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Thread: ANOTHER resistor check!

  1. #11

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    i was thinking this myself
    The All-Father wove the skein of your life a long time ago. Go and hide in a hole if you wish, but you won't live one instant longer. Your fate is fixed. Fear profits a man nothing.

  2. #12

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    Hello!

    I jump on this thread for resistor considerations I'm having at night.

    I'm making a color-changing saber using a Cree XM-L or a Tri-Cree, anyway. There are two voltages displayed in the specifications, one for 1000mA and another for 350mA. I understand the smallest is the minimum current to get the led turned on (correct please if I'm wrong ^_^ ) and the highest is for the highest brightness before the led burns...
    But applying the Ohm Law for both voltage and current, I come with two (quite) different resistor values... Because I want a color changing saber, the led may come down to the lowest current or up to the highest. As read above, a high resistor implies high resp... a dimmer led.

    Then, what to do? What is the "good" solution? Should I use the max resistor? A middle one? The lowest? And then I come to... What is a variable resistor? Will it Save Us All?

    Some mathematical examples... with XM-L (RGrB-White)
    Forward Voltage (@ 350mA)-Typical: Red - 2.25V, Green - 3.3V, Blue - 3.1V, White - 3.1V
    Forward Voltage (@ 1000mA)-Maximum: Red - 2.6V, Green - 3.9V, Blue - 3.7V, White - 3.7V
    Let's go with the red: (with 3.7V and with 4V, as I read somewhere around we may use this voltage to be sure nothing explode)
    --- R = (3.7 - 2.6) / 1.000 = 1.1 Ohm --- R = (4.0 - 2.6) / 1.000 = 1.4 Ohm
    --- W = (3.7 - 2.6) * 1.000 = 1.1 Watt --- R = (4.0 - 2.6) * 1.000 = 1.4 Ohm
    and
    --- R = (3.7 - 2.25) / 0.350 = 4.14 Ohm --- R = (4.0 - 2.25) / 0.350 = 5 Ohm
    --- W = (3.7 - 2.25) * 0.350 = 0.5 Watt --- R = (4.0 - 2.25) * 0.350 = 0.62 Watt

    => 1.5Ohm, 2W or 5Ohm, 2W ?

    (note that for the blue and green, I'm above the 3.7V from the accu, then I may not use resistors... But Erv's Law says "Always Use a Resistor, Always". What to use, then? 0.0001 Ohm? )

    Regards

    Paul
    Last edited by Anoril; 03-13-2017 at 02:30 AM.

  3. #13

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    The Tri-Cree LEDs are pretty robust. They're not going to pop if you calculate at 3.7v instead of 4v. I always use 3.7v with my calculations.

    Use the 1000mA value for your calculations. Giving these LEDs only 350mA will make for a terribly unimpressive blade.

    This: http://www.thecustomsabershop.com/47...stor-P947.aspx is what we generally use when the math says you don't need a resistor. It won't hurt the brightness, and it'll prevent certain wonky behaviors from the sound boards.
    We all have to start somewhere. The journey is all the more impressive by our humble beginnings.

    http://led.linear1.org/1led.wiz for the lazy man's resistor calculator!
    http://forums.thecustomsabershop.com...e-to-Ohm-s-Law for getting resistor values the right way!

  4. #14

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    Hi!

    Thanks a lot for the advice and for the "not-so-useless-resistor" I'll go with that!

    Paul

  5. #15

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    Quote Originally Posted by Mineral View Post
    Is there a simple list of colors and best resistor values? It seems like the variables are relatively few, so there should be a "right" answer for each color of each brand of LED given a certain power source.

    Telling everyone to do their own math is an excellent pedagogical philosophy, but if there's a best resistor for each color of Cree XP-E2 powered by a 3.7V source, it ought to be fairly easy to make a list and post it somewhere to stop the few "resistor check" threads every day.
    Nice in theory, to be sure, although there is no single battery solution that will be perfect for every single saber design (at least not yet). For some people just getting into the hobby, a stunt saber may be their chosen path, so if they choose a standard battery holder... just in this store that could be 2x or 4x AA, or even a 4x AAA holder. Compound that by the fact X of alkaline batteries will have a different voltage than an equal number of Ni-MH batteries... + the store sells a AA "dummy cell" ...basically a battery shaped metal rod covered in plastic (to prevent short circuits?) and that throws another variable into a saber's potential input voltage.

    In the end, there's no way the store could know which version of how many volts someone plans to apply to their saber, despite how obvious it may seem to our human brains. After all, since we bought the parts, we already know what power source we're going to use.
    Last edited by TwinMill; 03-13-2017 at 10:42 PM.

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