HOWTO: Fix Broken Christmas Lights Quickly

Now the reason that one blown, missing or otherwise broken bulb can take out the whole string of lights is down to the way they are wired. Christmas lights are often wired in series - so they complete a circuit. Electricity needs a source, called "Live" when talking mains, and a destination, called "Neutral" in mains. In AC positive and negative are not used, as the actual potential of live alternates during usage.

If you look at the lights carefully, you will see that one line goes straight through to the last bulb, and although it is intertwined with the cable going in and out of each bulb, it does not. The other line connects into each bulb, and comes out the other end. So if a bulb is missing, the electricity cannot complete the circuit. This is why they all do not light.

Remove the first bulb in the chain and test it. See HOWTO probe the bulbs for details. If it is okay, then it should show a closed circuit with the tester.

Remove the last bulb also - and test this. If it was either of those two - you are on a winner.

Looking at the first socket, one of the two contact cables will go to the plug, and the other to the next socket. You should put the first probe at the contact for the cable going to the next socket.

At the socket for last bulb, there should be two cables present - one going to the previous bulb socket, and one going back to the mains plug (which bypassed the other bulb sockets). Place the second probe on the contact that goes to the previous bulb. You will likely see no continuity here - so the dead bulb is clearly between the first and last.

Working inward one by one would be really time consuming, so here I introduce the divide and conquer method. Find the bulb roughly in the middle of the set, and remove it from the socket. Test this bulb to make sure it is not the one. If this bulb is okay, you may have the one, otherwise continue.

Laying out the bulbs as mentioned above, you should be able to tell which contact for this socket goes towards the first socket, and which goes towards the last. Probing towards the first bulb socket, and the first bulb socket towards this bulb, you are now able to tell if the problem is in the first half. You can then switch to checking the contact that goes towards the last socket and the last socket itself, which tells you if there is a problem with the last half of the bulbs.

As the diagram shows, each bulb has two contacts. Connect the probes to each contact as shown above. If the bulb is okay, then continuity will be shown.

Note - this only works on incandescent lamps. LED lamps must be probed only in the right polarity.