Why do Christmas lights all go out when one bulb blows? (and how to find the broken one)

The answer is rather simple: traditional Christmas lights (I'm ignoring newfangled LED varieties) were typically connected directly to the mains power supply and wired in series like this:

Only if the filaments of all the bulbs are intact will a current flow around the circuit; if one bulb breaks then the circuit is broken and all the lights go out. The reason the bulbs are wired in this, inconvenient, manner is that it's convenient for the manufacturer.

Although the supply voltage is 230v (or 110v) the bulbs are rated for a much lower voltage. At home I have a string of 20 lights like this with 12v bulbs. This works because of the rules of series circuits. In my home lights there are 20 bulbs each with some unknown resistance R. The total resistance of the circuit is 20R and the entire circuit is a sort of voltage divider.

The current flowing through the entire circuit is I = 230/20R and the voltage across any individual lamp is V= R * 230/20R or 230/20. So my 20 bulbs are each getting 11.5v. That's handy for the manufacturer because they can use cheap, small bulbs that use a low voltage.

BTW Some bulbs have a second piece of wire called a shunt that passes current when the filament breaks. With a shunt the manufacturer can still use series wiring and cheap bulbs, but a blown bulb doesn't stop all the lights from working.

Finding the broken bulb

A really fast way to find which bulb is broken is to perform a binary chop. To do that you need a multimeter (or similar meter to test continuity).

0. Unplug the string of lights from the power.

1. Remove the first and last bulbs and check that they are ok.

2. Remove the bulb in the centre of the string of lights. Using the multimeter check to see if there's an electrical connection between the contacts in the centre bulb socket and each of the end bulb sockets that you remove the bulbs from (you can actually look at the wiring to see which way the wires go and which contact that corresponds to).

3. Pick the half where there's no connection. The broken bulb is there. Remove the bulb that's in the middle of that half of the string and check it. If it's ok proceed to checking the electrical connection between the socket of the bulb you just removed the two nearest bulbs you removed (which will be the middle of the string and one end).

4. Proceed like that following where there's no electrical connection and dividing in half until you find the broken bulb.

This is a technique from computer science and you will find the broken bulb much faster (on average) than if you proceed checking each bulb in turn.

Chuck Masters said…
Just use the Light Keeper Pro to fix the blown shunt. Takes a few seconds!
Unknown said…
Well, that method, and I have tried it, but it will drive you nuts. Works, maybe if you have the string on the floor, and still you can get no holiday joy. Google, light keeper pro, this thing has a crystal in it, pull the trigger and a huge spark, i.e. voltage shoots down the wire, cleaning, clearing, the shunt, these shunts for the most part, don't work. Every year working lights, when put up working, fail to work the next year, out of 150 bulbs I average 30 to 40 bad one that this device clears and the string lights up. Very nice if the string is on the tree. No, it will not find a bulb that is not connected correctly, bad wiring, etc.
Unknown said…
Another method that requires a volt stick is just to put the volt stick at each cable going in to a bulb and check where it stops indicating the presence of electricity. Takes a mere fraction of time and you don't have to unscrew the bulbs. But you have to get a volt stick :-) (But its a handy tool for more things)

http://www.sagabelectronic.se/index.php?page_id=238&language=en
Unknown said…
Some would call the binary chop "Successive Approximation"
Anonymous said…
I find my bulbs the same way Paddy2005 does. You can use a "volt stick" to find your bad bulbs, bad wires, and bad connections. These "volt sticks" are usually called non-contact voltage detectors, and you can find them in most hardware stores in the electrical/electronic section.

Using them you don't have to unplug any bulbs. The non-contact voltage detectors are small and have pinpoint tips, that let you find bad bulbs in a jiffy. You can usually find bad bulbs in 30 seconds or so. . . once you get the hang of it.

Here's an article explaining how it works:

Find Bad Christmas Lights
Unknown said…
I had this same problem happen to me last Christmas. I had a couple hundred lights set up on my house, and when one went out the rest would go to. This go really annoying to find the one that was faulty. Any tips on easy ways to find which one it might be? http://www.ajpalmer.com.au

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