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color changing led problem

Discussion in 'Electronic Basics' started by Patrick, Oct 17, 2004.

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  1. Patrick

    Patrick Guest

    I bought some color changing led's. It works well with 9v batteries
    but when I got home and tried it with 12v (planning to use it on my
    car) and a 750ohm resistor it just lit red and didnt change color.
    Tried again with lower voltage starting at 1.5 gradualy moving up
    without resistor... it changed color at 6v - and then burned! What am
    I doing wrong? It worked well with the 9v battery so why did it burn
    at 6v? Any ideas on how to get it to work?

    Thank you very much and good day
  2. Blake

    Blake Guest

    If there is any possibility of getting a data sheet on the LEDs, or at least
    the recommended operating voltage and current, that would help you design
    the bias curcuit properly. Without a data sheet on these LEDs, all I can
    offer is a bit of guesswork.

    My guess is that the LEDs don't have an internal resistor, so an external
    resistor is necessary to limit current. Without a current limiting resistor,
    diode current rises very fast once the turn-on-voltage has been passed, and
    a burned-out LED is the usual result.

    Your 750 ohm resistor with a 12v source may have been a bit large. It would
    limit current to under 15ma. Maybe you need a more current to get into color
    changing mode. Try reducing that resistor to something smaller; maybe 600
    ohms or 450 ohms.

    As for why it worked with a nine volt battery even without a resistor, the
    battery may have been weak enough that it self-limited the current to a safe
    value. I'll bet that the LED would have burned out quickly enough when
    powered by a freshly charged 9v battery.

    Just guess work, but I hope it helps.
  3. Marlboro

    Marlboro Guest

    dont do that
  4. John

    John Guest

    LEDs are current-operated devices and need a limiting resistor to
    prevent excessive current. Connecting the LED directly to a power
    source is generally a recipe for failure...

    The reason the LED worked at "9 volts" is that the battery is limited
    in the amount of current it can deliver and the battery voltage
    dropped when the LED was connected.

    You need to limit the LED current to something around 20 ma
    (milli-amps) for testing.
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