How to Repair Christmas Tree Lights

What is a “Mini Light?”

The most common type of Christmas Tree Light in use today is the “mini light.” This article focuses on how to repair Christmas tree lights that use the "mini light" bulb. The mini light is a small, cylindrical light bulb that is about 1 inch (25 mm) in length. Two small wires exit the rounded base of the bulb. The bulb is seated in a plastic plug that has a somewhat rectangular base. The two small wires that exit the round base of the bulb are threaded through this plastic plug and are bent back along the length of the plug. The household current plug-in connector for the light set is fused with two 3 amp fuses, one for each wire.

What’s a Shunt?

Mini lights consist of a filament connected between the ends of the two small wires mentioned above. Closer to the bulb base, just above the bead, there are several turns of thin wire wrapped around both wires. This is called a “shunt” and is intended to conduct current when the filament fails. This is what keeps the rest of lights working when one fails. However, it is very common for shunts to fail.

Is there a “Mini Lights” Series?

Mini lights are wired in a circuit type known as a “series.” In a series circuit, each light is supposed to share an equal amount of voltage. In other words, a 50 mini light strand that operates on regular house current, 120 volts, uses bulbs that are rated for 2.4 volts each (120÷50=2.4), this allows the use of low voltage bulbs without the added expense of a transformer to adjust the electrical current.

Putting It All Together

When a mini light bulb “burns out” the shunt is supposed to kick in. If the bulb didn’t have a shunt, the instant the bulb blew out, the circuit to the rest of the lights in the set would be broken and all the lights would go out. The shunt allows the current to continue in the set allowing one bulb to blow out while keeping the others lit. However, each time a bulb blows out and is not replaced the voltage to the remaining bulbs increases. An increase in voltage means the probability of more lights burning out until eventually all the lights burn out. At this point, the strand would still conduct electricity through the shunts. If this happens, here are 3 amp fuses in the plug that burn out to prevent any overheating of the strand.

Now that you understand how the mini light set works, we can devise a step-by-step plan for repairing them.

Step-By-Step

How to Repair Christmas Tree Lights1) Examine the light strand wires and bulb sockets closely for any breaks in the line or any broken or missing bulbs.

  • If wires are broken splice them back together and cover tightly with electrical tape
  • Replace broken bulbs with new ones
  • Fill in any empty bulb sockets with new bulbs

2) Check the fuses in the plug-in connector.

  • Most plugs have a small panel that is slid outward to expose the fuses
  • Hold each fuse up to a light to see if the element in the fuse is melted or broken in two
  • If the fuse element does not stretch continuously from one end of the fuse to the other, replace the fuse

3) If Step 1 or 2 does not fix the problem, then you will have to test and/or remove and replace each individual bulb.

  • At this point the lights are not working because the bulbs are not making proper contact with the sockets or the shunts are failing (to remedy this see Repairing Shunts below)
  • This is lengthy and tedious, but there is no real shortcut to this step except by using a light tester tool that can test the bulb without removing it from the strand
  • Remove the bulb by lifting it out of the socket by the plastic plug, a flat edge screwdriver or your fingernail may suffice
  • Replace with a new bulb and repeat until the entire set works again

Light Keeper Pro

There is one way to speed up your repair time and that is by using a Light Keeper Pro.

  • The Light Keeper Pro can be purchased at most department stores or on- line
  • The device contains a continuity tester and voltage detector as well as a built-in bulb and fuse checker that can check bulbs without replacing them
  • This is the only device at the current time that can repair faulty shunts. It does so by sending a pulse of electricity through the strand to repair the shunts

Repairing the Shunts

If you have completed Steps 1 and 2 above and your light strand still doesn’t work, there is a very good chance the shunts are oxidized and not working properly. To repair the shunts with a Light Keeper Pro follow these steps:

  1. Locate a light that is not lit and then unplug the light strand from the house current.
  2. Plug the empty socket into the Light Keeper Pro and plug the light strand back into the house current.
  3. Pull the trigger on the Light Keeper Pro 3 or 4 times.
  4. Unplug the strand from the house current.
  5. Unplug the light socket from the Light Keeper Pro.
  6. Replace the bulb and plug in the strand to test for proper operation.
  7. If the lights still do not work, repeat the above steps pulling the trigger up to 20 times to clear the shunts.

Tips

  • Remember that most Christmas tree lights work off of household current and there is a risk of being electrocuted when working with light sets that are plugged into an outlet.
  • Most mini light sets are not very durable, so pack and store the lights with care.
  • Most Mini light bulbs are rated for 1,000 to 1,500 hours of use which is about 1 to 3 years.

More How to Articles