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B Christmas lights: Why in series?

  1. Dec 11, 2016 #1

    I helped my neighbour putting up (quite old, no LEDs) strings of Christmas lights and noted that some of them (different brands) are connected in parallel, others in series.

    Inevitably, we found several of the serial strings not working due to defective bulbs. We replaced the visibly broken ones and thus were able to fix some of the strings, but my neighbour trashed most of the non-working ones with 50+ bulbs, heavily swearing and promising to replace them with LED next year.

    I wonder why light strings in series even exist. Is there some essential advantage to this kind of wiring that makes up for the fact that one broken bulb interrupts the total current?
    I also found that apparently there exist newer products with bulbs in series that still work if some of them break. How does this work?
  2. jcsd
  3. Dec 11, 2016 #2

    Jonathan Scott

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    Gold Member

    For lower power incandescent bulbs, low voltage is more practical and cheaper to manufacture. Connecting an appropriate number of bulbs in series allows the string to be operated directly from mains voltage without the need for a transformer, but of course that means that a single failed bulb prevents the whole string from working.

    More modern products may use an antifuse shunt within the bulb which will short out the bulb if the voltage across it becomes abnormally large because the filament has burned out. This will of course increase the voltage to the other bulbs, and requires a fuse bulb or fuse somewhere in the circuit to ensure that the current cannot get dangerously large when multiple bulbs have failed.
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