How to fix Christmas tree lights in series?

December 9, 2008 8:01 AM

I have a Christmas tree with lots of lights on it. The lights are wired in series and are currently not working. So somewhere on this set of lights one or more lights are not functioning. It is a major pain, swapping bulbs for known good ones or peering into each and every bulb with a torch to see if you can spot whether the filiment is still there. What is the easiest way to find the non functioning bulbs and get the lights working again?
posted by mccartrey to technology (12 comments total)

Ta-Da Christmas Light Tester!
posted by Sassyfras at 8:11 AM on December 9, 2008

You can narrow it down to the defective strand by getting an extension cord and testing each strand. Start at the top/last strand and work your way down the chain. When you get to the point where it doesn't work anymore, you've located the defective strand of lights.

Also, most newer lights will still work even if the bulb burns out, but not if the bulb has been removed from the strand of lights. Sometimes it's just a loose bulb. So when you find the "dead" strand, make sure each bulb is pushed firmly into the bulb socket. If this still doesn't work, you can try replacing bulbs/fuses, or do what I do and replace the whole strand.
posted by AlisonM at 8:19 AM on December 9, 2008

Are these the little micro lights with the friction-fit sockets, or the larger bulbs that screw in?

Each friction-fit bulb has an internal short, only a portion of the current flows through the fillament, so that the strand will stay lit even when bulbs burn out, but not when a bulb is removed or poorly seated. Newer versions of the microlights add spring contacts in the sockets that come together when a bulb is removed so the strand keeps operating whether a bulb has burned out or removed. They can still fail if a functioning bulb is in place, but not properly in contact.

However, you say you've wired these all in series? How many strands are in series? The mains plugs probably has a couple of small fuses embedded in it. If you plug in too many strands in series, the fuses in the first plug might blow, which would leave them all unlit. That's where I'd look first.
posted by Good Brain at 8:35 AM on December 9, 2008

Have you ruled out replacing the current strands with either parallel wired strands or LEDs?
posted by electroboy at 8:44 AM on December 9, 2008

I had the problem Good Brain mentions a few years ago with outside lights. I tested all the strands individually with success, then mounted them on the house in series and plugged them in from one end. The fuse in the first plug kept blowing, so I ended up having to run two extension cords to reduce the number of strands in series.

LEDs are much lower-power, and you can run thousands in series with no problems.

There's not really a quicker method than testing the strands individually, and then once you find the bad strand, checking the bulbs.
posted by chazlarson at 9:09 AM on December 9, 2008

This is a great resource.
posted by masher at 9:35 AM on December 9, 2008

To add to the good advice already given, while you can daisy-chain many light strings, you shouldn't chain together more than about 350 (?). You can actually burn out a socket doing that.
posted by adamrice at 10:01 AM on December 9, 2008

The light tester looks like a waste of time and money. You still have to remove each bulb to test it. Might as well save 10 bucks and try plugging in a new bulb.
posted by pointilist at 10:25 AM on December 9, 2008

How can the bulbs light up and have a built-in short-circuit at the same time?
posted by cockeyed at 11:21 AM on December 9, 2008

Both Xmas lights and artifical Xmas trees have high lead in the plastic. Suggest to wash your hands after extensive tinkering. Don't handle food with dirty hands.
posted by ohshenandoah at 1:33 PM on December 9, 2008

How can the bulbs light up and have a built-in short-circuit at the same time?
Because it's not a superconductor. Its a few loops of wire at the base of the leads inside the bulb. Some current goes through the looped wire, some goes through the filament.

That's an oversimplification though. Upon further reading, the looped wire (called a shunt) is coated to create a higher resistance between it and the leads. If the filament fails, more current flows through the shut, heating it, which burns off the coating, lowering the resistance and passing more current.
posted by Good Brain at 12:25 AM on December 10, 2008

After all the wonderful comments I've looked through, I'm thinking of getting a proximity type bulb tester. This should light up an LED on the tool when we have live current and stop lighting the LED just past the bad bulb. That seems to me to be the easiest way to find bad bulbs in a string of Christmas tree lights.

p.s. I am using friction fit 2.5v mini bulbs on an indoor artificial tree.

Thanks everyone!
posted by mccartrey at 8:04 AM on December 10, 2008