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How To Fix A String Of Christmas Lights

Mukulika Mukherjee
How often have you found yourself plugging in a string of festive fairy lights only to discover that they do not glow? This is a common problem which requires a bit of DIY fixing. Here we provide a guide for identifying and repairing problems in a string of Christmas lights.

Did You Know?

The LightKeeper Pro is an awesome tool to have when you're repairing a string of lights. It is an all-purpose device that works as a hum detector, a shunt repairer, a circuit tester, and a bulb remover.
The holiday season is around the corner, and it's time to bring in the festive fervor with some decorations that reflect the spirit of Christmas. But when you check the string of lights from "last season", to know if they still work, you're in for a shock―they don't! What now?
Wouldn't Christmas seem incomplete without those pretty fairy lights that illuminate the Christmas tree and even our backyards and gardens? The only solution to this problem is to get a new string, or is it? Well, not if you know how to repair the one at hand.

What Goes Wrong With Christmas Lights

First of all, we shall look at the reasons which can keep your string of beautiful fairy lights from glowing at their full splendor. Here are the probable causes.
♦ A damaged or non-functional power plug
♦ Torn or damaged wires
♦ A damaged or non-functional fuse
♦ Damaged bulbs and/or sockets

Steps To Fix A String Of Christmas Lights

Repairing a string of Christmas lights is not as difficult as some might think, but the difficulty level depends on the complexity of the problem. For successful troubleshooting, you should be well-equipped to determine where exactly the problem lies. Here, we shall learn how to identify the cause of the problem, and to tackle it in the shortest time.

What You Need

Before you begin troubleshooting the lights, you must have a few tools and equipment at hand. Here's what you'll need.
  • Wire Cutter
  • Multimeter
  • Electrical Tape
  • Solder Roll
  • Soldering Gun
  • Voltage Tester

Step # 1. Check for Torn or Damaged Wires

Before you plug the string to check if it's working fine and there are no dud bulbs, examine the string for damaged insulation or torn wires. Problems in the wire, if not tackled beforehand, can cause leakage of current and result in electrocution. And if you do find exposed or torn wires along the length of the string, then get down to fixing the leak.
How To Fix It?
♦ Wherever you find the insulation missing, use the electrical tape to cover the exposed copper wire. This tape acts as an insulator and prevents leakage of current.
♦ If you find a broken wire, mark one inch on both sides of the breakage.
♦ Use the pliers to pull out an inch of insulation from both sides, which exposes an inch of copper wire.
♦ Twist the wires together till they are securely connected.
♦ Solder the joint to make it more secure. For this, plug in the soldering iron, hold it at the particular portion of the wire. Once the wire turns hot, apply solder to seal the joint.
♦ Repeat the procedure for the other side of the wire. Once the joint cools down, cover it with electrical tape.
Once you have checked and repaired the wire, you can safely plug the wire to an AC supply and check. If that is the only problem in the string, you'll see that it lights up! However, if that isn't the only problem, you'll either find that the entire string is dark or you'll see that only a section or part of the string illuminates.
If the entire string does not light up, then there might be a problem with the fuse and/or the power plug, and you need to check them. However, even if you find a single bulb in the string glowing, it indicates the fuse and the plug are both working fine, and the problem probably lies with the bulbs or the sockets.

Step # 2. Check for a Damaged Fuse

The fuse is a wire of low resistance that melts when excess current flows through the circuit, thus protecting the equipment and device/s that are a part of the circuit. If you have a damaged fuse, electric current cannot flow through the wire, and the bulbs do not illuminate. To check if a fuse is damaged, use the continuity tester or a multimeter.
♦ Place the two probes, one on each side of the area under test, and check for the indicator in case of a continuity tester, or the value of the resistance if using a multimeter.
♦ If the fuse is broken, it indicates an open circuit in electrical terms, which means current cannot flow between the ends of the probes when they are placed on either side of the fuse. This is indicated by a resistance value of "1" in the multimeter.
♦ However, if current does flow, it is an open circuit and is indicated by a light on the continuity sensor, or a resistance value of "0" on the multimeter.
♦ If the latter holds true, you have a perfectly functioning fuse, and you should move on to check the power plug.
How to Fix it?
In case of a faulty fuse, you need to replace it with one of the same resistance.

