Basic led set up help

Discussion in 'The Projects Forum' started by darky82, Jan 9, 2012.

  1. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Hi all, New to allaboutcircuits and in need of someones expertise.

    I am try to set up 20x 1w high power leds rated at 350mw 3.2v-3.5v and running from 3.3 running off a computer power supply.

    The problem is i have a basic knowledge of circuits and components.

    I have looked at a few posts and just confussed myself more on what type of circuit to use parrallel or series and what size resistor to use, i think along the lines of a CFR-1w 1ohm i have looked at other posts and some have transistors and other components i need to keep it simple.
    Any help would be great..
     
  2. darky82

    Thread Starter New Member

    Jan 9, 2012
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    No one wants to help the dumb new guy:confused:
     
  3. nerdegutta

    Moderator

    Dec 15, 2009
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    This is an international forum, and members from all over the world. When you are at the computer, some are asleep, and cannot answer your question. Give it some time.

    However, have you read Bill_Marsdens blog about LEDs. Start there.

    And yeah, Welcome to AAC. :)
     
  4. darky82

    Thread Starter New Member

    Jan 9, 2012
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    yeah i see lots of posts being made, isn't what this place is for to help, the experienced to help the people that have no idea. Oh yeah thanks (nerdegutta) your a good bloke..:cool:
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Your power supply output voltage must be higher than the Vf of the LEDs. You will not be able to run 3.5V LEDs from a 3.3V power supply. You should consider using the 12V output from the power supply. In that case, you should be able to arrange 3 LEDs in series with a suitable current limiting resistor. Then, duplicate the arrangement seven times and you can light 21 LEDs, or if you must use only 20 LED's, a different current limiting resistor will be required for the seventh string (because it will have only 2 LEDs.)

    If using a higher supply voltage seems viable, repost here and someone will help you determine the value of the current limiting resistor.
     
  6. russpatterson

    Member

    Feb 1, 2010
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    Give this a try: http://ledcalc.com/ If you Google 'LED resistor calculator' you'll get a bunch of hits.

    I put in 12V, 350mA and 2 LED's and it came up with a 15 Ohm current limiting resistor. I've read that you should have at least 100 Ohms for this purpose (I think from Bill's LED blog on this site, you should read it). So in that case you could just put one LED and one resistor.

    Usually the high power LED's are current regulated with a driver designed for that purpose. Like a Buck Puck. If you're not that concerned about power efficiency you can use an LM317 to limit current to 350mA (search LM317 current limiter, it's a simple circuit).

    LEDSupply.com sells all this stuff (drivers, LED's, heat sinks, optics, etc.). You will need a heat sink to run those LED's. I tested one once for a few seconds w/out a heat sink and it failed after a couple months of use. So don't run them w/out a heat sink. The LED's need to be mounted on a metal core circuit board that is epoxied to a heat sink.

    Also be careful not to look directly at the LED's when you turn them on or you'll get a gnarly headache and feel like a flashbang grenade went off inside your head.
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    You think the OP meant 350 mA instead of 350mW. I wondered about that.
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ya me too wondered on 1W running on 350mW :eek:

    As stated before, 3.3V will light jack using 1W LED's

    U should use the 5V if you wanna use them in parallel.

    Better suggestion is use them in series @ 12V line.
    U can series 3 of them ....

    Oh wait, 20 pcs u say.
    Then ya u can make 6 Strings of 3 series LED's and 1 string of 2 LED"s in series.

    For the 3 LED string use a 6.8Ω 1W resistor in series @ 12V
    For the 2 LED one use a 18Ω 2W resistor in series @ 12V
     
  9. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Many people get confused as to mA and mW, so you kind of need to do some deciphering. Luckily, the OP helped us out a bit.

    OP states they are 1W LED's, and I don't know of any that are marketed as 350mW, so 350mA is a good guess, especially since 350mA x 3.5V ~ 1W
     
  10. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Ok thanks guys, my mistake was ment to be mA, i had a look at the data sheet and they say a IF is 350mA and a Ipulse of 800mA will try post a data sheet. I had a look at the video's on this site and there is alot to learn..
     
  11. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Ok if i use the 12v line its rated at +8.0A, and +5v at 22A.

    So would it be better using the 5v line so that the 12v is not running near full capacity.

    And the LM317T, Is this used on the - side, or it does not matter and does each globe need one if i run in a parallel circuit.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Does not matter what line u use for the supply. They are rated for it's capacity.

    Mind you an ATX won't properly regulate if all the lines are not at minimum loading. Specially the latest ones.

    Very old ones are the ones u can get away with without secondary unused buses.
     
  13. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Yeah its an old model supply, Is the LM317T a better option.
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U won't need it. See post #8 and try it first.
     
  15. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Ok thank you :)
     
  16. darky82

    Thread Starter New Member

    Jan 9, 2012
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    Ok, can i ask how you came up with 6.8ohm in post #8

    18ohm seems right, but 6.8ohm to run 3, i worked out 588mA.

    Does an 12ohm resistor seem right.

    And my jaycar store only had the square ceramic type 5w resistors in 18ohm, are these ok to use, can they be used for all the strings.

    Also i tryed one globe with the two 18ohm resistors and they get quite hot.
     
  17. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Show me ur setup.

    Show me how u have connected the LED's.

    Show exactly what u did.
     
  18. darky82

    Thread Starter New Member

    Jan 9, 2012
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    I have nothing setup up yet still trying to get the right current, I only used a 12v motorbike battery with two 18ohm resistors and one globe
     
  19. darky82

    Thread Starter New Member

    Jan 9, 2012
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    dont know how i got 12, but when i put 6.8ohms in the ohms law calculator it gives me 1.76471 Amp divide that by three is 588mA

    And i put 36ohm in and get 333mA, so i had two 18ohm resistors and put them together and used my multimeter and got 36ohm of resistance, so i wired it to the led and got alot of heat after about 4-5 min.
     
  20. darky82

    Thread Starter New Member

    Jan 9, 2012
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    look never mind hey. Simple yet to hard, will find another place where people dont treat you like a <snip>.
     
    Last edited by a moderator: Jan 19, 2012
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