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LED Driver delivers constant luminosity

Discussion in 'Beginner Electronics' started by Simon Dyer, Jun 16, 2004.

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  1. Simon Dyer

    Simon Dyer Guest

    I very much like the circuit by Israel Schleicher in EDN Jun 12 2003 for its
    ability to drive a 3.6 volt Superbright LED off a 1.2 to 1.5 volt cell with
    very minimal and inexpensive components. Since it pulses the LED, greater
    perceived brightness can be acheived at great efficiency. See
    http://www.reed-electronics.com/ednmag/article/CA302239?pubdate=6/12/200
    3&spacedesc=designideas

    According to the formula, changes to R, L or C could thoretically provide
    more current to the bright white LED - I am looking for 30ma average, but in
    practice I have not been able to acheive this.

    Can anyone tell me where I am going wrong? I would be happy to use three
    cells if it helped.

    --
     
  2. Activ8

    Activ8 Guest

    By believing that pulsing an LED produces greater perceived
    brightness.
     
  3. Ban

    Ban Guest

    Simon,
    if the LED current should be 30mA average, then it has to be with 20% duty
    cycle(which is varying with battery voltage) 150mA. For this kind of current
    the right transistor should be something like a BC337 with more current gain
    at high currents. The inductor probably needs to be of bigger size and less
    inductivity too and you can try to solve the equation given, which IMHO is
    too simplified. The best would be to simulate the circuit with the free
    LT-spice.
    I think the first article gives a better circuit, in the pdf the connections
    to the driver transistor are not drawn or got lost in some pdf-conversion,
    but it is clear where they go.
     
  4. Simon Dyer

    Simon Dyer Guest

    Bonjorno
    Thanks for the practical response Ban.
    I'm not up with Spice models, but will try swapping out Q2.
    Can you suggest values for the inductor?
    Simon
     
  5. Tam/WB2TT

    Tam/WB2TT Guest

    Simon,

    If all else fails, try this: 3 AA batteries in series with 10 Ohm resistor
    (for current limiting) and 50 Ohm potentiometer (connect to 2 terminal only)
    to LED. 30 ma corresponds to the potentiometer at about the center of its
    range.

    Tam
     
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