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trying to adapt warm white LED to candles

Discussion in 'Electronic Repair' started by bubbas, Sep 16, 2012.

  1. bubbas

    bubbas Guest

    I recently purchased the light string shown here:

    http://www.amazon.com/Transparent-White-Clear-Christmas-Lights/dp/B0069VI8ZS/ref=pd_sim_sbs_hg_5

    The hope was to replace existing 120 V lamps in window candles with an LED
    light from the string, but I am having some difficulty. As I suspected when
    I cut off one of the lights from the string and, still fused, plugged it
    into the 120 v outlet, the fuses blew. So, it looks like a voltage drop is
    needed. The total voltage/ current of the entire 25 bulb string, according
    to the link above, is 0.027 amps (3.24 watts) @ 120 V. A tag on the string
    says that each bulb is 3.1V @ 0.062 amps. So, which approach is best:

    1) Use a dropping resistor for each bulb I want to use? What value and
    wattage if so? Would the resistor stay cool enough to hide it in the
    candle?

    2) Use an AC plug in the wall type transformer (rated for 3.1 V AC), which
    I assume would be quite hard to find because most of them are DC (I tried
    placing 3 V DC across the LED lamp and it didn't work)?

    Appreciate any help here. The candles I placed in the windows recently all
    use what I thought were cool, smaller standard 120 V incandescent bulbs but
    there was enough heating to discolor the window blinds they are up against.
    The wife liked the "warm white" of the LED string, so I wanted to replace
    the incandescents with those and I also want to use a much thinner power
    cord to the candles if possible, something I couldn't really get away with
    using the higher powered incandescent.

    Thanks,
    Bud
     
  2. N_Cook

    N_Cook Guest



    If you like the flicker effect of candles try mixing some orange neons ,
    perhaps overdriven, fed from DC rather than AC.

    Reminds me of the fad for mock leaded glass windows - strips of lead laid
    over plate glass. Missing the point entirely , that the visual effect of
    leaded glass windows is not the lead itself but the multi-facit reflections
    off the individual small panes.
     
  3. bubbas

    bubbas Guest

    These original candles don't flicker (they have a standard incandescent) and
    the hopeful LED replacements won't work from DC, which is why I am asking
    the best way to power them.
     
  4. First, & very importantly, LEDs are INHERENTLY DC devices. You have a
    string that uses 120v AC because there is a rectifier built into the
    string, or each bulb has 2 LED's, in opposite orientation.

    So, you need to determine which it is. Here's how: get a DC supply of
    more than 3.1v. You can use 3 AA or AAA batteries, end to end, or a 6v
    DC wall wart. Connect a 100 ohm resistor in series with the supply.
    This limits the current to 30 ma, so as to not burn out the LED.

    Then connect the supply to the LED, both ways (both polarities). If the
    LED lights with both connections, there are 2 LEDs in your bulbs and
    you'll need an AC supply. If it only lights on one polarity, there is
    just 1 LED and you'll need a DC supply.

    Report back with your findings and we'll go from there.

    Bob
     
  5. bubbas

    bubbas Guest

    Ah, the walwart I initially tried wasn't plugged in completely so that's why
    the LED's wouldn't light. My findings are that each bulb has 1 LED as it
    only lights the one way.

    Looks like I should just be able to use a cheap walwart to power these if
    I'm not mistaken. Should I include the 100 ohm resistor to each LED?

    Thanks,
    Bud
     
  6. hr(bob)

    hr(bob) Guest

    Do you want just one bulb in each candle? Are these individual
    candles, or are they the "8 candles in a row" of 4 ascending and 4
    descending height?
     
  7. bubbas

    bubbas Guest

    "Do you want just one bulb in each candle? Are these individual
    candles, or are they the "8 candles in a row" of 4 ascending and 4
    descending height?"

    Thanks, one bulb per candle. I'll have two candles per room and I hope to
    use a separate power source for each candle pair as there are 3 rooms.
     
  8. OK, here's the next thing about LEDs: they're current devices. I.e.,
    you design your circuit for the current through them, rather than the
    voltage across them. And LED brightness is a matter of how much current
    you put through them. For your lights, I would use the .027A (27 ma).
    The .067 is bogus: how can a bulb draw more than is going through the
    whole string? Although the .067 might be a maximum current.

    So, assuming a 6v DC supply and 3.1v dropped across the LED, there has
    to be 2.9v dropped across the resistor. With .027A needed, a 2.9/.027 =
    107 ohms. 100 ohms is a common value - use that. .027A & 2.9v gives
    ..08 watts dissipated by the resistor - it will not heat up the candle
    stick. And you can use the smallest resistor (1/8W).

    Each LED will need a resistor.

    BTW - it you want to use a separate wallwart for each, you can get them
    for $2.59 each (shipped) here:
    http://www.ebay.com/itm/170898516293

    If you want to carry this further, you can experiment with the
    brightness by changing the 100 ohm resistor. Smaller resistor equals
    brighter LED. But shorter life. Too small means zero life. If the
    ..067A LED rating is a maximum current, you couldn't use a resistor
    smaller than 2.9/.067 = 43 ohms. And at that, the life would probably
    be very short. IME

    Have fun,
    Bob
     
  9. mike

    mike Guest

    There are a bunch of issues here.

