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Christmas Tree Fairy Lights Confusion.

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Season's Greetings all,


I have several sets of fairy lights by my front door. There are either 20 or 40 light sets with traditional filaments, not L.E.D.


 I only have 20 or 40 light sets and spare lamps for them.


One or two lamps had failed at the front door. There are several sets all tangled up and left there from last year. I can not be bothered to count the number of lamp holders in the sets with blown lamps.


When I put in new lamps they failed immediately, but maintained the set's other lamps due to the internal shorting wires in the new lamp.


If I put in a new 12 Volt lamp  all should have been good. (20 light set with 12 Volt lamps).


I took some 12 Volt lamps from a new set for the spare lamps to replace the blown ones at the front door.


If the new 12 Volt lamps were accidently put into a 40 light set they would just glow dimly as they would be supplied at 6 Volts.


So, why did the new 12 Volt lamps blow instantly?


Z.


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  • 0
    The 40 lamp set has to have a higher constant current to get the same brightness, so the 6V lamps have a lower filament resistance.

    The other 39 lamps at 6V each r whatever in series with the mains are more or less a constant current source, certainly not a source of a constant voltage.



    Think of a simpler example, put a single 100W lamp, and a single 40W lamp in series across the mains - which is brighter ? -

    The lower wattage one - the same current flows though each, but the higher resistance one gets most of the volts and so gets hotter.

    I used to use that very example as a test question in candidate interviews actually.

    Put on the spot quite a few folk choose the 100W one forgetting that the series world and the parallel world are complimentary.


    So, put the higher resistance 12V lamp in a series chain where a 6V one was expected and it will get more current than it is designed for (perhaps almost twice). Given they run so near the knuckle anyway I'd expect near instant failure, which it sounds like you have..


    regards Mike.
  • 0
    I thought that we were back to one of Zoom's weekend quizzes. ?


    Then I had to see for myself.

    100 W lamp 240 V => I = 0.42 A => R= 576 Ohms

    40 W lamp 240 V => I = 0.167 A => R = 1440 Ohms



    Total R = 2016 Ohms

    240 V across both => I = 0.119 A



    Power = I²R

    100 W lamp => 8.16 W

    40 W lamp => 20.4 W


    So yes, the lower wattage lamp glows brighter. Interesting!


    ETA: Now for the fairy lights.


    Let's assume a current of 1 A in the 12 V ones

    Power of each lamp = 12 W and R = 12 Ohms

    To have the same power in the 6 V ones, I = 2 A and R = 3 Ohms

    39 x 6 V lamps + 1 x 12 V lamp = 129 Ohms => I = 1.86 A and there she blows! ?
  • 0
    Chris Pearson:

    I thought that we were back to one of Zoom's weekend quizzes. ?

    I was thinking that was what the 12 volt lamp was thinking and decided to fail at an inopportune moment......... ?


    My wife has some old Christmas sets and I struggled last year trying to find a suitable lamp to connect in series. It was a sort of a lamp inside a glitter bulb, ended up piecing together the outer bulb with super-glue.

    Clive

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