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Thread: Yet another resistor question. Haven't seen this one asked yet, though.

  1. #1

    Default Yet another resistor question. Haven't seen this one asked yet, though.

    Hey all. I've built sabers before, but they've all been current driven via a PC or a CF with a Color Extender. I haven't had to use resistors for high power LEDs yet.

    I'm planning an NVv4 RGB saber with a PEx using the R/G/rB Tri Cree in the store and an 18650 cell. My main question is: should I use 4.2v or 3.7v to calculate what resistors to use? I don't know why I wouldn't use 4.2 (max), but I get weird values that aren't available in the store. That strikes me as weird, because I feel like they should be in the store.

    with 4.2v:
    R - (4.2 - 2.65)/1 = 1.55 ohm (I'd probably use the 1.5 ohm 5W resistor in the store, even though it's unnecessarily large. I only need 1.5 - 2W resistor)
    G - (4.2 - 3.7)/1 = 0.5 ohm (Easy: 0.5 ohm 3W would be fine)
    B - (4.2 - 3.4)/1 = 0.8 ohm (I'd probably use a 1 ohm 2W for this, but I'm worried that would be to much.)

    with 3.7v:
    R - (3.7 - 2.65)/1 = 1.05 (Probably safest to go with the 1.2 ohm 3W, but you could also probably get away with 1 ohm 2W)
    G - (3.7 - 3.7)/1 = 0 (I know i can just use the drive parameter to limit the green a bit, but I'd be much more comfortable throwing in a small resistor)
    B - (3.7 - 3.4)/1 = 0.3 (0.47 ohm 0.5W is probably the most appropriate thing for this)

    What do you NB veterans use to limit the current in your Tri Cree R/G/rB? I'd be worried only using 3.7 v to calculate the resistors, as that would lead to them being overdriven when the battery is fully charged. My gut is to use 0.5 ohm 3W on the G and the rB and use the 1.5 ohm 5W for the red.

  2. #2

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    I use the .5 and 1.2 all the time, works great
    Darkmatter73

  3. #3

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    Quote Originally Posted by Darkmatter73 View Post
    I use the .5 and 1.2 all the time, works great
    Darkmatter73
    .5 on both the blue and green?

  4. #4

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    Quote Originally Posted by cvsickle View Post
    .5 on both the blue and green?
    That’s what I did on mine.

  5. #5

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    Awesome. Thanks for all the help everyone!

  6. #6

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    You use the nominal (average) voltage rating 3.7 or 7.4V when calculating resistor values.
    TCSS MODERATOR
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    "Yeah, yeah, I've heard it all before... you want blindingly bright, super loud, running 1138 blinkies off of the cheapest sound card you can find AND you want all of it to run on a battery the size of a dime, and run for a very, VERY long time. That one cracks me up every time..."
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  7. #7

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    Quote Originally Posted by Forgetful Jedi Knight View Post
    You use the nominal (average) voltage rating 3.7 or 7.4V when calculating resistor values.
    ^^^^ Yes. This. ^^^^

    The battery will quickly settle out of the maximum charge voltage down to it's rated nominal voltage (3.7V or 7.4V). You won't get as bright a blade as you should if you use the maximum voltage for the calculations. In stunt setups some people don't even use a resistor at all for green LEDs with an 18650 because the forward voltage of the LED is so close to 3.7V.

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