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Flickering LEDs

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Mar 6, 2005.

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  1. I mounted 15 cheapo Hong Kong white LEDs on a protoboard last year and
    ran it for awhile with about 25mA thru each LED. I didn't have it
    running for months like I usually do, and it's been sitting around for
    awhile.

    I connected it up to a PS and with the current set at about 20mA,
    several of the LEDs started flickering. First one, then a few more
    started flickering at a few times a second, some completely on and off,
    others just barely perceptible. Finally a few of the flickering LEDs
    just quit completly, so I replaced a couple with some of the same
    (unused) HK LEDs. The unused ones are much brighter than the rest, so
    it's plainly evident that the LEDs deteriorated with whatevr use I gave
    them last year.

    I've run other white LEDs for long periods, and this is the first time
    I've ever sen them flicker. Must be those cheapo Hong Kong LEDs.
    Weird.


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  2. "Watson A.Name - \"Watt Sun, the Dark Remover\""
    You seem to know a lot about LED quality and lifetime, what's a good
    choice for night-light LEDs around my house. Cost is (essentially) no
    object, as mounting them and wiring them will suck up enough of my
    time that the cost of the LEDs will pale in comparison. Lumiled
    Luxeon Star Hex is my current choice...

    Thanks!
     
  3. R.Lewis

    R.Lewis Guest

    'Hong Kong leds' is a bit vague.
    Who is the manufacturer, what is the type number (etc.)?

    I came across a datasheet for a 5mm white led (a bit under a year ago but
    now can't put my hands on it) that had a max continuos DC current rating of
    19mA.
    You don't forget numbers like that too quickly.
     
  4. Michael

    Michael Guest


    As a former Mfg Engr and Assurance Engr I've often thought it would be really
    interesting (and mebbe humorous) to visit the mfg lines of certain eastern
    companies and observe the various processes. It wouldn't surprise me at all to
    see a worker eating lunch in the clean room or stacks of PCB panels on a bare
    floor before etch. It truly amazes this western observer that the lowest-tech
    houses ever manage to ship functional product; their MEs must be wizards.
     
  5. Yeah, most people don't realize that their time s worth a certain
    something. Then they try to build a few more after the 'one-off' and
    find with stark realization that they really should be counting their
    hours as dollars.

    I have several LS lights around, mostly NW98s. I think I'm gonna order
    a couple Luxeon Star side emitter replacements for the incandescent
    lamps in my Mini Maglites. But I think they're a bit expensive for
    night lights. And I think that it would be better to have a wider,
    distributed light source for a night light; I'd use several 5mm LEDs
    like the Nichia NSPW510s with the LED mounted at different angles to
    distribute the light evenly over a wide area.
    http://www.nichia.co.jp/specification/led_lamp/NSPW510BS.pdf but you can
    buy them for a buck apiece here. http://www.whitelightled.com/

    The two Hong Kong LEDs that I've had on 24/7 since July (over 5500
    hours) have pretty much gone dim, but the two Nichias in series with
    them are still fairly bright. I've never had a Nichia go flakey like
    these HK cheapos. I got them on Ebay, and I think the sellers sell a
    lot of seconds there, even rtho they claim otherwise.
     
  6. To ne honest, I don't know who made them, but the seller doesn't sell on
    ebay anymore, probably because of the reputation for the cheap crap they
    sold. They were rated at 3000 mCd, 20mA, but the 3000 was, instead of
    typical, more like maximum, with the typical something like 1500 to
    2000. So the seller lied. And I've run them at 20mA and they are going
    dim after only 5000 hours. So they lied about howe long they last. :-(
    Another number I came across the other day. Back in March 26, 2001 I
    purchased my first white LED from Rat Shack, paid an outrageous five
    bucks for it. I soon began to look for more reasonable prices for them.
    I went by one electronics store and they wanted even more than Radio
    Scrap(!) So I started looking online for them. Now the shyster sellers
    on ebay will sell them for nearly nothing, but they hit you with a ten,
    fifteen or more dollar shipping charge, which brings the price up to
    about a quarter apiece. Turkeys.
     
  7. Yeah, a walk-through can tell you a lot. I estimate that 80-90% of
    problems could be anticipated by having an experienced (in the
    production processes) engineer, not even a QA guy, poke around the
    facility. I've seen PCB panels on open-air racks in dirty environments
    (resulting in hairline shorts between traces), fine mechanical
    assemblies being done by subcontract housewives in uncontrolled
    conditions, poorly maintained equipment, lax qc control etc. etc.
    There are also some very well run factories, and the catch it that
    it's hard to tell the difference from just looking at the samples
    (which may or may not be made in the same conditions as the production
    run). They are usually quite willing to have foreign buyers look at
    their operations, but are much more suspicious about locals- with some
    justification (I've actually had offers to do industrial espionage for
    competing comanies, which of course were declined).
    One of my friends in HK who has their production on the mainland says
    he must deal with much cheaper locally invested competition that ships
    10-20% bad product. That's acceptable in 3rd world countries, where
    the labor to find the bad ones is very cheap. Similarly, one of the
    semiconductor suppliers I deal with has a cheaper product with no QC
    for the India etc. market. I also know a well-known US semiconductor
    company that sells their chips "untested" at very low prices for the
    China market. There are probably more of them doing it.


    Best regards,
    Spehro Pefhany
     
  8. Guest

    i too believe the ones on ebay are seconds. i picked up a bag of rgb
    leds. lined them up on a board in a straight line. as you power up
    each color, it is easy to see that the chips are all over the place
    inside the package. red seemed to be skewed the most causing the
    viewing angle to be way off to one side.
     
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