, some quick specs:
    Cree MC-E LED (4-dice star) in cool white 6000K
    @ 350mA, Vf = 3.2V, Luminous flux: 370 lm to 430 lm
    Max current @ 700mA, Vf = 3.4V, Luminous flux: 160% increase (about 590 lm to 690 lm)

    Li-ion battery:
    Either 14500, 16850, or 26650.
    Resistor needed per die at 400mA input: 2 ohm, >1W resistor per die
    Total current draw ~ 1.6A
    Total lumen output ~ up to 1780 lm.

    So two things...
    1) GOAL: Use a 5V 5000mAh power bank with a 1.5A output
    Reason: I can solder a simple USB connector to connect the battery to; simple plug in/removal; cheap and multifunctional battery.
    Problem: the 1.5A output... Do I need resistors???

    I know batteries don't usually output a specific current and that it's usually the devices that draw it out instead... But since this powerbank has an output value in the specs and manual, I'm wondering if that actually means the output is controlled where I don't need resistors. After all, the current gets split up between the dice when everything is wired in parallel, right? That would give about 375mA per die.

    2) Is ~1700 lumens too hot? I've never worked with a high-output light like this before so I don't have anything to compare with.
    If this is too hot, what would be acceptable and safe?

    ---------------------------------
    Edit: Correct me if I'm wrong but I might have just realized that if the 1.5A output of the battery exceeds what the LEDs are trying to draw, then the appropriate resistor for 5V battery should work just fine as it will resist the excess current. So if I use only 2 dice instead of 4 and intend to run it with a 400mA draw, this resistor per die should be fine.

    So if I only use 2 dice w/ 2ohm resistor...
    Total current drawn from battery = 900mA
    Luminous Flux is about 1000 lm
    Can anyone verify if my thoughts/calculations are correct?
    Last edited by fdesa12; 05-30-2016 at 08:17 PM. Reason: Update

  2. #2

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    Ok, as far as I understand (and anyone feel free to correct me too) the specs on the power supply aren't a concern, except the voltage. The 5000mAh means that the battery can last for 1 hour with a 5000mA (5A) pull, but that is limited by the output circuit to 1.5A max, but that is only the amount it CAN put out, not how much it normally does. But again, none of that is a concern unless you are trying to pull more than 1.5A.
    Your main concern with resistors is dropping the voltage to what the LEDs can handle, which is 3.4V @ 700mA, which.
    Watch this (http://forums.thecustomsabershop.com...-5V-Power-Bank)
    According to the formula - R = ((5V - 3.4) / .7) or somewhere around the 2.6ohm range minimum (don't quote me on that, do more research).
    Again, I could be completely off on this, but its what I have figured out looking around.

  3. #3

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    Okay, so the 1.5A is nothing to worry about unless I'm trying to draw more than that. Thanks for the clarification

    With that said, I was also considering Cree XP-L single die, which has a max current of 3 A at an unlisted Vf. Luminous Flux is 1034 lm.
    At 1050 mA, Vf is 2.95, and luminous flux is 523 lm.

    If I wanted to for the XP-L, could I just use a direct-drive setup with the power bank since max current is 3 A? Or would it be wise to use a resistor?

  4. #4

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    If its this one http://www.thecustomsabershop.com/Co...ova-P1017.aspx then the forward voltage is what you wanna look at, which looks like 2.95-3.25V (3.25V max) so you would need a resistor to drop the voltage to that.
    As far as the amperage goes, with the setup you described earlier, the LED would never hit peak brightness, seeing as the power supply can only give 1.5 amps, not the 3A (3000mA) listed (unless you bypass the output circuit).

  5. #5

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    Oh, and the watch this link was supposed to be Madcow's vid: https://youtu.be/afJEx_Wk-HA

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