leds in parallel choosing resistor

Discussion in 'The Projects Forum' started by panayiotis, Mar 5, 2014.

  1. panayiotis

    Thread Starter New Member

    Feb 25, 2014
    16
    1
    dear sirs
    i want to connect 20 leds in parallel.
    my source is 2x 4.2v batteries parallel
    led voltage white 3.4v 20ma
    in the following page i can find that i need 20 resistors 47ohms
    http://led.linear1.org/led.wiz

    now in other page
    http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator
    i find out that i need resistor 2ohms 03w
    does it mean that i need 1 resistor for all circuit? or 20 resistors, one to each led as the previous page?

    what i want to do, is to connect in the beginning of the positive wire only one resistor to limit the voltage to all leds. instead 20 resistors , one to each led
    is it possible? i am little confused!
    thanks
     
    Last edited: Mar 5, 2014
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,387
    497
    You are making a current divider. The best way to accomplish your goal is to have each LED have its own resistor. So. 20 LED, 20 resistors. The reason this is the best way to do it, is because each LED will be slightly different from other LED, this means that in your current divider some LED will get more current, some LED will get less current. LED that will get more current will be brighter. LED that will get less current will be dimmer. To solve this little problem you can select individual resistors for individual LED, this way each LED will get almost the same amount of current and be as bright as other LED.

    This is wrong. Resistor here does not limit voltage. The job of the resistor is to limit current.
     
  3. radiohead

    Active Member

    May 28, 2009
    474
    31
    If you parallel LEDs, each LED will require its own resistor. If you connect LEDs in series, you need only one resistor for the series string. Series strings may be paralleled.

    For your circuit, you have a source voltage of 8.4VDC. The LEDs are 3.4V @ .02A. If you put two LEDs in series: 8.4V - 6.8V = 1.6V that the resistor must drop. 1.6 x .02A = 32 Ohms per two LED string. Since 32 Ohms is not a standard value, 33 OVhm resistors will suffice. This way you will use half as many resistors.

    I hope this helps.
     
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Case 1. Two batteries in series.

    [​IMG]
    -----------------------------------------------------------------------------------------------------------------------------




    Case 2. Two batteries in parallel.

    [​IMG]
     
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    Last edited: Mar 5, 2014
  5. pwdixon

    Member

    Oct 11, 2012
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    Perhaps a little excessive unless the guy is super sensitive and can manage to kill himself with 8.4V.
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    It is the nature of internetz that you can be anything you want to be.
     
  7. pwdixon

    Member

    Oct 11, 2012
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    I use the internet a lot, I hope it's not going to make me prone to electrocuting myself on a 9V battery, you've got me worried now.
     
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    A healthy sense of paranoia is always healthy.
    ☺7
     
  9. bertus

    Administrator

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  10. panayiotis

    Thread Starter New Member

    Feb 25, 2014
    16
    1
    i told i have 2x 4.2v batteries.yes but they are in parallel.
    you did not ask you just have big mouth man!
    dont be the smart one!
     
  11. panayiotis

    Thread Starter New Member

    Feb 25, 2014
    16
    1
  12. pwdixon

    Member

    Oct 11, 2012
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    Now you might have a problem, putting two batteries in parallel can be a problem that might cause them to overheat and that could be a danger. I think anyone would have assumed you were putting the two batteries in series.
     
  13. panayiotis

    Thread Starter New Member

    Feb 25, 2014
    16
    1
    hmm ok i want to learn why the cause overheat?

    ( i am just a learner.i want to learn from members in here.i dont know about electronics thats why i ask.please be patient with me!)
     
  14. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I apologize. I assumed that you were going to use two batteries in series. A bit later I thought to try two batteries in parallel and I got results that you posted. I have updated my post, please look here: http://forum.allaboutcircuits.com/showpost.php?p=704312&postcount=4
     
  15. panayiotis

    Thread Starter New Member

    Feb 25, 2014
    16
    1
    thank you sir
    just an advice!
    some people can not accomplished their dreams! i work from 16 and i am 29!
    i never had the money to even think for a university! so electronics for me is google and this awesome page you all create here! so be patient with some people that you understand they dont know well and advice them
     
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