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LED lighting project - suitable 12V adapter

Discussion in 'LEDs and Optoelectronics' started by flashdom, May 21, 2010.

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  1. flashdom

    flashdom

    18
    0
    May 21, 2010
    Hi guys,

    This is my first post so go easy. :) I am also not experienced at all with electronics.

    What I want to do is make a suitable lighting system from led's. I would like to keep it as simple as can be. The biggest hurdle is finding a way to connect led's straight to AC mains power. I know led's are 12V(at least the ones I am planning on using) so my question is will any adapter possibly from an old router or laptop work(if it's 12V of course)?

    I am planning to connect between 20-60 led's. I've seen adapters for sale for led strips that are 12V 150W or 100W so thats more than enough watts.

    Thanks in advance for any help!
     
  2. doctaq

    doctaq

    8
    0
    May 21, 2010
    if you are powering low powered (25ma) leds, just about any power supply can work, just find the appropriate resistor size to drop the voltage to the total vf of all the leds in series, for instance laptop bricks tend to be about 18v, you can run 5 leds off of that then resistor to eat up the rest of the voltage.
    this is basicly a throw away method of using leds, since they need to be current driven, for small cheap leds, this is not a problem, but for larger more expensive high powered ones you should build some kind of current limiting driver, search up transistor based drivers and you will be able to build one for about a dollar and change.

    as for selecing a power supply, add up all the foreward voltages of any leds in series, subtract that from the power supply current and find a resistor for the remaining voltage. that is for a low powered led setup, for high powered get a driver
     
  3. flashdom

    flashdom

    18
    0
    May 21, 2010
    Thanks for the swift reply.

    The led's I was looking at are 25-30ma. You say I could only run 5 led's from 18V power supply, what is the reason for that? Would I be able to run alot more led's if I connect them in parallel(because Voltage stays the same i parallel if I'm correct)?



    Would this work?: 12V 150W power supply connects to transistor based driver which connects to 50 led's in series(or parallel)? I hope I don't need a a driver for each individual led' coz that would be way too much trouble..

    Is there any suitable ready made ones available, or kit's that would do what I want minus led's themselves. Circuit should also be dimmable(future upgrade maybe).
     
  4. doctaq

    doctaq

    8
    0
    May 21, 2010
    you can run strings of 5 in parallel to the 18v source

    if you are looking to actually drive low powered leds with a driver running them in parallel within a driver would be kind of pointless since they may fluctuate and some leds may draw more current and so some would get brighter then they should and some will get dimmer than they should.
    id say just run them with a resistor, building many drivers would not be cost effective
     
  5. flashdom

    flashdom

    18
    0
    May 21, 2010
    So what is the reason for 5 led limit?

    is it possible to run it without power adapter just by connecting many led's in series and remainder voltage used by transistor? eg. 18*12=216 + 4v transistor =220V This wouldn't work since it's AC power I guess but would it work from lets say pc power supply, then I would only need led's 1 transistor and power supply...?

    Would running them with a driver in series be better? or still resistor is better idea?
    Thanks.
     
  6. doctaq

    doctaq

    8
    0
    May 21, 2010
    the reason for the 5 multiple is that the vf of leds are about 3.3, divide your power supply by 3.3, if you are using 18v like i mentioned that would leave you with 5 leds and some left over voltage, about 1.5v, then add in a 6 ohm resistor to eat up that 1.5v and you are all set.

    so say for instance in your application, take 5 leds and 1 resistor and wire them all up in series, you can put over 80 of these strings on any average laptop charger.

    a resistor is generally cheaper than a transistor.
     
  7. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    It is not as simple as that in real life. Not all LEDs are spec'd at 3.3V. For starters you need to look at the actual LED you are using and get the voltage. Then, the voltage varies from LED to LED. Then, the voltage varies a lot with temperature. If you have 5 LEDs in series and each of their voltages varies by 0.3V due to temperature, you are looking at 1.5V over the string. If you started out with just 1.5V across the resistor, you now have 3V and are looking at twice the current.

    In real life, if you have less than 30% of the voltage being dropped across the resistor, you need a transistor or IC based driver. For an 18V power supply and low current LEDs, I would no consider anything less than 6V across the resistor. Depending upon the LED being used, that will leave you between 3 and 5 LEDs in series in each string.

    ---55p
     
  8. lates

    lates

    1
    0
    Dec 19, 2010
    make up 20 sets of 3 leds wired in series with a 100 ohm current limiting resistor then wire each of the sets together in parallel

    try looking at this hope it helps http://led.linear1.org/led.wiz

    have fun lates :)
     
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