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Discussion in 'Electronic Basics' started by zooz, Oct 5, 2007.

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  1. zooz

    zooz Guest

    Hello
    I need to know about controlling current through LEDs in order to
    control the illumination
    I've searched, but all I found was of commercial content, I need to
    know the theory.
    Thank you
     
  2. Tom2000

    Tom2000 Guest

    PWM is simplest and pretty much the standard these days.

    I suppose you could get away with a pot, in series with a fixed
    resistor, in series with a LED. That might work for small LEDs,
    drawing 20 mA or less. If you go this route, check the power
    dissipation of the pot. Otherwise you might build a variable current
    source from a regulator IC or a transistor.

    To figure the LED's current limiting resistor requirement, subtract
    its forward voltage drop from the supply voltage. Then determine the
    current you want to push through the LED. The dropping resistor value
    will be (V - Vf) / I. (For small LEDs, you'd typically drive them
    with 5 to 20 mA. Often less than that will serve you well.)

    For manual control, a pot, feeding the ADC input of a Picaxe08M chip,
    driving a small LED via its PWM output through a series resistor is
    pretty simple and reliable. Just a few lines of simple code makes it
    happen.

    I guess it all comes down to what you want to do. Change the
    brightness of a small indicator LED or control 100 3 watt RGB LEDs to
    light a nightclub? Vastly differing requirements.

    HTH,

    Tom
     
  3. There are two common methods.

    One is a DC method that uses either a resistor or a more
    complicated active current regulator circuit in series with
    the LED. Changing a resistor value changes the current
    setpoint.

    The other uses a pulse width modulation scheme that varies
    the duty cycle (percent on time) of a fixed current
    amplitude pulse to vary the average current. In either
    case, the intensity is pretty close to the average current
    through the LED.

    Either scheme can be varied with a variable resistor (having
    the resistor vary the duty cycle of some timer circuit for
    the PWM scheme), but the PWM scheme is most often used when
    a digital processor will be in control of the intensity.
     
  4. There is no "one theory" for this. Theory comes in many varieties.
    (1) Any one of a number of current source/sink topologies exist --
    some requiring a resistor change to adjust the current, some
    controllable by setting a control voltage or current, etc. I'm no
    expert on these, but I know a few ways, and they require somewhat
    differing ideas to follow them. (2) There is PWM (pulse width
    modulation) that can be used. Might be an electronic circuit or
    controlled from a microcontroller (with software.) Different theory
    here. (3) You might be interested in controlling the change in
    illumination from the point of view of a human viewing it... if that's
    the case, there is more theory on that subject, as well.

    What do you intend? Be as detailed as you can, so that others can
    make judgments about what theory you need to understand. Or else
    state categorically what commercial example you find meets your needs
    and ask about that. Or?

    Jon
     
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