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Thread: Wiring up LED's

  1. #1
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    Question Wiring up LED's

    I'm making a dust shoe for my router and I'm putting a ring of LED's around the spindle.
    I have 12 LED's wired in parallel, what I need to know is what is required for me to hook them up to
    my 48 volt power supply.

    Regards,

    Kiwiken

  2. #2
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    Default

    I guess that would depend on the voltage requirement of your LEDs
    Every day is better than yesterday

    Cheers
    SAISAY

  3. #3
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    Default

    They are;

    - Voltage: DC 3~3.6V
    - Current: 15~20mA

    Regards,

    Kiwiken

  4. #4
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    I would put a resistor in series with each led.

    Try one with a 2.4K resistor in series & measure the drop across the resistor.
    Cliff.
    If you find a post of mine that is missing a pic that you'd like to see, let me know & I'll see if I can find a copy.

  5. #5
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    Cliff.
    If you find a post of mine that is missing a pic that you'd like to see, let me know & I'll see if I can find a copy.

  6. #6
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    I have areadly soldered my LED's together and was hoping
    to only put something on the positive wire going to the LED's.

    Regards,

    Kiwiken

  7. #7
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    13 leds at 3.6V each would be 47 v if done in series (+ve of one to -ve of the next etc).
    Better to put 14 in series to be between the 3 and 3.6 volt.
    no pull down resistors needed.
    Otherwise you'll need 1 to pull the 48 volt down to 3.6V

  8. #8
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    Those LEDS with those specs need a resistor of 220Ω(Ī30), power rated to suit. The page Cliff linked to explains it well enough.

    You say you've already soldered 'em up in parallel... personally, if I couldn't 'undo what's done' I'd go for two 500Ω resistors, one to each end of the "+ve loop." So in effect they're in series.

    But if I could start again, I'd definitely wire one per LED. If physical room was an issue, I'd consider 1 resistor per group of 2 or 3 LEDs. (Then again, my 'tronics skills are even more rustic than my woodwork... )
    I may be weird, but I'm saving up to become eccentric.

    - Andy Mc (AKA "Ghost who posts." )

  9. #9
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    Get the resistor value wrong, and you'll either get a dim light for a very long time, or a really bright light for a very short time indeed......

  10. #10
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    Default A long time ago in another life...

    I was involved with building a large LED clock (ie each number segment was made up of lots of individual round LEDs). We initially tried a single resistor in series with a bunch of LEDs in parallel. Due to the manufacturing differences, the LEDs lit at different rates. It looked quite poor, for a clock display. Agree with most above. Use a resistor for each LED. We cured our problem in the end by using parallel LED driver chips, which were available then. I suspect you probably don't want that option.

    Regards
    SWK

  11. #11
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    Cliff is spot on. You need a resistor in series with each LED. 2.235k ohms 2.2k is a common size and would be close enough. This will give you a Vf of 3.3V and a current of 20mA per LED.
    Those were the droids I was looking for.
    "just because I donít need the lathe doesnít mean the beer isnít cold" - Grand Master Flett

  12. #12
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    I've done strings of LEDs for a few things, and, IMO, by far the best option, if you can, is series strings. As kinda mentioned, if you had done this with your 12, you'd only need a single 250R resistor with very little current going through it and thus little power dissipated (100mW).

    As it is, you've got a 48v supply and LEDs that require 3.6v@ 12 x 20mA = 240mA which will need a 180R resistor. The catch here is that you'll need a 10W resistor, and will be getting very hot, and wasting a lot of power, and as mentioned, LEDs in parallel often don't behave well.

  13. #13
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    As I haven't enough room to put a resistor on each LED and don't want something that's going to
    get hot, it looks like I will make up a ring with the LED's in series.
    The only reason I was go the parallel way was in case one failed.

    So what do I have to ask for when I go in to the local leading edge store to get one as they
    are as dumb as dog crap. I would have more luck asking someone at McDonald's and getting the right part.

    Regards,

    Kiwiken

  14. #14
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    Well, if you wire them in series, that's 12 LEDs each with 3.6V drop across them, or 43.2V total drop across the string. For a 48V supply and a LED current of 20mA, the resistor required is (48V - 43.2V)/0.02 = 240R.

    You can't buy a 240R resistor, only a 220 or 270. If you use a 220 the LED current will be 22mA, if you use a 270 it'll be 17mA, either will be fine.

    As mentioned, power dissipation will be low - for the 220R it'll be 0.02^2*220 = 0.1W, so a cheap 1/4W carbon resistor will be fine.

    So go get a 220R or 270R 1/4W carbon resistor.

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