Led circuit help using parallel array of leds

Discussion in 'General Electronics Chat' started by scott2la, Dec 29, 2008.

  1. scott2la

    Thread Starter New Member

    Dec 28, 2008
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    0
    I am having trouble designing a schematic for a led setup I am working on.

    I will be using 56 white leds. Here are the specs.

    Source voltage I would like to have it at 12 volts.....
    diode forward voltage
    diode forward current (mA)
    number of LEDs in your array

    3 x 19 array uses 57 LEDs exactly

    • each 68 ohm resistor dissipates 61.2 mW
    • together, all resistors dissipate 1162.8 mW
    • together, the diodes dissipate 5814 mW
    • total power dissipated by the array is 6976.8 mW
    • the array draws current of 570 mA from the source
    I need to be able to dim the leds by use of a pwm.

    Is it possible to use transistors to achieve 12 volts from 2 AA rechargeable batteries which together equal 2.4 volts or is it better to use a 9 volt battery" rechargeable 9 volt would be 8.4 volts.

    I need to keep the battery source small to fit in the enclosure. I am not sure how long the batteries will last although i have seen lights using only 2 aa's powering similiar lights.

    Any help designing a schematic with the pwm and any other info would be greatly appreciated.

    I have some basic knowledge of electronics but I am struggling with the power source.

    Thanks so much.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    [​IMG] [​IMG]

    At a transistor going to ground and use it to switch the base of either to ground for PWM.

    The current mirror isn't really recommended, it is for illustration only.
     
  3. MikeD_72

    Active Member

    Nov 11, 2008
    46
    0
    I don't mean to hijack this thread, but the circuit Bill posted caught my interest. What is the purpose of the two diodes going to ground?

    [​IMG]
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
    Hello,

    The two diodes to the ground are keeping the bases of the transistor at ca. 1.4 Volts.
    The base- emittor diode is ca. 0.7 volts.
    The current through the resistor will be 0.7 / 36 = 0.01944 A ( 19.4 mA ).
    The leds will get about 19 mA through their bodies.

    Greetings,
    Bertus
     
  5. MikeD_72

    Active Member

    Nov 11, 2008
    46
    0
    Thanks, that was a great explanation :)
     
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