LED Circuit Layout Question

Discussion in 'General Electronics Chat' started by Jack Bourne, Jan 28, 2015.

  1. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
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    Hi,
    I'm trying to create a UV LED light box using stripboard and wondering if there are any problems with hooking the LEDs in parallel and series.
    See the pictures
    http://imgur.com/ChdaWcK,vTyF7wI#0
    and
    http://imgur.com/ChdaWcK,vTyF7wI#1

    The first is with each arm current regulated, the LEDs in series then each arm in parallel. The second, and what I want to do, involves putting each LED in parallel and each of those in series and applying current regulation to the enitre network. In my calculations I've obviously accounted for the increased current, resistor power rating, and decreased resistance.

    This saves a lot of space with having to solder in less resistors and time from not having to cut the track.

    All clear?

    Many Thanks,
    Jack
     
  2. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
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    0
    Oh yeah, they're identical LEDs (cheap UV LEDs from eBay, sent from HK with no datasheet) Only rating I have is 20mA @ 3.2V. I think a slight overcurrent of 1mA isnt gonna hurt
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Do the first..
    Never a good idea to put LEDs in parallel without current limiting each one.
     
  4. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
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    For what reason is this?

    Theyre not going to be identical but will it make that much of a difference if theyre close (to identical)?

    Can I do this instead?
    http://imgur.com/JKe9RdL

    Also excuse the drawings, they're done on mspaint
     
  5. Jack Bourne

    Thread Starter Active Member

    Apr 30, 2008
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  6. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    What do the wavy lines represent? More LEDs in series for a given set? I've modified your image slightly, to show the best way to wire the LEDs.

    ChdaWcKMod.png

    In each series string, the total of all LEDs Vf must be less than the supply voltage, plus some headroom for the current limiting resistor.

    In your image where the series strings are paralleled and has one current limiting resistor will run into problems. Since LEDs never totally match, one string will take more current than the others. This will result in premature destruction (See thermal runaway). Then the next string and the next and so on.

    UPDATE: While I was writing this, you posted an excellent article!
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    yes it can make the difference between a functioning circuit and one that dies quickly.
    LEDs can be well behaved.. BUT it doesn't take much at all for them to go psycho and start becoming bullies and stealing others current.

    A "properly designed" LED device will NEVER use LEDs in parallel without some form to "force/require" current sharing.

    If you would like more detail you can google "led thermal runaway" (edit.. or that article you already posted and I didn't read :) )
     
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