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Cheap multi-color LED Christmas light strings.

Discussion in 'Electronic Design' started by Winfield, Dec 13, 2007.

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  1. Winfield

    Winfield Guest

    My wife bought some multi-color LED Christmas-light strings,
    50 lights for $10 per string. We used them to decorate our
    traditional wreath over the door, but I was disappointed with
    their brightness. Checking them on the bench, I found that
    having no bridge rectifier, they operated on only one half of
    the AC line cycle, and with 50 LEDs in series, they needed
    125 to 135 volts to draw a even modest 10mA current.**
    Their conduction angle was also less than one might like.

    BTW, half-cycle operation makes a 50 or 60Hz flicker, not
    a 25 or 30Hz flicker as many web-sites state.

    Clearly, adding a bridge rectifier would double both the LED's
    flicker rate, and light output. But I elected to add a storage
    capacitor after the bridge (200uF, for 2 to 3 strings), and feed
    the LED string with DC. This would make the conduction
    angle 100%, and further increase the light output.

    My in-situ measurements: 162 volts and 42mA per string,
    about 5x the original estimated current. Yep, they looked
    about five times brighter. At least that's what I told my wife,
    after she said they didn't look _that_ much brighter.

    ** The string has 20 yellow LEDs, and 10 each red, green
    and blue. I guess the band-gap voltages add up to ~ 110V.
  2. 42mA is a lot of average current for a small LED, especially the blue
    and green ones.

    Best regards,
    Spehro Pefhany
  3. T

    T Guest

    I hooked up 10 high brightness LED's to my power supply. They're all run
    through 1K resistors and it's drawing 30ma at 5VDC, so if if you divide
    it out by the 10 LED's it would mean approximatley 3ma per LED.
  4. Jim Yanik

    Jim Yanik Guest

    42 ma seems a bit high for the LEDs,doesn't it?
  5. Guest

    Where did you get the lights? Price sounds good.

    You might be buying a few more strings if the lights burn out from the
    higher current. Do a burn-in test yet?

  6. D from BC

    D from BC Guest

    Maybe for indoors, but chilled LED's (outside in the cold) can
    probably last longer with higher current.

    D from BC
  7. Fine, now suppose you were to put them in series at 42mA as in the
    above situation, what would you calculate the current per LED to be?

    Best regards,
    Spehro Pefhany
  8. Jim Yanik

    Jim Yanik Guest

    most common,small LEDs max at around 25ma,would cold allow almost double
    that current? It doesn't seem likely.
    But a pulsed LED would allow a greater max current,as the duty cycle would
    be less than for 'pure' DC.
    less than 50% for half-sine.

    I guess it all depends on what LED chip they are using.
  9. Given a fixed maximum power dissipation, the average current has to be
    *less* for pulsed current because there is more voltage drop at higher
    And on the packaging.

    Best regards,
    Spehro Pefhany
  10. "The light that burns twice as bright burns half as long ..." -- Eldon
  11. could you use something like this?

  12. WJLServo

    WJLServo Guest

    My wife sweet talked me into making her an LED XMass string, some
    years back. She had tagged along with me on a visit to the old Eli
    Heffron / Solid States Sales surplus shop in Cambridge, MA, US, and,
    while I was looking at used test equipment, bought herself a couple
    bags of red and green LEDs. She raided my stash of blues when we got
    home (Eli's didn't stock them) and demanded that I make them into
    XMass lights for her.

    So, got a 5 volt wall-wart and a bag of 150 ohm resistors, one ballast
    resistor per LED, and soldered it all together. She wrapped the string
    around a small evergreen tree that she's had growing indoors,
    "bonsai" style, for a good many years, and lit it up for the holidays.
    Looked pretty, but, took up way too much time soldering....

    W Letendre

  13. This trick dates from at least the 1960's, when you could drmatically
    improve an electric shaver's performance with a diode and a capacitor.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  14. The little incandescents (~1mm diameter) are more attractive than most
    current LEDs on a mini tree.

    Best regards,
    Spehro Pefhany
  15. 42 mA would usually appear less than 4.2 times as bright as 10 mA

    a) Nonlinearity in human vision. Keep in mind that a 60 watt
    incandescent lightbulb looks 60% as bright as a 100 watt one, while being
    only 50% as bright.

    b) The LEDs would run hotter and their efficiency would suffer

    c) Efficiency of the LEDs vary with instantaneous current. I expect the
    reds and yellows to mostly operate fairly linearly, possibly be
    extremely slightly improved except for their higher temperature because
    their efficiency is less at less than a few mA, and without the smoothing
    capacitor a small percentage of the energy going into the LEDs goes in
    during low current operation that impairs the reds and greens.
    However, I expect this to be outweighed slightly by higher temperature.

    Same story for the green ones if they are the dimmer very yellowish
    green. If they are brighter non-yellowish green, then they will behave
    like most blue ones - see below:

    Most blue LEDs, especially turquoise blue ones and very deep blue ones,
    have a different nonlinearity where efficiency is maximized at a few mA
    (for the lower power size ones, as opposed to ones rated 150 mA or more).
    With a smoothing capacitor, they will be receiving current near the
    maximum instantaneous current received without a smoothing capacitor, so
    their efficiency will probably be reduced measurably (though maybe as
    little as a few percent) by use of that capacitor.

    Of these factors A, B and C, I would give A the most weight.

    - Don Klipstein ()
  16. In general, modern high brightness low power LEDs, when given 25 mA
    average current, give maximum efficiency with steady DC as opposed to
    pulsing. This is especially true with the InGaN ones, and really
    especially with blue and green InGaN ones.
    - Don Klipstein ()
  17. That's true, but these LEDs really do look much brighter
    in-situ, I suppose because the LED light output is being
    compared with the surrounding reflected light from street
    lights, porch lights, the moon, city sky light, auto
    lights, etc., where the absolute light level certainly
    does count, as opposed to being compared with no light.
    Hasn't been to be too bad at 40mA. I'll take measurements
    and report back.

    BTW, many years ago I made a very intense light source from
    hundreds of "ordinary" red LEDs (with the exact wavelength
    we needed to process bags of blood), and ran them at 150mA.
    The light output vs current began to drop above 80 to 100mA,
    but it was still effective to run them at 150mA for maximum
    light. I soldered the LEDs to a copper bar and used a huge
    fan to remove heat as we could manage. It worked quite well,
    lots of light, far more than you could stand to look at (and
    we got very rapid blood processing, minutes instead of hours).
    We called it the "LED Blaster" and my technician affixed a
    sign, like the laser warning signs, saying DANGER LED LIGHT.

    It worked well, except every now and then one of the LEDs
    would explode. No warning or dropping light level, etc., it
    would just explode into many small bits of plastic. (The
    guys at the Red Cross, who were performing the blood tests,
    were not amused.)
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