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Discussion in 'Electronic Basics' started by terryS, Mar 15, 2009.

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  1. terryS

    terryS Guest

    1) An LED is diode; right? So that it conducts in one direction only?

    2) LEDs are normally operated by DC, from a DC supply or battery such
    as in an LED flashlight?

    3) What happens if I apply AC? Keeping in mind that the AC will have
    peak voltage 1.4 times that of a DC supply the AC supply could be at a
    lower RMS voltage?

    4) So, for example if the DC in item (2) was at 12 volts with
    suitable dropping resistor etc. the AC could be at around 8.5 volts
    RMS, right? But in the reverse or nonconducting state of the LED that
    would result in 12 volts peak (backwards) across the LED.

    5) However if the reverse voltage was limited to to no more than the
    forward voltage of the LED would that be OK?

    6) No particlar application at moment; just curious!
  2. Tom Biasi

    Tom Biasi Guest

    The LED is indeed a diode.
    The reverse peak voltage it can tolerate is not what a diode designed to
    rectify could handle.
    Depending on the LED there is no guaranty that limiting the revese to the
    forward voltage would be OK.
    So you could exceed the reverse peak of the LED and burn it out.
    Why don't you look at some specs of LEDs?

  3. Sjouke Burry

    Sjouke Burry Guest

    If the AC is more tahn a few volts, you will blow the LED.
    The reverse allowed voltage is quite low, but one of the
    tricks for using AC is to use two LEDS parallel, but one of them
    in the reversed direction.
    Each will conduct for half a cycle, and protect the other LED.
  4. John G.

    John G. Guest

    Or just a normal diode in the reverse direction has the same effect.

    John G.
  5. I think most LEDs will tolerate about 5 VDC in reverse, and probably will
    not blow out unless the current is not limited to the usual forward
    current. There are also LEDs with built-in antiparallel diodes, sometimes
    one red and one green, so if DC is applied it will tell you the polarity,
    and with AC it will light yellow.

  6. Jasen Betts

    Jasen Betts Guest

    it conducts better in one direction, forcing it to conduct in the other
    direction will damage it.
    yeah, continuous, or pulsed, DC is the norm.
    the led will blink really fast :)
    not good for many leds.
    most LED datasheets quote 5V as maximum reverse voltage, and
    forwards voltage will be less than that, so yes that would be OK.

    they make some 2-colour LEDs like that, with a red and a greed led in
    connected one way it lights red, the other way green, and with ac you
    get both red and green and the combination appears yellow,
  7. neon


    Oct 21, 2006
    LEDS are diodes with a very low voltage reverse breakdown. Can you operate with ac sure.just make sure that your peak voltage does not exceed the breakdown and if add them in series then make sure a resitor is placed across each to make sure the same voltage breakdown is met. Actualy by using AC a total of more then a 100 LEDS can be lit. Asuming 3v leds.
    Last edited: Mar 20, 2009
  8. bos

    bos Guest

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