Lots of Leds, please help

Discussion in 'The Projects Forum' started by E-Newbie, Jun 28, 2007.

  1. E-Newbie

    Thread Starter Member

    Jun 28, 2007
    10
    0
    Hi Folks,

    Im a complete Newbie to electronics, although I do repair a lot of electrical items and am really good with a soldering iron, I dont really know too much about resistors and so on.

    I really hope someone could help me please? I am trying to connect several leds, I have made led strips, which I need to all light up at the flip of a switch. There are 10 strips in total, 8 strips have 12 leds the other 2 only have 8 leds, the leds are Blue and require a voltage of 3.4-3.6V the strips are for decorative purposes and there isnt much room on the strips for even the wiring so I didnt use any resistors, all the leds are connected in parallal. I was just wondering is there any way to power all these led strips from a single power supply, or to modify a PC power supply to power all the strips (I have several working spare PC supplies sitting around...)

    Would there be any better way to get these to all work please without placing a resistor on each single led? at the moment, I dont want to use 10 adjustable power supplies, set to 3v, one for each strip as this would take up too much space and be difficult to maintain.

    I really hope someone can help, as I said I am really new and dont know much apart from how to solder.

    Thank You :)
     
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
  3. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    To get the proper current through each LED in a parallel circuit you need a current setting resistor on each one. Usually when there are a lot of LEDs to control you would wire
    them in series and use a single current setting resistor (or an LED controller).

    Since all of the LEDs (in a parallel group) are the same you may be able to get away with a single resistor. The problem is that you will not know how much current is going to each LED. If an LED fails then the current will increase in the LEDs that are operating.

    Also the LED color changes with current. I am not sure how much of change you will get or how noticeable this is in your application.

    Check out http://www.linear.com/pc/viewCategory.jsp?navId=H0,C1,C1003,C1094
    for some LED driver application notes.

    (* jcl *)
     
  4. E-Newbie

    Thread Starter Member

    Jun 28, 2007
    10
    0
    Hello,

    Thank you so much for your replies and help, im really trying to find a power supply that can supply the leds with the 3.4-3.6V that they require, I was thinking that a PC power supply could be adapted with a few resistors to reduce the voltage and supply the needed power to the leds? The leds themselves once in place will be covered by metal strips so they wont be visible only some of their light will pass along the wall, so im not too sure how they would look if some end up brighter than others.

    If they where wired in series wouldnt that mean if one of the leds fails all the leds would go off please?

    I have been testing the strips using a Nokia phone battery, which has 3.6V and they look quite good and really bright, however using a 3V power supply (500 mah) they are a lot dimmer, im just struggling finding a decent power supply for them all.
     
  5. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    You are better of with a 5-6V wall transformer and an adjustable LDO regulator.
    Go to Digikey and find the appropriate power wall transformer. You could probably
    use something like an On-Semi NCP1086 1.5A adjustable output LDO.

    A PC power is a lot more power than you require. It may require minimum loads
    on some of the unused outputs to operate.

    In series if one LED fails they all go off. The LEDs do not fail often and you can
    use jumpers to isolate the failing LED.

    (* jcl *)
     
  6. E-Newbie

    Thread Starter Member

    Jun 28, 2007
    10
    0
    Hello,

    Sorry for sounding really dumb, I have a 6V power supply and I assume I could add a resistor on each strip to lower the voltage to 3.6V, however what is a LDO regulator please??? Sorry.

    Mentioning the jumpers, is it possible to purchase PC jumpers with leads attached anywhere please??? Like the ones you get in PC cases to connect the hdd leds, on/off switch, etc. Im in the UK and I cant find these anywhere but if I could it would make the strips much easier to put together as im having to cut up network cables into 20CM pieces and solder an led to each one, having a jumper style cable I could just insert the leds into it to make the strips?

    Maybe it would help if I explained what im trying to do, I have shop slat boards like http://www.displaycentre.co.uk/image.aspx?ImageID=2708&Width=400&Type=Category
    The boards are a beech finish with silver inserts where the boards meet on the wall I want to add leds on some of the panels, so that light shines through the inserts.
     
  7. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    The voltage across a resistor changes with current. For your application (which is
    constant current) it would work. The better way is to use a Low DropOut (LDO)
    voltage regulator to provide a constant voltage source. Checkout the
    datasheet of the On-Semi NCP1086 for details.

    The best way would be to use a constant current source since you want to maintain
    a constant current through the LEDs. A voltage source and current setting resistor
    will work.

    (* jcl *)
     
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