    Have you verified that the optical properties are acceptable?
    Light comes out of a LED in one direction. It's non-trivial to
    adapt an incandescent fixture to LED. You might need to do some
    grinding on the LED plastic or use more than one led to get the look
    you want. Those lamps look like the right stuff, but can't be sure
    from the picture. Try it.

    This project appears to be all about aesthetics.
    Make sure you like the look
    before you invest in a bunch of power supplies.
    Even number of LEDS means you can use AC.

    Your power choices depend more on what power supplies you can get.
    Without going into all the gory details...
    Go to a couple of garage sales and pick up some 25-cent cellphone chargers.
    Use a 5V charger and a resistor at least 68 ohms 1/2W. Larger resistor
    for lower brightness is no problem. For the nitpickers reading this,
    you're probably gonna put the resistor in heat-shrink without heat
    sinking inside some plastic thingie, so a higher wattage resistor is
    warranted. At 100 ohms, 1/4W should be big enough.
    When you're done with the prototype, run it a while
    and make sure nothing gets hot.

    READ THE LABEL. Not all cellphone chargers are 5V. Some are 5.4V and
    need larger resistors.
    Check the current rating on the label. One charger should easily
    supply two LED-Resistor combinations...or more...check the label.

    You mentioned smaller wire. The size of the wire is relatively
    unimportant.
    BUT
    You want something that won't be damaged and shorted by flexing,
    slamming the window on it, cat chewing on it.
    If it shorts and your wall-wart is plugged in behind the curtain...
    have your fire insurance paid up. Chances of a short AND just
    the right combination of PS factors to cause overheating are very
    small, but the consequences are catastrophic. Save your
    good luck for something else.
     
  10. bubbas

    bubbas Guest

    Sounds like a good plan, thanks, except I'm a little concerned about one
    thing: actual wallwart voltage. Very often, they're marked as 6V, but
    output will be a volt or two higher. In that case, I'm assuming I'd just
    need to increase the resistance slightly to each bulb, correct? If 7V, then
    3.9v dropped across the resistor so 3.9/.027= 144 ohms (I would use the
    closest standard resistor, probably 150 ohms). So power would be a little
    more, but I could still use a 1/4 watt resistor. Please correct me if I'm
    wrong. Been years since I've done much with electronics calculations, but
    they're coming back to memory somewhat.

    Bud
     
  11. That's all correct.

    Bob
     
  12. bubbas

    bubbas Guest

    Working out good so far. I've had whatever walwarts I've had in the junk
    box and have had to readjust the resistance accordingly. You'd be surprised
    how much more some of the actual voltages are than are stamped on the units.
    One I have said "12 volts", but in actuality, it was 19.5V! Makes me wonder
    how much electronics in general burn out because of mislabeled adapters.

    Bud
     
  13. Some over-voltage is to be expected: it's the compensation for the
    supply bogging down under load. But 19.5 vs 12 is a bit much. If
    you're curious, you could load it to its rated current & see what
    happens to the voltage.

    If you are going to be using 12v supplies, you might need higher wattage
    resistors. E.g., (12 - 3.1) * .026 is just a 1/4 watt, so using 1/2
    might be a good idea. Of course, if that "12v" supply is really 19.5,
    you'd need an even larger one.

    Bob
     
  14. bubbas

    bubbas Guest

    Yes, I used resistors up to 1 watt today. I aimed to use double the wattage
    required as a safety margin. All the candles are now in the windows and are
    lit nicely. My only concern now is LED life. I understand it varies
    greatly according to manufacturer. I had really hoped I wouldn't have to
    touch these for years. Perhaps so, but I'm not sure.

    Bud
     
  15. gregz

    gregz Guest

    It's 12 volts at specified amps.

    Greg
     
  16. bubbas

    bubbas Guest

    Oh, ok, well that makes sense now. Didn't think about it that way. Thanks.
    In other words, the label is correct when the unit is, say, plugged into
    whatever device it was made for, and the device drawing the same current as
    on the label. Bud
     
  17. I'm sure that life does vary with mfg'r. It also varies greatly with
    the current used to drive them. But that affects the brightness, too.
    You could try this: use larger & larger resistors until the bulbs are
    too dim. A pot would be the easiest. It might surprise you as to how
    low you can go & still get the desired affect.

    Bob
     
  18. bubbas

    bubbas Guest

    Today, I went back and measured the current for all the lights. All were
    from about 20-27 mA. I then increased resistance for each one until current
    was from about 13-16 mA. There was slight dimming, but I think still
    acceptable. I'll find out for sure after dark.

    It has been fun modifying the original incandescent candles into these.
    I've also had a chance to revisit electric circuit study, something I
    haven't done for nearly 15 years.

    Bud
     
  19. That's really good. If the dimming was only slight and it's acceptable,
    your bulb life will be greatly extended. How much, exactly, I couldn't say.

    Bob
     
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