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UPDATE LED Life Test at 2 Months

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Jul 6, 2004.

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  1. I I've had the 4 LEDs operating at 25 mA for all of May and June, and
    the first few days of July. This is about 1500 hours so far. The LEDs
    are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
    that I got off Ebay. All four are in series so there is no doubt that
    they are all running at exactly the same current.

    The two Nichias are going strong, with no noticeable signs of dimming.
    The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
    good panel lights, so can't even consider them suitable for
    illumination. They're really quite dim.

    Of the hundred Hong Kong LEDs I bought, all I've used have had this
    dimming problem after a few hundredds of hours, so I decided to put the
    four on a PS to control the conditions and verify that I'm not imagining
    things. I think one problem might be that the Hong Kong LEDs use an
    organic phosphor that degrades faster than Nichia's, and might
    contaminate the LED chip. But I'm not an expert on this at all, so I'm
    open to any other ideas.

    I posted an update of the progress of the dimming at the beginning of
    last month, and now this one shows that the hong kong LEDs are so dim
    that it's probably not worth continuing the test. I may solder two more
    unused hong kong LEDs into the circuit, and start the test again, just
    for the halibut.

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  2. Jim Adney

    Jim Adney Guest

    I know I've seen references to the fact that normal LEDs, driven at
    their rated current, will be dimmer by half after a year. I don't know
    what the mechanism is, but I've seen this for myself on LED displays
    that were clearly left on for a long time, displaying the same
    characters.

    I could imagine that the rate of dimming would be related to the drive
    current and the rated drive current. It sounds like you may be driving
    yours a bit hard. OTOH, it's quite possible that the "after 1 year"
    test that I read about, meant a full year of normal use, rather than
    left on 24/7.

    -
     
  3. Tim Wescott

    Tim Wescott Guest

    In laser diodes it's aluminum migration. I assume it's the same in
    LED's but don't know -- and I don't know how the heck white LED's are
    made, for that matter (gotta catch up).

    Its worse at high temperatures, probably exponentially with absolute,
    but I know when we were trying to buy a solid-state laser for
    rangefinding it was still mad science stuff -- our mad scientist talked
    to the laser manufacturer's mad scientist and they decided it would work
    just peachy keen. Then it got kicked over to their production engineers
    who said NO! Pissed off people all around.
     
  4. "Watson A.Name - \"Watt Sun, the Dark Remover\""
    I've had problems with several white LEDs from Fry's Electronics failing
    too. They dimmed after a few months of use then they started blinking.
    It sucks because some are inside a night hiking flashlight to keep it
    glowing while switched off. It's hand crafted inside a tough acrylic
    tube like a ship in a bottle. The LEDs can't be removed. I didn't
    think I'd need to make LEDs replacable.

    I'm going back to green LEDs for the next flashlight.
     
  5. AFAIK, LEDs don't have aluminum in them. At lease not in the mechanical
    makeup.


    [snip]
     
  6. What color are they? I bought some cheap blue LEDs, and they were just
    like that atraight out of the package. After five minutes on the PS,
    they started to get erratic.
     
  7. Warren

    Warren Guest

    My 2 cents worth>> Have had a "Hong Kong" white LED on for 70
    days. Runs off of 2 pen cells (3.12 volts when fresh) no series
    resistor and draws 26.4 mils. After 70 days battery voltage down to
    2.628 volts. Drawing 7.92 mils. LED is dimmer with this current draw.
    Going to let it go util led won't light. Gave a quick check with fresh
    batterys and led still very bright. Warren
     
  8. What's claimed and what's actually going to happen are two different things.

    There've been statements here that some LEDs start to lose a lot of
    birghtness at less than 10k hours. That's not much different than a
    fluorescent light.
     
  9. I t hink your measurements are off. Seems to me that a pair of AAA
    cells couldn't put out that much current for that long. How are you
    monitoring the current? If you're inserting the meter in series, then
    that meter is adding series resistance, and it is changing the readings.
    Best to use a resistance permanently in series and monitor the voltage
    across that resistance. I've checked white LEDs and found that the
    current drops off rapidly below 2.8V, to almost nothing.
     
