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Finding the blown bulb in Christmas Lights

A Christmas Tree With Lights
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A Christmas Tree With Lights
Source: My own - Toshiba Camileo H20

Finding the dead bulb is easier than you think!

If you have a string of Christmas Lights, at least the older kind not using LED's, then one of the hardest things can be to identify which bulb is actually gone. 

The traditional method, is to check each bulb in turn to see if it is broken, which on a string of any more than 10 bulbs, is going to be a tedious and unpleasant job. 

Also, even once you know this handy method, you may need to find out how to actually check a bulb, or a string of lights, and how to do it safely. 

If all else fails, there are also a few links to buy yourself a new set of lights. Of course - I hope you'll give the replacement a good go, and on some lights, even bigger sets, then it may not be viable to simply buy new ones. Repairing is a great way to save a little money.

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Be sure to fully unplug the lights from mains voltage before attempting any diagnosis or repair. Mains electricity is dangerous and can kill.

Being plugged in but turned off does not mean the device is safe to work on. Being broken and dark also does not mean it is safe. I repeat - be sure to fully unplug the lights.

Tools you will require

If you are checking these light strings, you will need a continuity checker of some sort.

For this, you can use a multimeter - but that may be a little overkill for this, and a cheaper option is a simple continuity checker, which essentially has a couple of probes, a battery and a light or beeper which will go on when a circuit is made.

You can also make one if you have a light, small battery (providing enough voltage to light the bulb), and a few wires.

Wait a minute! Whats continuity?

In an electrical circuit, you have a positive and a negative. Simply put, there must be a path from one to the other for a circuit to work, and if it is broken then the circuit will stop working.

A continuity test normally puts a very small amount of electricity (low voltage and low current) through a circuit between the two points upon which you place the probes, and will light or beep if there is a clear path for the electricity to flow between them.

If there is no path, then it is likely there is a broken bulb between the two points.

See all 5 photos
See all 5 photos

The basic method

The simplest way to tackle many problems is to divide and conquer. So how can this be applied to light bulbs?

This method assumes one thing - that the lights are the kind that are in series. They may be multiple strands, or a single strand, but one strand is dark. This usually means they are in series.

So from here on - you either found the dark strand, or there is one strand of none-working lights.

The way you will work is instead of testing each light, you test the whole strand. Once this shows as having no continuity, you then take out a bulb roughly halfway alone. Test this bulb to ensure it works, and if it is blown, you can immediately replace it and retest the whole strand.

If it works, then the blown bulb must be either one side, or another of that bulb. Starting with one of the sides, probe from the cable end to the middle - you will have to find the contact that corresponds to that side on the socket for the removed bulb.

If the side tests, then you now know that the blown bulb is in the other side, if it fails then you know there is a blown bulb in this side. If it is the other side, check the other side in the same way and continue.

Once again, you take out what is roughly the middle bulb of a failing side, test the bulb as above, and if then test either side. 

As you can see, you can continue repeating this until you get right down to a single blown bulb. Each time, you are halving the number of potential candidate bulbs, and that means that with around 3 tests per step, you can check 240 bulbs in around 24 tests (1 more if you check the fuse too). Neat?

 Last updated on December 6, 2010

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