1. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    Bill,

    I snagged this from a post you did earlier.

    This is the basic idea of how to light all of the LEDs? It's that easy?

    Anyone can reply. I'm not singling Bill out

    Jim
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    It can be that easy.

    What do you want to do?
     
  3. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    I am in my 3rd Quarter at ITT for electronics. I just wanted to light all my LEDs in my toolbox. (just messing around) but I couldn't quite get it. Now that I seen this post by Bill I feel like an idiot. My only question is...

    How do I get more current so the LEDs are brighter?

    This is probably a dumb question too. lol.

    If I raise the voltage the current increases. I don't want this. I will use only 12 V. If I lower resistance the current increases. This, however, will effect the voltage drop on the LEDs. I can't do that either. The choices of resistors is limited so I am forced to use:

    4 x 5 rows (20 LEDs)
    4 - 82 Ω Resistors
    2 - 56 Ω and 27 Ω in series (don't have enough resistors to be consistent)

    12 V industrial heavy-duty lantern battery

    LED specs:

    3mm Red
    Vf = 2.6 V
    If = 20 mA
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    I tend to focus on 20ma because that is a max for many makes. You can buy LEDs that use 1.4A, and there are or will be units that use even more current with corrisponding increase in brightness. Such units need heatsinked, since they generate over 3W in heat, but compared to light bulbs or fluorescent they still represent a major improvement in efficiencies. There are concepts just over the horizon that will be even better.

    LEDs are pretty easy and cheap, which makes them fun. They also have some other characteristics that I find pretty useful, such as the Vf, which is a pretty good constant voltage I occasionally use for other jobs.

    You will find the voltage drop on the LEDs doesn't decrease that much with a reduction in current, this is both a blessing and a curse. The max current rating is just that, a max. They can use less, and still be plenty bright. The amount of light generated is pretty nonlinear compared to current, which makes PWM a very good option for controlling LED brightness, since it is linear.

    LEDs also have a long lifespan that will shorten considerably if you overdrive them. Less is usually better than more.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Look at the datasheet for an LED to see the very small change in voltage that is caused by a change in current.

    Here is the voltage/current graph of some ordinary red LEDs that I have. The max allowed continuous current is 40mA or 200mA peak. At 25mA they are very bright.
    At 2mA the voltage is 1.6V.
    At 10mA the voltage is 1.7V.
    At 50mA the voltage is 2.0V.
    At 200mA the voltage is 2.75V.
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
    1,146
    16
    They also have LED drivers that provide a simple connection of series and parallel LEDs. I've seen drivers that will drive 60 20mA to 30mA LEDs.

    Austin
     
  7. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    I have similar.

    [​IMG]
     
  8. a7med

    New Member

    Jan 30, 2010
    1
    0
    you can use parallel voltage sources, it will increase the current while the voltage is the same

    best regards,
     
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