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Running leds on AC

Discussion in 'Misc Electronics' started by cpemma, Aug 4, 2003.

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  1. cpemma

    cpemma Guest

    Received wisdom is that one should use an inverse-parallel diode or perhaps
    a series diode of adequate PIV, but what actually happens if you don't but
    current is limited?

    I found this at
    talking about diodes in general.

    "Thus the peak inverse voltage (p.i.v.) is quoted in their specification as
    the voltage below which they will not breakdown.
    The reverse breakdown of diodes has its uses as a voltage reference or for
    protection purposes. It is possible to recover from the breakdown situation
    as long as the diode has not been overheated, i.e. burnt out, by excessive
    power dissipation. Thus a series resistor must be used to limit the current
    flow through the diode."

    True or false?

    Boldly going, I've a 2v yellow (5v max PIV) running off a 55v transformer,
    with 2.35k of series resistance. Calculated If about 22mA. Still alive after
    4 hours, but will it do long-term damage?

    Meter reading shows 35v across the resistor, so average current (both ways)
    15mA and a fairly high avalanche(?) voltage.
  2. I do know that most GaN and InGaN LEDs (UV, violet, blue, non-yellowish
    green, white, pink, and anything non-red by Nichia) suffer damage in the
    form of partial shorts (resistance often around a hundred to several
    hundred ohms) when they break down. And that one aging mechanism in some
    chemistries appears to be from a combination of heat and electric field in
    the chip - and this may progress faster during reverse breakdown than
    during normal forward-biased use.
    I would add a parallel diode, and in addition a series one to reduce
    current consumption and heating of the dropping resistor. I would not use
    just a series diode since it may leak enough current to cause breakdown or
    result in excessive reverse bias that some types may be aged excessively
    by if the temperature is elevated. Or put two LEDs in antiparallel.

    - Don Klipstein ()
  3. In the past year or so someone posted a URL to a study regarding LED
    use in auto applications. It said that some LEDs survived a very
    harsh environment that had high voltage spikes in the reverse
    direction. But that doesn't mean that they should be exposed to those
    high reverse voltages. Like Don K. says, use another diode (even a
    much cheaper red LED) in parallel to the LED, with its cathode
    connected the the LED's anode (antiparallel). If it is run off an AC
    power line, use a capacitor to filter out high voltage spikes such as
    those from lightning.

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  4. cpemma

    cpemma Guest

    Don't get me wrong, I'm not advocating this as a recommendable practice ;-)
    Just a bit surprised the led didn't quickly put its legs in the air. (It's
    still running :)

    It arose from somebody who'd fitted a bell-push with a led, and it didn't
    work. I thought it was the 12v ac supply that had killed the led. Turns out
    his 430R dropper resistor was a 430k and the led was fine. Monkey curiosity
    took over.

    Without a 'scope I can't see exactly what's happening, but from the meter
    readings I'm guessing a fairly high reverse voltage drop and a more modest
    reverse current, still giving considerably more VI heat in the reverse
    direction than in the forward.
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