  10. Today I read an article that claims the die heat contributes to colour
    change and long term reduction of light output. So I gots to thinking,
    if you pulse the led at some reasonable rate, would this reduce the
    die temperature and prolong the life of the led? Maybe you could bung
    two of you're cheap hong kong leds next to each other, on pulsed at
    the lowest duty cycle possible to prevent visible flicker, and the
    other on dc.

    The article I read also stated the UV tend to degrade the lenses.
     
  11. "The Real Andy"
    The resistance of the bond wires is always there, and if you put high
    current pulses thru these wires, they get hot. Just one more heat
    source to cause degradation. Same also applies to the chip itself.
    Well, I haven't noticed any discoloration or change in clarity of the
    lenses so far. I would say that most of the dimness comes from the blue
    LED chip fading to nearly nothing.
     
  12. You could tell if it was the LED chip or the phosphor degrading by shining a blue led into the
    degraded and un-degraded white LEDs and comparing the glow.
     
  13. Graham W

    Graham W Guest

    Errr...no. The clear lens is an attenuator of the UV which stimulates the
    phosphor. The UV LED lens may be a different fomulation. There is
    certainly little response from a white LED's phosphor to external UV
    stimulation compared with a straight reflection of the light from an
    external white LED.
     
  14. R.Lewis

    R.Lewis Guest

    Since 'pulsing' the leds would, for a given output, increase the die temp.
    there would appear to be little mileage in this approach if die temp is the
    critical criterion.
     
  15. White LEDs tend to require a higher average current and have a higher
    voltage drop (for equal average klight output) when pulsed than when
    operated continuously. Unless the average current is less than a few
    milliamps, you get more light and less heat with continuous operation than
    with pulsed operation at a frequency high enough to apear continuous.

    By and large, only LEDs that are more efficient at higher instantaneous
    current benefit from pulsing. This was especialy true of LED digital
    displays with GaAsP on GaAs substrate, where a segment had an average
    current of only a few milliamps but the efficiency was maximized at
    instantaneous currents of 50 milliamps or more, often at least 100 mA.
    Many people were not aware of this nonlinearity of those LEDs and believed
    that the benefit of pulsing was due to a quirk of human vision.

    Some LEDs, namely at least some InGaAlP ones, have a degradation mode
    that is a function of temperature and duty cycle. I suspect this is from
    some sort of diffusion of an ingredient from where it belongs to someplace
    else and dependent on electric field around the boundary or zone that the
    diffusion occurs across. Maybe a boundary between different layers
    (having/lacking whatever diffuses) should be sharp but gets "blurred".
    LEDs with that chemistry, primarily at higher temperatures and lower
    average currents, can have a life extension from pulsing. One
    manufacturer, Agilent, publishes an application brief where they encourage
    pulsing of their LEDs with that chemistry if the average current is less
    than 10 mA, and recommends instantaneous currents 10-100 mA (but average
    current not exceeding 30 mA).

    I discuss pulsing LEDs in an attempt to make them brighter (or appear
    brighter) in a web page of mine, http://www.misty.com/~don/ledp.html

    By and large, if the average current is already close to the maximum
    rated constinuous current, there is at beast little to gain from pulsing.

    - Don Klipstein ()
     
  16. The phosphor is intended to be stimulated by visible blue light. In a
    usual white LED, a blue-emitting die (chip), typically with a peak
    wavelength around 460-470 nm, is used.

    - Don Klipstein ()
     
  17. But you would still be able to compare the (attenuated) level between a new and a 'worn' LED.
    .....and I thought it was blue, not UV, that most white LEDs used....
     
  18. Graham W

    Graham W Guest

    Don, Martin:

    Arrrrgh... I thought it was UV. I've re-done the experiment with a
    visible blue LED and it does seem to externally stimulate the
    white phosphor at a level (with a new white LED) which should
    be measurable with a simple photometer. Making a jig and setting
    the drive current would be critical. [I wish I had access to a Perkin
    Elmer Spectrophotometer like the one I used to use!]
     
  19. These are the lovely bits of information that professional eng. mags
    seems to skip over so often. I am no designer of LED gear, but the
    info is interesting to read none the less.
     
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