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Using LEDs in a rectifier bridge in a suspended light?

Discussion in 'Electronic Basics' started by Adam Funk, Nov 8, 2013.

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  1. Adam Funk

    Adam Funk Guest

    A kid's ceiling night-light, which shines planets, stars, &c., over
    the walls & ceilings, has failed (after several years & changes of
    tiny 12 V halogen lamps). I've been asked to see if it can be fixed.
    The lamp-holder inside the globe has corroded & is no longer usable.
    The whole thing consists of the following:

    a wall-wart with a British plug & AC output, labelled

    PRI 220-240V~ 50 Hz 0.1A
    SEC 11.5V~ 10-20 W

    a plug that fits into the wall-wart, cabled through an in-line
    switch, to the lamp-holder, which clicks & twists into the globe

    the globe, which is just a plastic sphere (more or less) with
    colored disks to shine various shapes on the wall & ceiling

    My first thought was to put a rectifier bridge on the SELV output &
    then a few white LEDs with series resistors, but then I wondered ---
    why not use either

    1. a suitable resistor in series with a rectifier bridge arrangement
    of LEDs,


    2. a rectifier bridge arrangement of series resistors & LEDs


  2. Tom Biasi

    Tom Biasi Guest

    You can get this done but you may also be able to buy an LED replacement
    for your bulb.
  3. A LED is not a good rectifier, therefore you will always find
    diodes in series to the LEDs.

  4. Jasen Betts

    Jasen Betts Guest

    Or leds facing both ways,

    the reason not to is it'll flicker at 50Hz if you do that, but at
    100Hz (which is much less noticeable) if you use a real bridge
    recitfier and keep all the LEDs pointing the same direction.
  5. Phil Allison

    Phil Allison Guest

    "Helmut Wabnig"

    ** Are you for real ???

    Rhetorical question alert.


    LEDs are run from AC supplies all the time, with no additional diodes.

    White LEDs normally have reverse breakdowns that exceed 20V so there is no
    issue with 12VAC even when using just one.

    .... Phil
  6. Phil Allison

    Phil Allison Guest

    "Jasen Bleatts"

    ** Not true.

    There is normally no visible flicker from a LED operating with 50Hz pulses.

    Try it and see.

    In the OP's night light app, it will be a complete non-issue.

    But getting an array of LEDs to act like a point source ( for projecting
    stars etc) may be one.

    .... Phil
  7. Phil Allison

    Phil Allison Guest

    "Tim Wescott"

    ** You could have expressed that one better ...

    .... Phil
  8. Adam Funk

    Adam Funk Guest

    To clarify the "corroded" bit above: I'd only just replaced the lamp
    again when it started going off intermittently. When I took it apart
    & examined it closely, I found that the stranded copper wire connected
    to the "squeezers" that the lamp legs get pushed into was falling
    apart (with greenish bits), so I think I need to strip back to clean
    wire & work from there.

    I came up with this crazy idea as a more permanent solution --- if
    used well (!), LEDs should last nearly forever. But maybe I could
    splice a new lampholder in.

    A recent study conducted by Harvard University found that the average
    American walks about 900 miles a year. Another study by the AMA found
    that Americans drink, on average, 22 gallons of alcohol a year. This
    means, on average, Americans get about 41 miles to the gallon.
  9. Tom Biasi

    Tom Biasi Guest

    Well Adam you need to fix it. Maybe LEDs will do what you need and
    maybe, as was mentioned, they won't give the same light characteristics.
    Replace the socket and put in a new bulb, how long will the kid need
    this thing? Play with the LEDs if that is fun to you. I might:)
  10. default

    default Guest

    I think the part where you say it "projects" the planets etc. makes
    the led idea a non-starter. Assuming they just have some holes in the
    sphere to "project" light from, the light must be a point source to
    work. Using a large light emitter or several would make the
    projection fuzzy at best, or in the case of focused leds, multiple
    iterations of the same planet.

    Halogen lamps in a plastic globe sounds like a fire hazard to me.
  11. Adam Funk

    Adam Funk Guest

    Thanks for the interesting replies. I'll play with the LED rectifier
    bridge idea sometime soon just for fun.

    What I hadn't realized when I thought of it was the simple solution:
    strip back to clean wires & splice in another G4 lampholder (leaving
    intact the plastic fitting that locks into the top of the sphere). It
    hadn't occurred to me at first that I'd be able to find the part, but
    it exists & is not expensive.
  12. Adam Funk

    Adam Funk Guest

    Well, I understood his point!
  13. Adam Funk

    Adam Funk Guest

    I'm going to splice in a new G4 lampholder instead.

    It's just a G4 in the middle of an empty globe about 15 cm across,
    with some vent holes in the top.
  14. Phil Allison

    Phil Allison Guest

    "Adam Funk"

    ** He did not make one - smartarse.

    Clearly, a LED is a smaller light source than the 12v lamp filament.

    .... Phil
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