constant current, increase in load ? LED

Discussion in 'General Electronics Chat' started by shiva007nand, Feb 24, 2016.

  1. shiva007nand

    Thread Starter Member

    Sep 25, 2015
    38
    1
    Hello,
    I am doing experiment on LED array.
    LED , 0.5 watt required 150 mA forward biased current and 3 volts forward voltage.

    eg 1, I have connected 14 LEDs in series
    instead of 150 mA I am giving 133 mA current and rest is same.
    My led array is working fine. It is giving me the required light intensity.
    Now eg 2,
    I am connecting same LEDs in series but the array size is 24,
    I have not change the current it is still 133 mA.
    my question is dose same amount of current will affect in light intensity output ??
    or I have to increase some current or make change in LED array design
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,004
    745
    Post your circuit diagram, any leds in series will have the same current through them, but may develop different voltage drops so your total supply voltage must be able to meet their needs,

    more current means brighter leds upto its maximum point then it doesn't have any effect, after that they will burn out.
     
  3. shiva007nand

    Thread Starter Member

    Sep 25, 2015
    38
    1
    Thanks KOP
    I have attached schematic diagram.
    I have enough output voltage and constant current output. that will not affect brightness of LED.
    each LED will draw same amount of current and will give light intensity according to it.
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Whats the purpose of L1and L2, they short out the bottom row of leds?
     
  5. shiva007nand

    Thread Starter Member

    Sep 25, 2015
    38
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    L1 and L2 are like jumpers to adjust LEDs in array. can be open or short according to product requirement.

    (Off Topic :- I am big fan of LFC what do think can LFC will win League cup final against city ??)
     
  6. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,004
    745
    They might just pull it off, need plenty of luck..
     
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  7. RichardO

    Well-Known Member

    May 4, 2013
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    At the same current, the light per LED is the same. So, more LED's all operating at the same current will, in total, be brighter than fewer LED's.

    A side note. 24 LED's in series needs nearly 100 volts, worst case. Do you really want to do that? I would consider more than one string to lower the voltage requirement.
     
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  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    The theoretical ideal constant current is infinite voltage limited by an infinite resistor

    In the real world, you have a finite voltage to work within..

    As long as the load doesn't require more voltage than your voltage source can supply; any constant current circuit worthy of the name should maintain the specified current.

    Most current limit circuits have a voltage headroom requirement - there must be at least a few volts across them for them to work.

    Generally speaking; LED intensity is pretty much proportional to current.
     
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  9. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    You seem to have figured this out but, what you have here is the maximum allowable current, not the required current. There is no law that says you must operate LEDs at their highest survivable current. Many applications work well at 1/2 or less of the maximum current, for instance, if you are merely doing an, "on" indicator.
     
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  10. shiva007nand

    Thread Starter Member

    Sep 25, 2015
    38
    1
    LED is semiconductor device, it does not obey Ohms law. so I get little confuse about
    its calculations.
    If LED intensity is proportional to the current then I have to use maximum current.
    I do not have any problem with output voltage as my driver provides me output voltage up to
    100 volts.
    Its simple question that LED current for array of 14 LED with constant current of 133 mA will give
    let 'x' amount of intensity per LED.
    can it gives the same amount of intensity i. e. 'x' per LED, for array of 24 LED in series and constant current of
    133 mA ??
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    4,808
    Why do you then have to use maximum current?

    Why don't you want your LEDs to last very long?

    Fine. But have you laid out your PCB to be consistent with 100 V on the board so that you don't cause any arcing?

    YES!!!!!!!!

    It's a simple question that has already been answered several times.

    How many ways does this have to be said?

    The light output of an LED is a function of the current through it. If an LED has 133 mA of current in circuit A and 133 mA of current in circuit B, the LED is putting out the same amount of light in both circuits. Why? Because the light output of an LED is a function of the current through it!
     
  12. shiva007nand

    Thread Starter Member

    Sep 25, 2015
    38
    1
    Thanks for your reply I got all of my answers about constant current of different LED array structures.
    Now, this question might sound stupid but you have to answer it.
    as LED intensity is proportional to LED light output,
    with forward current of 150 mA I am giving 133 mA it will give me 'x' amount of intensity
    and if I give 115 mA , what amount of light intensity it will give ??
    'x' or less than 'x' ?? and is there any formula for it ??
     
  13. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Less current causes less light. The, "how much" of it is provided by the manufacturer of the LED, not some universal formula that we all know.
     
  14. WBahn

    Moderator

    Mar 31, 2012
    17,788
    4,808
    And just how is it that you figure that I HAVE to answer it?

    [/QUOTE]
    as LED intensity is proportional to LED light output,
    with forward current of 150 mA I am giving 133 mA it will give me 'x' amount of intensity
    and if I give 115 mA , what amount of light intensity it will give ??
    'x' or less than 'x' ?? and is there any formula for it ??[/QUOTE]

    What is it about the concept of "the LED intensity is proportional to the LED current" that you are not comprehending?

    Look up what "proportional" means.
     
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