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
    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


    Mar 24, 2008
    [​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
    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?

  4. bertus


    Apr 5, 2008

    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.

  5. MikeD_72

    Active Member

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