Connect with us

Do LEDs become older after being used for 1 year?

Discussion in 'Electronic Components' started by Q Gang, Oct 28, 2004.

Scroll to continue with content
  1. Q Gang

    Q Gang Guest

    Hi

    I found a strange things in my LEds device.

    I made a "Batterless Bicycle Safety Lights" 12 months ago.
    (http://www.freelights.co.uk).

    The LEDs on the device are opened to the air, sometime raining, water
    will touch the LEDs. Few days ago I found the red LEDs not working. I
    checked all system, nothings is broken. Then I uses a 3v battery to
    test the LEDs, the LEDs are working, but I feel the red LEDs are
    little darker than the new LEDs. When I put the red LEDs back to the
    device, the red LEDs still not working.

    Finally, I change the red LEDs (replaced by 2 new red LEDs), the
    system working again.

    The problem is: Will the LEDs become older after using a while? Such
    as become darker, resistence will go up.....

    Someone who are know LEDs well can give me a answer. Thanks very much.

    This is very important to me, because I plan to mass manufacturing
    this product in low cost. If the LEDs will become older, I have to
    change the designs for mass manufacturing. (To be honest, I do not
    know how the do the redesign job if the LEDs do get older).

    If you are interested see my redesign works for mass manufacturing, go
    to:
    http://www.helpyou.freewire.co.uk/redesign.html

    Thanks for your help.
    Chin
     
  2. On 28 Oct 2004 06:50:40 -0700, the renowned (Q Gang)
    wrote:

    Chin:-

    Especially some cheaper white LEDs will lose brightness after a long
    time operating near maximum current, but I don't think this is your
    problem. (we call it "aging"-- of course they are a year older after a
    year whether they are used or not. ;-) )

    Zai jian.

    Best regards,
    Spehro Pefhany
     
  3. Dave Platt

    Dave Platt Guest

    I made a "Batterless Bicycle Safety Lights" 12 months ago.
    LEDs do tend to lose some brightness over the years. This is a
    significant problem for some of the "white" LEDs, which use a phosphor
    layer to broaden the spectrum from a blue LED emitter... I've heard
    that some of the phosphors "burn out" quite a lot after a year or so
    of steady use.

    Red and other monocolor LEDs don't use phosphors. They can "age" with
    use, but this is normally a fairly slow process... I think they are
    usually rated as having lifetime-to-50%-brightness of 100,000 hours or
    so, very roughly speaking.

    Now, this applies if and only if the LEDs are being used within their
    ratings. LEDs do not have a fixed "resistance" - like other diodes,
    they conduct very little current at voltages below their "threshold",
    and then conduct very high amounts of current once the voltage exceeds
    the threshold. Threshold for red LEDs is in the neighborhood of 1.2 -
    1.5 volts, depending on the specific color.

    LEDs are usually rated for both a maximum continuous current, and an
    absolute-maximum pulse current. You can usually get away with running
    an LED at current levels higher than its maximum continuous current
    *if* you're feeding it pulses with a short duty cycle, but there's an
    upper limit beyond which even a short pulse of high current will
    either degrade or destroy the LED.

    The sharpness of the current-vs.-voltage curve means that it is almost
    always necessary to have _some_ form of current control or current
    limiting, in order to drive an LED safely.

    Also, LEDs are not very tolerant of high inverse voltages. If you
    present an LED with a reverse-polarity voltage, it's likely to "break
    down" (start conducting current in the reverse direction) at a
    relatively low reverse voltage, and I believe I've read that this
    tends to degrade the LED junction.

    I looked at the "freelights" page, and I wasn't able to see whether
    the little magnetic generator contains [1] any form of current
    limiting, or [2] any protection against reverse voltages.

    If the Freelight doesn't provide a good discharge path for the
    "reverse" half of the voltage cycle generated by its little
    rattling-magnet dynamo, the reverse voltage (the output of the coils)
    is likely to spike up quite high, and I suspect that it'll probably
    exceed the inverse-breakdown voltage of the LEDs. This is bad for 'em.

    You *might* be able to fix the problems by adding a zener diode right
    across the coil. Wire it so that it zeners in the forward direction
    when the coil's output voltage rises up high enough that the current
    through the LEDs is approaching the safety limit. The zener will also
    conduct when the voltage across the coil drops below -0.7 volts (i.e.
    one diode drop in the reverse direction) and this should keep the LEDs
    safe from excessive inverse voltage.
     
  4. I've seen LEDs get weird, the resistance increasing, and then becoming
    intermittent. But I haven't seen this happen with red LEDs. Usually
    it's blue or white LEDs that have problems.

    You might have a problem with sunlight overheating the LEDs when they're
    outside. Or the LED maker made a bad batch of LEDs.

    Also, how do you solve the brightness variation with speed?
     
  5. I was gonna say the same thing, too. ;-)

    Well, I guess it's time for a periodic update on my white LED
    experiment.

    I've had four white LEDs, two Nichia NSPW500BS whites, and two cheapo
    Hong Kong white LEds in series on a CC power supply, running at 20 mA
    since July 11. It's been about 2700 hours (for the HKs, the Nichias are
    the same ones from the previous test and have another couple thousand
    hours at 25 mA). The Nichias are still very bright, might have dimmed a
    bit. But both HKs have dimmed very noticeably, they're putting out much
    less light. They are not as dim as the pair I tested previously at 25
    mA, and which dimmed very badly. The pair I'm now testing have lost, by
    eyeball estimation, probably 2/3 or more of their output. No, I don't
    have a light meter, this is just a simple lifetime test, with my
    eyeballs, that's all.

    From this and the previous test, I would say that the cheapo HK LEDs
    have very poor tolerance of overcurrent, and their lifetime is severely
    shortened as the current is increased beyond 20 mA. At exactly 20 mA,
    their lifetime is still a lot less than the advertised 100,000 hours.
    The Nichias seem to hold up well, and should be capable of sustained
    high light output for more than 5 to 10 thousand hours. But the HKs
    will be lucky to make it to 3000 hours (~4 months) and still have enough
    light output to be considered useful. Sometime in January, at 6 months,
    or about 4500 hours, I expect the cheapo HKs to be nearly useless for
    even indicator grade LEDs. I'll keep everyone informed.

    http://www.speff.com
     
  6. [snip]
    The threshold of IR LEDs is about 1.6 volts, but for red LEDs, it's
    about 1.8 to 2.1 volts. That doesn't sound like a lot of difference,
    but when you put them in series across a power source, it is very
    important.


    [snip]
     
  7. Jim Adney

    Jim Adney Guest

    An LED run at its max current for a year will give off about half as
    much light at the end of the year. I suspect that if you run them well
    below their maximum current the effect will be much less.

    -
     
  8. Ken

    Ken Guest

    That's very true.
     
  9. René

    René Guest

    3V - what current limiting device? With red leds the limited life span
    could be a self fulfilling prophesy if exposed to a "hard" 3V?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-