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Thread: Wiring LEDs with differing voltage/current

  1. #1

    Default Wiring LEDs with differing voltage/current

    I'm a total newbie when it comes to this stuff, but I'm working on my kiddo's Halloween costume and had some questions about wiring two different types of LEDs together to a single on/off switch.

    As of right now I'm using 4 x 5mm Super Ultra-bright white LEDs:


    • Forward voltage: 3.4 VDC (typ), 4.0 V (max)
    • Peak forward current: 70 mA
    • Average forward current: 30 mA


    and 6 x 5mm Ultra-bright blue:


    • Forward voltage: 3.4 VDC (typ), 4.0 V (max)
    • Peak forward current: 30 mA
    • Average forward current: ?


    I want them to turn on using a single switch, but I'm not sure how to calculate what I need. If I wire it in 5 arrays of 2 LEDs each, using a 9v to power it, would I simply wire the SUB Whites in pairs with the appropriate resistors, and then the blues in pairs with the appropriate resistors? Ideally, at least for ease of putting things together, I'd drop to eight LEDs and the white LEDs would be wired in pairs, and the blues in sets of 3... but I can't make the wizard do that, and obviously I'm clueless

    Thanks for the help!

  2. #2
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Wiring LEDs with differing voltage/current

    I think you are going in the right direction with making 2-LED strings, each string with its own resistor, and putting all strings parallel so that you can control them all with one switch. You cannot use 3 LED strings, though, because in series the Vf adds up to be 10.2V, which is more than your 9V battery can supply. This is probably why the wizard won't let you do that.

    Perhaps this will help you think this through: A white LED is simply a blue LED with phosphor slathered on it. Electrically, they are the same thing.

    Perhaps if you just try to run both the blue and white LEDs at 20mA (low enough to ensure that it won't damage either the blue or white LEDs, based on the provided specs), it would simplify the design process, although I'd recommend testing it before actually arranging it in the costume, to ensure that the LEDs are bright enough for what the costume needs.

  3. #3

    Default Re: Wiring LEDs with differing voltage/current

    Thanks for the feedback. I think I am just going to end up running them all at 30mA, and hoping the whites are bright enough.

  4. #4
    Flashaholic* bshanahan14rulz's Avatar
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    Default Re: Wiring LEDs with differing voltage/current

    You might also consider carrying an extra 9V cell or two. I don't know how much current the average 9V cell can supply, but I think they are usually 6 carbon-zinc cells in series, packed into one pack. I feel I probably should have caught that from the first post, sorry 'bout that. I'm probably worrying about nothing, though. Test it out and let us know how it goes!

  5. #5

    Default Re: Wiring LEDs with differing voltage/current

    If you use a NiCd 7.2 volt PP3 battery, you can avoid using wasteful resistors altogether. Alternatively, you could put all LEDs in series and power from three NiMH 1.2 volt cells in series?

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