How do Christmas lights work, and what's the easiest way of finding the one that is out?

Discussion in 'General Electronics Chat' started by Roger at CCCC, Dec 2, 2016.

  1. Roger at CCCC

    Thread Starter Member

    Jun 8, 2009
    How do Christmas lights work, and what's the easiest way of finding the one that is out?
    Dumb question I know:
    I bought a string of Christmas lights but the string didn't work.
    The box says: 100 lights; Total wattage: 40.8; use 2.5 volt lamps; series-connected string.
    The string is intended for 120 volt AC, and I believe that they are incandescent lights, not neon or LED.

    Believing that these bulbs are in fact incandescent bulbs, I assumed that they ought to work with DC, so I hooked up a DC voltage divider and applied 2.5 volts DC to a couple of the lights (individually) but neither one of them lighted up. I raised the voltage to 3 volts but still nothing happened. Furthermore, the voltage across one of the lights remained at whatever the supply was, indicating that this bulb wasn't conducting any current. But the voltage across the other bulb went essentially to zero, indicating a short circuit, but that bulb also still didn't light up.

    So what's going on here? Shouldn't these lights work on DC as well as AC? If not, why not?

    What do I need to do to get these 2.5 volts to light up ?

    Thanks for any info or sources of further information.
  2. MaxHeadRoom


    Jul 18, 2013
    As long as you are sure they are incandescent, then 3 or 4v should light them and it should not matter if it is DC, do you have a meter to measure resistance?
    There is a law of physics at work here, it is if you packed last years light away and they were working, guaranteed next Xmas they will not work!.:p
  3. Roger at CCCC

    Thread Starter Member

    Jun 8, 2009
    Well, duh, one of the bulbs was apparently burned out, but the other one did in fact light up when I changed the voltage divider. I think I probably wasn't giving it enough current. But then my cheap volt/ohm meter is giving anomalous results even while the bulb is lit. I measured the voltage across the lit bulb, then the current flowing the lit bulb (although not at the same time, since I only have one meter), then the resistance of the bulb (with an ohmmeter) . Measurements were:
    voltage across lit bulb: 2 volts
    current through lit bulb: 145 ma
    Resistance of (unlit) bulb, as measured with ohmmeter: 2.3 ohms.

    So I'm guessing that the resistance of the lit bulb increases from 2.3 ohms (unlit) to 13.8 ohms (lit) (=2 volts/145 ma), although I thought lamp resistance DECREASED with increasing temperature/current.

    Anyway, I DID get a bulb to light up, so now I can test the rest of the string to find any other bulbs that are out. Thanks for the encouragement!!
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    The resistance of most conductors increases with increasing temperature.
  5. wayneh


    Sep 9, 2010
    My brute force procedure for fixing these light strings is to put one lead of the voltmeter in the neutral or ground hole of a wall plate, then probe the string for AC voltage, skipping a few bulbs at a time, as you move away from the wall. Any voltage less than full 120V (whatever you see at the wall) indicates that you've gone past a bad bulb. Back up until you identify where the voltage drops, and then switch out that bulb.

    The commercial LightKeeper tool is also incredibly handy for fixing incandescent strings. It zaps the string to make the filament-bypass wiring in burnt bulbs work like it is supposed to. It can also test bulbs, scan the wire for bad bulbs, has tooling to remove bulbs from the string, and more.
    joeyd999 likes this.
  6. Bernard


    Aug 7, 2008
    I listen for bad bulbs. Using a slightly modified R-S battery powered amplifier, running the tip of
    a shielded cable down the light string gives a slightly different buzz when passing the dead bulb.
    My situation is slightly different as there are 20 strings of 13 bulbs operating on full wave rectified 24 V AC.
  7. joeyd999

    AAC Fanatic!

    Jun 6, 2011

    My wife bought one of these in spite of my objection (for the price, I was sure it was a gimmick). We fixed about a dozen strands and nets of incandesents -- all of which were many years old -- in about a single afternoon. Consider me surprised, and quite satisfied.

    Oh! And if you order now, they'll throw in a free set of Ginsu knives...
    ErnieM and wayneh like this.
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    One oft neglected item about Christmas bulbs is when under power the burned out ones actually have a lower resistance than the illuminated ones. This is due to there being two different mechanisisms for conduction.

    One is a standard filament wire. For a 50 bulb string off 120 VAC each bulb sees about 2.4 volts.

    The next comes into play once a filament burns out. Off the posts supporting the filament is a short S shaped piece of enamel coated wire. When the filament burns open the full line voltage of 120 volts appears across this open. That is more than the enamel insulation can isolate and this S wire welds itself across the terminals.

    Repair zappers work by placing an even higher voltage across the burned out filament to get it to short, usually by a piezo element to keep the zap energy low so you don't put your eye out.

    Thus one bulb is dead but shorts itself to keep the rest on. A quite ingenious concept if I do say so.

    Further dead bulbs short in turn but provide a higher and higher voltage across the remaining bulbs, so they burn out faster and faster, eventually leading to a zipper effect where the rest of the string just shorts out, unless one build fails the fuse open. Thus you may want to keep on of the dead bulbs and replace them ASAP.