Step # 3. Check for a Faulty Power Plug

If you find that the fuse is fine, the next step is to check the power plug. Sometimes, a damaged power plug can be the cause for absence of current in the wire, and detecting it early can save you a lot of work.
♦ Place the probes of the continuity sensor or the multimeter at each end of the power plug.
♦ Note the indicator light or the value of resistance displayed on the indicator.
♦ If it indicates a closed circuit, then the plug is functional. However, if it does not, it means you have a faulty or damaged plug.
How to Fix it?
In case you find a non-functional power plug, get it replaced immediately.

Step # 4. Checking for Damaged Bulbs and/or Sockets

Now that you have made the necessary repairs in the wire and verified that the fuse and plug are fine, the last job is to check the bulbs and sockets, and replace the faulty ones. This can be slightly time-consuming depending on the number of bulbs on the string.
Depending on the type of circuit, you'll be able to find the blown out bulb as soon as you turn on the string. However, that is the ideal scenario, which seldom holds true. Why? To get a better understanding, let us learn a little about the flow of electricity within the string of Christmas lights.

Flow of Electricity Through a String of Lights

Christmas Lights Connected in Series

Most Christmas lights have a series circuit in which each individual light is connected to the next one sequentially or in series. This means that if one bulb blows out, current cannot flow to the rest of the circuit and the remaining part of the string does not illuminate. To prevent this, each socket has a shunt placed below the bulb.
The shunt is a wire of low resistance that allows electric current to flow through it and the rest of the circuit. However, you're more likely to come across non-functional or damaged shunts, which makes it difficult to trace the bad bulb amongst the several ones that don't glow.
Bulb With Functional Shunt
Bulb With Non-functional Shunt

Two Series Circuits Connected In Parallel

In this type of a circuit, a bad bulb or shunt in one part of the circuit means the other part works fine. This is the reason why you sometimes find only a section of your Christmas lights glowing while the other half doesn't.
Parallel Connection
A voltage tester is a device that detects the alternating AC voltage, which is commonly known as "hum". In theory, you can manage the task using a multimeter or continuity tester by checking each bulb, but this is time-consuming and not practical, especially for longer strings. To use a voltage detector to locate blown out bulbs, follow the steps given here.
♦ Locate the bulb wire, which is the wire that enters and leaves bulb socket.
♦ Once find it, run the voltage detector along the wire it into and out of each bulb socket. As long as the voltage detector can detect the "hum", the circuit is fine.
♦ When the voltage detector fails to detect "hum" in the wire that exits the socket, you have the bad bulb/socket.
How to Fix it?
All you need to do now is replace the bulbs. When buying replacement bulbs, ensure that they are of the same resistance and wattage as the bulbs that will be replaced. This is because placing bulbs of a higher resistance in a series circuit can cause the bulb to blow out.
The safest idea, therefore, would be to replace bulbs from a similar string that you had bought along with the string you're repairing.
♦ Use a pair of wire cutters and pull the broken or bad bulbs out. Don't forget to TURN OFF the power before you do so.
♦ Once you have new bulbs of a similar make, place them back into the empty sockets.
♦ Switch on the power supply, and check if the bulb and/or the rest of the string glows.
♦ If you find that the new bulb doesn't glow, and neither does the rest of the string, then you must replace the socket.
♦ To replace the socket, TURN OFF the power supply and use the wire cutter to cut the wires from both ends of the socket. Now, take a new socket, connect the wires to it and secure both the ends with insulating tape.
♦ Once you've replaced the bulbs and sockets, turn the power supply on and check if all bulbs in the string are glowing. If not, start moving the voltage detector along the rest of the wire to detect more bad bulbs and repeat the process till you've found all of them.
In the end, if you still are unable to fix the Christmas lights, then don't despair. Just get one or more new strings from the market and wind them around the Christmas tree and watch it glow. Merry Christmas!