Troubleshooting Christmas Mini-Lights

The Christmas season is upon us. Along with the season comes decorating. With decorating, you have 100’s of little mini Christmas lights. Christmas lights are great when they work, and sometimes troublesome when they don’t. You are likely to run into two different breeds of mini-lights. One that just goes dead causing the whole series of 50 bulbs to go dead, and the newer style that goes dead but allows the others to continue to glow.

Mini-light only draw 2.5 volts, but we plug them into 120 volts. However, the light are wired as a series of 50 bulbs. When voltages are in series they are additive. If we do the math (2.5 x 50 = 125), we find that hits the correct voltage with a little extra for line fluctuations. If one of the older or cheaper style bulbs goes out, you break the voltage going out to the rest, thus the entire string of 50 goes out.

With the newer style, there is a second pathway through the bulb other than the light filament. This creates a path through the bulb allowing it to burn out, but the rest of the bulbs to glow. Testing these types of bulbs with an ohm meter is not reliable because of the extra path. The only way to test whether the bulb is good it to plug it into power.

This is where I got the idea for the mini-light tester. It is a simple and easy to make, and successfully tests both types of bulbs.

Building a Tester

What you need:

Items Needed

Items Needed

  1. 9 volt battery
  2. 9 volt battery connector
  3. Mini light receptacle (I cut one out of an old set of Christmas lights I no longer use)



Assembly is very simple. Attach the two leads of the 9 volt connector to the two leads of the mini-light receptacle. If you have a soldering iron available, solder these connections. Wrap the bare wires using electrical tape or shrink tube.


The finished tester

The finished tester

Plug a 9 volt battery into the battery connector and a bulb to test in the receptacle. If the light is good, it glows brightly. Since mini-lights only need 2.5 volts and you are applying 9 volts, they will heat up pretty quickly. I wouldn’t recommend leaving them in for move than the few seconds it takes to make sure they glow. This short period at a higher voltage will not damage the bulb. I left one plugged in for over 20 minutes and the bulb was not damaged.

Troubleshooting a String of Non-Glowing Mini-Lights

Most strings of lights have more than 50 bulbs. This is made possible by paralleling strings of 50 bulbs . This means you have sections of 50 bulbs in series. If one section goes out, you can quickly troubleshoot that section using the following steps. You only need to follow the steps until the problem is resolved.

Lights in series-parallel

Lights in series-parallel

Step 1: Make sure all the bulbs are securely seated in their receptacles. The fit of the bulbs into the receptacle can easily wiggle loose, causing them to break connection but still remain in the receptacle.

Step 2: Divide your section in half. Use a wire or bread tie to mark your center point. Test the center bulb with the tester or by trying it in a working section of lights. If the bulb is bad, replace it with a tested good bulb. Continue through the half section until you have resolved the problem or have tested every bulb in that half. If still not resolved, move on to the second half and do the same.

Step 3: If the above two steps do not resolve the problem, you have a problem with one of the receptacles or a wire. You should replace the entire string of lights. Trying to find and repair the bad receptacle or wire is more work than the cost of the string of lights. Besides, in trying to repair the string, you might create a fire or shock hazard.

Troubleshooting Christmas lights is much like troubleshooting anything. Have a plan and take your time. If you rush through it, you may miss something. Having a tester that works for both types of bulbs helps a lot. While the new style bulbs can fool an ohm meter, they can’t fool the mini-light tester.


7 comments on “Troubleshooting Christmas Mini-Lights

  1. David says:

    Not all lights are like this. The key variable is the group size. Some group 10… Others 25… And I have also seen some do 25 and the 50 for 75… So they used 2 different bulbs on the same light set!

    How to tell what your “group size” on a particular strand is easy if they are all working… Just pull a bulb and see how many go out. Generally, lights from similar group sizes are interchangeable, but the connectors may not be… Most of the time it is OK to swap the bulb into a different holder as long as the bulb is destined for a set with the same group size.

  2. Alex says:

    It would be difficult to use a mini bulb socket for the test since there is more than 50% chance that the bulb assembly will not fit the socket. personally i will just touch the battery wires to the bulb wires

  3. Jean says:

    I have a string of 150 bulbs ( 3 sets of 50 strung together). The set in the middle is out. I used the Lightkeeper Pro and it appears there is voltage running thru the dark section. I tested each of the 50 none working bulbs, and they were all burned out. What could have caused this? Should I just trash the entire string?

  4. scottd says:

    One thing that is missing is that many newer mini-light sets use energy saving bulbs. When these are put into older sets (typically non-energy saving) – they blow out immediately and vice versa – they will burn very dimly. The cause of this is the resistance of the bulbs is different. The voltage is still 2.5V but the current draw is less or more. Best way to test bulbs to see which they are – is to find an energy saving set then test bulbs with it – if they burn normally – they are energy saving – if they burn dimly (even barely noticeable sometimes) – they are non energy saving. This way you know without the bulbs destructing during the testing phase. Another point is that if you are planning to replace ALL (typically 50 at a time) bulbs as long as they are all either Energy Saving or ALL non-energy saving it will work. it is the mismatch for one or a few which causes them to either blow out or burn dimly. This works as long as the bulbs came from a set with an identical number of bulbs (mostly 50).

  5. Scott says:

    Google found this article for me, very helpful, however I see it was written in 2013 and last comment was 2015, hope someone can help me (dec 2017). I bought a (used) 4 ft Christmas wreath I don’t want to throw away. First season using it. I’m just like reader “Jean” above, I have a set in the middle of many that is out and all bulbs are burned out. This just suddenly happened after it was working for several weeks. I’ve tested both fuses, they have continuity. There is continuity from one end to the other of the bad set. Current is passing through the bad string to a good string and lighting up the good string plugged in. This string is a series-parallel of incandescent lights. I have taken working lights from the wreath and put onto the burned out set and nothing lights up not even the good light, however only doing one light at a time. The only semi understandable issue is that all lights on the set are burned out (why didn’t the fuses blow). Replacing the strand would be a nightmare as they are twisted and wound so well. any response would be helpful, thanks

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