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Thread: Resistance is Futile - NBv3 Resistor Math Question

  1. #1

    Default Resistance is Futile - NBv3 Resistor Math Question

    Hi All,

    I'm sorry to be that guy (or that thousandth guy) to ask a resistor question when there are entire sticky threads devoted to the subject, but I've read all of them and I'm still having some trouble and figured there was no better place to turn.

    So I'm building a custom Yoda for my son with an NBv3 sound board, Green/Green/White CREE XP-E2 Copper Nova LED, a 3.7volt 18650 Li-Ion battery, and a 12mm antivandal momentary switch with green illumination. Here is where my issue begins with the resistor math:


    For the main Tri-CREE Green LEDs
    3.58v @ 1000ma
    =(3.7v-3.58v)/1a = 0.12ohms
    Power = 0.12ohm * 1^2 = 0.12watts

    I think that's all well and good but when I check myself against the online calculators it doesn't even seem close, and friends of mine that have built Graflexes say the standard for a green led is a 1ohm2watt resistor, but I just can't make that math work.

    Online calculators:
    http://www.led.linear1.org/1led.wiz
    This one says 1ohm 2 watt

    http://www.ohmslawcalculator.com/led...tor-calculator
    This one says 0.12ohm 3.58watt


    Similar issue for the Illuminated switch though I certainly come closer to the online calculators:
    3.0v @ 20ma
    =(3.7 - 3.0)/.020 = 35ohms
    Power = 35 * .020^2 = 0.014

    Online Calculators
    http://www.ohmslawcalculator.com/led...tor-calculator
    This one says 35ohms 0.06watt

    http://www.led.linear1.org/1led.wiz
    This one say 39ohms 0.125watt

    Any help or guidance here would be greatly appreciated!

    - Red Hawk

  2. #2

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    The online calculators generally round up the values. Doing the maths yourself is the best way, as you have and you are correct.

  3. #3

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    Keep in mind that a single cell 3.7v battery can produce 4v to 4.1v volts when it is fully charged. LEDs are current devices, which means the voltage determines how much current they draw. A small increase in voltage can mean a VERY big increase in current. You can easily exceed 1A to the LED a 4v! Not a huge deal with the Tri-Cree Green as they can typically handle being over driven a bit with a proper heatsink. The LED spec sheet numbers are a "guide", each LED can actually be slightly different from this "guide".

    To be safe and also eliminate any chance of false swings from the NB it is recommended to use 4v for you calculations Which would put you at a 0.5ohm resistor per Green dice.

    There are several excellent videos on TCSS YouTube channel on the subject.

    -Denzil

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