Doubt on led characteristic

Discussion in 'General Electronics Chat' started by b.shahvir, May 9, 2010.

  1. b.shahvir

    Thread Starter Active Member

    Jan 6, 2009
    444
    0
    Hi,

    Pls. refer the link below;

    http://www.optiled.com/LightingKnowledge.aspx

    In the link, point no. 9 (Dimmability) refers to current Vs. input volt charac. for both dimmable & non-dimmable LEDs. What I fail to interpret from the chart is, how can current thru LED increase with decrease in input voltage in case of non-dimmable LED, especially when an LED, like a simple diode, obeys Ohm's law beyond threshold voltage of say, 0.7V? Pls. clarify.

    Regards,
    Shahvir
     
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Those are not 'simple' LEDs, they are units designed to replace filament bulbs.

    These often include a switched-mode power supply to reduce the input voltage to that needed by the actual LED element.

    As with any switched-mode regulator supplying a constant load, increasing the voltage reduces the input current as the *power* (W = V * I) remains constant.
     
  3. b.shahvir

    Thread Starter Active Member

    Jan 6, 2009
    444
    0
    Thanx, but I don't think non-dimmable LEDs, in general, would include an SMPS as they are generally simple in construction & work on any 230V / 12V Xmer. They do not necessarily require electronic power supplies. This is from practical observation.

    If one traces the charac. backwards, reduced voltage indicates increase in LED current.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Single LEDs are diodes with a conduction voltage of around 1.8V - 3.5V depending on colour and type.

    How do you think they produce the actual LED supply from the input voltage?

    Using a simple resistor on a 12V suppy, there would be around 9V across the resistor and it would be dissipating three times the power of the actual LED - in other words, 75% of the input power would be wasted.

    Very low power ones for indicator lamps may just use resistors, as the overall power is still small.

    High power LEDs for filament lamp replacement in illumination applications virtually always a switched mode regulator.

    Even the little LED module I got for my 2 cell AA Maglite flashlight has a switched mode regulator built in!
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    If you look at the graph on your link-- point 9, it says
    "Current is regulated by IC inside"

    The non-dimming line on the graph is read from right to left.
    (for all who may not have noticed)

    And the non dimmable, still use switch mode regulators.

    The dimmable uses a switchmode that also adjusts PWM in comparison to voltage.
    (but only if you go full bright first, then dim) (And only dim to 50% of full)
     
    Last edited: May 9, 2010
  6. b.shahvir

    Thread Starter Active Member

    Jan 6, 2009
    444
    0
    OK, but still one doubt.. if the product W= V x I is held constant by the SMPS, then how does the light output intensity made to decrease in case of dimmable LED Ltg. systems...since light intensity of LED depends on magnitude of current thru it. :confused:

    Going by the current discussions, an LED would not 'dim' as at every step, the SMPS would try to maintain W = V x I product constant and keep intensity of light constant. Pls. clarify.

    Thanx & regards,
    Shahvir
     
  7. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    That's why there are two different classes, dimmable and none-dimmable.

    It's the none-dimmable type that has an increasing current with decreasing voltage.
    (Your original question was specific to the none-dimmable ones).

    The dimmable ones will have some form of additional power control for the output to the LED, based on comparing the actual supply voltage to a reference level.
     
Loading...