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White leds for backlight

Discussion in 'Electronic Design' started by Michael, Sep 15, 2006.

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  1. Michael

    Michael Guest

    I haven't followed all of the progress with LED technology and the
    impressive brightness figures some of these are now achieving.

    Just about every LED site is promoting some sort of high brightness, high
    power, white led(s) on a chip.

    Most are very pricey and hard to find.

    I've always wanted to try and mimic the light output I get from a couple of
    4 watt (4C7) (nightlites), which is about
    60 lumens and thought maybe it's time to give it a whirl.

    I'm so unfamiliar with all the latest and greatest that I thought I'd ask if
    anyone that has been closer to this can give me a head start.

    I have a 6" wide pc board that up till now has had the (2) 4 watt bulbs
    mounted on it.

    I thought about breadboarding not only a multiple discreet series LED system
    but may try some of these hi brightness chips as well.....at the lowest cost
    possible.

    Any part numbers anyone can recommend?

    Like I say, I'm just after my 60 lumens in a white light that
    backlite a shallow 7 x 7 box.

    Thanks
     
  2. Guest

    Can't offer any application-specific advice, but I can tell you that
    I've found some REALLY bright (just received some 7500mcd green 5mm
    LEDs, myself) at http://superbrightleds.com/

    Hope that helps,
    -Drew
     
  3. Tim Auton

    Tim Auton Guest

    You might want to look here:

    http://candlepowerforums.com/

    It's a flashlight forum and the guys and gals there are well up to speed
    on high-power LEDs. They're a friendly bunch too. You might have to
    search around a bit, but there are links to places which sell high-power
    LEDs in small quantities and discussion of all the latest and greatest.

    Promise yourself you won't buy a new flashlight before you go. I'm not
    joking. Repeat after me: I do not need 80 lumens on my keychain. I do
    not need 500 lumens by my bed. One brightness level is fine. My Maglite
    is fine. I just want info on LEDs.


    Tim
     
  4. Don Foreman

    Don Foreman Guest

    The Luxeon III will produce 65 lumens at 700 mA, 80 lumens at 1000 mA.
    Partnumber for a white Lambertian pattern Luxeon III star is
    LXHL-LW3C.

    Google on "Luxeon" for sources. One source
    http://theledguy.chainreactionweb.com/index.php?cPath=48_52_29_33
    offers LXHL-LW3C for $9.00. They're less at Future if you want
    several, but just their small-order handling fee and shipping will
    be more than $9.00.
     
  5. Jim Yanik

    Jim Yanik Guest

    (Tim Auton) wrote in

    I believe a 1W Luxeon LED has 25 lumens,a 3W ought to do better.
    They only use ~3.5VDC,so some sort of circuit to lower line V to that level
    and regulate the current is necessary.
    FYI,I've seen a 3W(advertised) LuxeonLED replacement "bulb" for Maglites
    for sale at Wal-Mart for $20 USD. There's a different model for 2,3,or 4
    cell Maglites.

    I also have a 3 LED replacement assembly for my Mini-Maglite that I like
    much better then the standard bulb.That cost less than $5 USD.at W-M.
     
  6. Michael

    Michael Guest

    Wow....thanks....great info.

    Luxeon looks like the winner and the prices are very reasonable....you can
    even order on the their site.

    Like that star configuration, looks like it may not require an external heat
    sink (although probably a good idea anyway).

    I'll have to get a variety to play with.....I may want to do 2 or 3 across
    rather than one 3 watt......I'm curious about how well the light
    diffuses......can't stand those "hot spots".
     
  7. Rich Grise

    Rich Grise Guest

    I'd just get a whole bunch of the smallest cheapest ones I could find,
    and see what they add up to in a dense matrix. ;-)

    Although, you could get a page-sized plastic Fresnel lens - just
    look up "full-page magnifier" with the quotes - it'd make a nifty
    diffuser for one very bright one, I'd think.

