Help with LED rope lights.

Discussion in 'General Electronics Chat' started by reno12469, Jan 19, 2009.

  1. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    Does any one know how LED rope lights work? Ive been looking every where and i cant find it. And does anyone have a schematic?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    What do you mean by LED rope light?
     
  3. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    Like this. I know that they are either wired in parallel or in series but websites say never to use parallel.
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    I think these string LEDs are wired in parallel inside the tube in case on goes off the other will stay on. By parallel, you mean that you want to connect more strings at the end of the first string?
     
  5. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    This is what i mean

    [​IMG]
    And make it into a long strip
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The LEDs would most likely be wired in strings of three, with a current limiter; either an active semiconductor device, or more likely a simple resistor.
    60/3 = 20 strings
    400mA / 20 = 20mA, which is a typical LED current.
    Blue LEDs typically have a Vf of between 3.2v to 3.8v.
    3.4v x 3 = 10.2v total.
    So, the resistor would have to drop 1.8v across itself allowing 20mA to pass.
    1.8v/20mA = 90 Ohms. 91 Ohms would be the closest standard value.

    See the attached schematic.
     
    Last edited: Jan 19, 2009
  7. mik3

    Senior Member

    Feb 4, 2008
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    The idea of using one resistor for all the leds is not a good idea. If One leds goes off ten the other will receive more current and will burn too. Or the voltage of the leds, even if they have the same specifications, can vary during their operation slightly and causing the current in other leds to increase and burn them. You can parallel many leds as you want if you use one resistor for each led and if your supply can provide the required current.
     
  8. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    so your saying every 3 leds put a resistor?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    See the schematic attached to my prior post.

    As Mik3 suggested, this schematic:
    [​IMG]

    is a very bad idea.
     
  10. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    Thank you.
     
  11. mik3

    Senior Member

    Feb 4, 2008
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    Sgt's idea works well. However, if one led of the three in a branch goes off, then the whole branch won't light. If you use one resistor for each led this problem will be eliminated but you increase the components and cost. Its your choice.:)
     
  12. italo

    New Member

    Nov 20, 2005
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    Wrong assumption if one fail_ how short? then there is no more light. If it fails open then there is 1/3 more power available. He shows a source of 3v so for LEDS in parallel that is proper design. What i don't understand how can you get a ROPE of only 3 LEDS.
     
  13. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    I think ill go with a resistor every LED. How many do you think i can put on with a 12v dc car battery?
     
  14. mik3

    Senior Member

    Feb 4, 2008
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    If each led needs 20mA to work then you can put up to 1000 leds! However, if you use Sgt's idea you can put up to 3000 leds with the same power consumption. Its a compromise between with what I mentioned before.
     
  15. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    So with Sgt's design i would have to run a positve and a negative wire all the way down the strip?
     
  16. mik3

    Senior Member

    Feb 4, 2008
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    The answer is yes, you have to run a positive and negative wire along the whole string.
     
  17. reno12469

    Thread Starter Active Member

    Jan 17, 2009
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    Thank you.
     
  18. mik3

    Senior Member

    Feb 4, 2008
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    You are welcome ;)
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    Manufacturers of LED lights measure and sort the LEDs into groups that have identical forward voltages so that they can be safely connected in parallel.

    But this rope light probably has 3 blue LEDs in series and other strings of 3 LEDs in series with all the strings in parallel and they are powered from a single resistor.
     
  20. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Take a look at this PDF on grouping leds.

    Greetings,
    Bertus
     
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