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:
- 9 volt battery
- 9 volt battery connector
- 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.
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.
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.