    Have Fun!
    Rich
     
  8. Dollar store.
     
  9. Don Foreman

    Don Foreman Guest

    That's Future -- they'll sock you with about 9 bux shipping and a $5
    handling charge for orders under $25. There are other sources with
    higher nominal prices that will net less for a couple of devices.

    I'd use four 1-watt (45 lumen) Luxeons. You don't have to run them at
    wide open throttle, and I think four of them might be quite
    acceptable for your application. They are infinitely dimmable with
    no noticable color change.
    It is a good idea anyway, but it doesn't take much. A 7x7 sheet of
    aluminum would be way plenty ample. You could even use the emitters
    (not stars) on a sink like that, but it's more trouble than it's
    worth. Then you must epoxy the emitters to the sink, and connect wires
    to rather fragile leads. Might be OK on a PC board with the emitters
    in holes in the board so the backs could be heatsunk to a plate either
    directly or with little aluminum posts. I've done this even with 3
    watt Luxeon emitters. Works fine.
    The Lambertian pattern Luxeon isn't greatly different from a bare
    lighbulb. It isn't quite as uniform as a bare bulb, but it spreads
    its radiation over most of a hemisphere with intensity about
    proportional to the cosine of the zenith angle. (That's why they call
    it Lambertian!) The "batwing" model spreads the light out even more.
    The more common 5mm white LEDs have much narrower beamwidths. They're
    very cheap in significant volume, but you'd need a lot of them to
    avoid hotspots over a 7x7 area and in small qty you get a lot more
    lumens per buck with Luxeons.

    Luxeon isn't the only show in town, they're just the easiest to get in
    small qty. Osram and others also offer similar parts. I've never
    played with any but Luxeons.

    Example: http://users.goldengate.net/~dforeman/ringlight/

    Another approach migh be to mount two Lambertian Luxeons on the sides
    of the box near the viewing surface but aimed backwards at a bright
    white backdrop -- or even a textured mirror backdrop like crinkled
    aluminum foil or peened bright aluminum, polished stainless, etc.
    You'd lose a little luminous efficacy that way, but you'd probably
    still have lumens to spare and you certainly wouldn't have any
    hotspots.

    Have fun!
     
  10. The 3W Luxeons put out significantly more heat and will require some type of
    "real" heat sinking. They may also be too difficult to diffuse depending on
    the distance between the LEDs and the object being backlit.

    I second the input to go browsing through the Candle Power forums. Lots of
    good information on the high-intensity LEDs there.


    --
     
  11. Ordering on the lumileds site actually redirects you to Future Electronics.
    So far that's where I've bought all my Luxeons and have had no complaints.

    I'm experimenting with the various Luxeons for auto lights.

    I think you're on the right track getting a variety of them to play with and
    also agree that you will do better with a couple of 1W lamps instead of a
    single 3W for use as a diffused light source.

    Some images of how well various Luxeons diffuse can be found here (yeah -
    they're mine and this week I'll build an illumination target to get better
    objective comparisons):
    http://www.daemony.com/Personal/CarAudio/Comp-Diffused/

    Distance from LED to diffuser is about 2" - less distance diffuses less,
    more distance diffuses better.

    The stars are very easy to work with but you may find they require
    additional heat sinking for long-term use depending on how well your
    enclosure dissipates heat. For my testing, I'm mounting them on strips of
    1/8" thick aluminum. The strips get warm in open air with the 1W lamps and
    get too hot to pick up with the 3W and 5W lamps.



    --
     
  12. The page-sized Fresnel won't diffuse a Luxeon - it was the first thing I
    tried. The Fresnel lenses used to diffuse Luxeons are scaled down to
    capture the rays in the range of an inch from the LED. The page-sized
    Fresnels are not a fine enough pattern to act as a diffuser - the LEDs
    retain their point source appearance.

    You can get some neat projections of the LEDs themselves on your ceiling,
    however :)

    --
     
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