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reducing the length of BigW lights

Discussion in 'Lights - Store Bought and Home Made' started by BundyRoy, May 22, 2014.

  1. BundyRoy

    BundyRoy Senior Elf

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    I have a string of BigW led lights that is 30m long (600 leds). It currently runs on a 24v AC transformer. I am planning on cutting of the multi function controller and running the string on 27V DC through one of Alan's 2811DC15 controllers.

    My question is can I cut the string in half and turn it into two 15m strings (300 leds each). I understand that I have to work out where to cut it as the leds are in groups. What I want to know is do I have to add some extra resistance to each string to stop the current going up. My thinking is V=IR, I have halved the resistance so one of the other two variables has to change. Or doesn't it matter if current goes up.

    This now has got me thinking. Are leds voltage dependent or current dependent. If it is current then maybe their is the potential to drop the voltage instead of adding resistance. I realise this is not as practical as all the other strings on the controller will still need 27V to run.

    Now if I do need to add a resistor, is this as simple as measuring the resistance through the existing full length string (with multimeter) and then measuring the resistance through the shortened string and adding the required amount (with a resistor) to get back to the original total.
     
  2. smartalec

    smartalec Im a SmartAlec what can i say! Community Project Designer

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    if your realy lucky at the half way point, there will only be 2 wires an not 3 wires, so that should also be your safe cutting zone as well
     
  3. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks Alec. I'd watched one of your videos on how isolate the bigw strings into the different groups. I was also hoping that there may be a section where the lights come back to two wires.

    Do I still need to add the resistor or can I just plug the shortened string straight into the controller?
     
  4. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    LEDs are current devices, so the best way to control an LED is with a constant current driver as this will maintain the correct current.
    Now with a DC controller board then this will control the lights via voltage control, this is sufficient for most low current LED applications, its when you get to high wattage LEDS that the constant current driver starts to play a more important role due to heat and longevity.

    With LEDs the basic rule applies

    Series - add voltage together to make a circuit
    Parrallel - Add current together to make a circuit

    So you will find that the string is made up of a bunch of LEDs in series to make an efficent circuit and then to make it longer it is then paralleled.

    So if you find the point between the parallel circuits then you will not need to add or change resistor values, its only when you start removing LEDs for a series circuit that you now have to add/change the resistor value
     
  5. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks Eddy. Great explanation. I can follow the concept now.
     
  6. David_AVD

    David_AVD Bite my shiny metal ass!

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  7. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks David. It always helps to see a diagram. So the 680R shown on the diagram is a 680 ohm resistor I gather.
     
  8. David_AVD

    David_AVD Bite my shiny metal ass!

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    Yes, 680 Ohms. I've cut down strings before. It's very easy to do - just cut where the number of wires drops from 4 to 3. If you cut whole segments there's no need to fiddle with calculating and adding resistors.
     
  9. Pircy

    Pircy Full Time Elf

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    Great timing with this thread. I am about to do the same thing with my BIg W red, blue and white Leds to make 6 arches for this christmas. Awesome timimg!
     
  10. Pircy

    Pircy Full Time Elf

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    Hey Bundy, where can I find that video on isolating Big W strings?
     
  11. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Right Here. All credit goes to Smartalec though. It's his video, I just found it useful. I think it should do exactly what you are trying to achieve.


    http://youtu.be/0ef9fF4HKDw

    The thread is here in case there is other info that could be useful.
     
  12. smartalec

    smartalec Im a SmartAlec what can i say! Community Project Designer

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    Personaly after spending $$$'s of dollar's making new gear an getting pixels..
    the different effects you can do after the icle conversion with the string was super great.
    one of the best hacks ive done
     

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