Stuck on a LED project

Discussion in 'The Projects Forum' started by raz0r, Aug 19, 2011.

  1. raz0r

    Thread Starter New Member

    Aug 19, 2011
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    So this is what I've done already:

    took a PCB board, wired 5 LEDs in series, added a 100 ohm resistor to the positive end. I did this 12 more times so now I have 60 LEDs on the PCB board connected in series of 5 each and one 100 ohm resistor per 5 LEDs.

    I want to use this with a power supply that gives 12-18volts. I did not know about this variation in voltage when I started this project and at that time assumed that the power supply would give constant 12 volts. But now I've got to know about the variation.

    I asked some of my friends about this issue and here are the replies:
    1. Use a PWM
    2. Use a resistor
    3. Use a transistor (reply 2&3 is from the same guy)
    4. Use a voltage regulator.

    What shall I do now?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    You need a constant current source. You have an example HERE.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Easy to say, hard to prove. I'm holding out for a schematic myself. Schematics are the language of electronics. While LEDs have variations, most cases it isn't sever enough to shut down a circuit.
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    raz0r, congratulations on following the Allaboutcircuits tradition on your first posting. You've succeeded in describing your problem without giving away any useful information. Keep it up!

    But if you want to break your perfect record, you could ruin things by saying:

    What is the desired current through the LEDs?

    What is the voltage drop across a string of 5 LEDs when the desired current is flowing? What is the voltage for reasonable maximum and minimum currents? (This relates to how accurately you need to set the current.)

    What is the behavior at 12 volts (i.e. all lights on, some lights on, no lights on)?

    Ditto at 18 volts (lights are too bright, lights are failing, catching fire, etc)?
     
  6. JingleJoe

    Member

    Jul 23, 2011
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    LOL:D

    He's right though, we can't help you if you don't provide the necessary information.
     
  7. KJ6EAD

    Senior Member

    Apr 30, 2011
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    I'll just assume based on the limited description of the circuit that it's a bunch of red or yellow LEDs and tell you to use an LM317L voltage regulator in current regulating configuration on each string of LEDs. You can get rid of the 100Ω resistors and use 68Ω 5% resistors to program the current (62Ω if you're a risk taker). The circuit topology is in the datasheet (figure 906413). You can use small LM317LZ (TO-92 package) if you can get them.
    http://www.national.com/ds/LM/LM317L.pdf
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    The LM317L idea will work OK once the applied voltage gets up to 13v; otherwise the LEDs will be dimmer than they should be.

    If you remove one LED per string, then the brightness will be even from ~11v to 18v.
     
    KJ6EAD likes this.
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I'm holding out with Bill on this one. A 100 ohm resistor seems to low regardless of the LED color or forward voltage even at the low end of his power supply. I also don't see the need to use multiple LM317's, one for each string of LED's.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If the LEDs have a Vf of 2v @ 20mA, and the supply is 12v, then it works out perfectly to use a 100 Ohm resistor with 5 of those LEDs in series.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    I suspect that it has a cheap wall-wart AC-DC adapter that is rated at 12V (but he didn't say its current rating) and is 18V without a load.
    But it is never used without a load so the 18V doesn't matter.
     
  12. John P

    AAC Fanatic!

    Oct 14, 2008
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    Maybe, but the wall wart would deliver some voltage between 12 and 18 that's dependent on current, and the current will vary depending on what the voltage is, so--trouble.

    And what if the LEDs (which of course he was careful not to describe) are white ones?

    He's wasting everyone's time, including his.
     
    KJ6EAD likes this.
  13. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Guys, I think you are being to hard on him. He made quite clear how he connected the LEDs, 12 strings of 5 LEDs with a 100R resistor in series, probably all in parallel. The only thing that's missing is the type of LED and current he wants through them.
    He can surely give this information in his next post.
    Anyways, with the information we've given him so far he has his problem almost resolved.

    There were other threads that started MUCH worse. :D
     
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