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LED Christmas Lights

Discussion in 'Electronic Basics' started by Nikki, Nov 17, 2005.

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  1. Nikki

    Nikki Guest

    Hi Guys
    I have a very limited knowledge of electronics but understand the basics

    I have a question about those new LED Christmas lights. I know that an
    LED does not take very much current or voltage 2-3 volts. How is it
    that they can plug those things into 110v. The reason I ask is my
    Daughter wants to dress up as a Christmas tree for a party. Is there
    any way to rewire a string of say 25 led's and run them off a battery
    pack from a drill. My husband has several of them at 9 volt or 12 volt
    or a 18 volt
  2. Impmon

    Impmon Guest

    Those LEDs are in series. A string of 70 LEDs would require close to
    150v. When you figure that 110v AC is merely a measurement of RMS and
    not peak, adjust the value for peak volt to about 155v peak, it is
    just enough to drive all the LEDs without burning them out or
    requiring resistor.

    Do keep in mind that all LEDS absolutely must have the same facing
    polarity. All LED must connect anode to cathode of next LED, anode to
    cathode of next LED, etc. If one LED is in reverse, none of them will
    light up.

    If you use smaller LED, it has lower voltage and you will need more to
    light them. If you use big LEDs, they may have higher voltage
    requirement. Check the specs before comitting to buy x LEDs.

    LEDs are more than just lower electric bills, they are also safer.
    Bulbs don't shatter like regular bulb so no exposed wiring or sharp
    glass. Also little heat so little fire hazard as well. Finally with
    proper care, those LEDs would outlast the wires they are mounted on!!!

    As for the battery pack, a 9 v will probably work with 4 or 5 in
    series. 12v can do 5-6, and 18 should be able to handle about 8 or 9.
  3. Nikki

    Nikki Guest

    What if I rewired all the LED's on that string to parallel
    Would that work
  4. Impmon

    Impmon Guest

    Well, yeah but only if you have a 2v source.
  5. Dave

    Dave Guest

    I think she meant wire groups of 8 or 9 LEDs in parallel.
  6. Bill Bowden

    Bill Bowden Guest

    There is an LED resistor calculator at

    There are several examples of connecting a few LEDs to various
    batteries. For example, you can hook 4 red LEDs in series and add a 270
    ohm resistor to connect to a 12 volt battery. Connect another 4 leds
    plus resistor for a total of 8, etc.

  7. Rich Grise

    Rich Grise Guest

    Only if each has its own series dropping resistor, which shouldn't
    be too hard. But, as has been mentioned, you can put strings of 2
    or 3 or 4 (or whatever, depending on the voltage) LEDs in series
    with one dropping resistor for the set.

    LEDs don't cooperate well in parallel - one will hog the current,
    and you'll get a cascade failure.

    Good Luck!
  8. Nikki

    Nikki Guest

    Thanks guys
    I tried the leds in parallel about 25 of them and only a couple of them
    came on. I ended up using the mini lights with a 4 volt 6.5 a/h that
    should last a little while but not as long if it were the led's
    Thanks for your help
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