parralell leds getting dimmer?

Discussion in 'General Electronics Chat' started by solexious, Mar 12, 2008.

  1. solexious

    Thread Starter Active Member

    Nov 20, 2007
    55
    0
    hello,

    I'm adding some leds to a circuit, ive got a 7805 hooked up to a 9v battery and im adding leds in parralell, but every time i add one i see a slight dip in blightness. I thought adding them in paralell wouldnt do this?

    How come I see this and is there a way to make it stop?

    Thank you

    solexious
     
  2. raffter

    Active Member

    Feb 28, 2008
    113
    0
    current IS divided everytime you add another .... add resistor PER LED ...



    Ralph
     
  3. antseezee

    Active Member

    Sep 16, 2006
    45
    0
    Your 9V batter has a max output current. You have so many LEDs in parallel (which are probably drawing 10-20mA) that the max current is being drawn, and a proportional amount of current is only being provided to each one. Let's say the max output current is 100mA. You hook up 5 LEDs with the same resistance - now they're at 20mA a pop. You keep adding LEDs, and as a result, that 20mA a piece now gets lowered to 15, 10, and so forth.
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If as raffter has mentioned, with each additional led placed in parallel with those already connected, the available current will be distributed among the leds connected in parallel.

    By connecting each of the leds with its own series current limiting resistor between the output of your voltage regulator and ground, each LED will get a prescribed amount of current which will not vary as long as the output of the voltage regulator remains constant.

    hgmjr
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The minimum input voltage for the 7805 regulator is 7.5V.

    Measure the voltage of the battery. When its voltage drops below 7.5V from being overloaded then the 7805 regulator's output voltage will drop.
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    You shouldn't be using the voltage regulator anyways, depending on your configuration. It's just wasting power :(

    You should string as many as possible together so that their forward voltages are a bit less than 9V, then use an appropriate limiting resistor. (9V - Vf X #LEDs)/(nominal current for LED) = Rlim

    Steve
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Then when the battery voltage drops a little, the LEDs turn off.
    You will actually see them dimming.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Yes, you're right... I looked into battery discharge curves, they're quite dramatic, which would show a lot of dimming over it's life. The only good way to fix this is to do a switching regulator. Something along the lines of the following link, except modified to accommodate more LEDs/.

    http://www.maxim-ic.com/appnotes.cfm/an_pk/3532

    Steve
     
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