How string lights (5mm Mini Lights) work
Each string typically contains two parallel circuits joined at the middle and ends.
One circuit powers the diodes, and the other passes power through to the next string.
Because of this design, a single bulb will not fail in isolation. You’ll see either half the string out or the entire string out.
3 common reasons lights will not turn on
The coaxial adapter fuse is blown
The adapter includes a small fuse that protects the lights from power surges.
Start here. Replace the fuse in the adapter and reconnect your lights.
If the new fuse blows immediately, the string likely has a severed wire or a faulty rectifier.
Severed or exposed copper wiring
Look for obvious breaks or small cuts in the insulation that let moisture reach the copper.
Common causes include wear, rodent damage, or stapling through the wire during installation.
Any cut or exposed copper can interrupt the circuit and trip fuses.
Malfunctioning rectifiers
Rectifiers are located at the beginning, middle, and end of each string.
If their components fail, power flow can stop and sections will go dark.
Troubleshooting scenarios
1) The whole tree is out
Verify the power source first. Try a different outlet or power supply to confirm power is available.
Check the coaxial adapter’s fuse:
Slide open the fuse panel on the adapter.
A healthy fuse has a thin, unbroken silver line.
A shattered or broken line indicates a blown fuse.
Replace the fuse and test again:
If the replacement fuse blows immediately, there is likely a severed wire or moisture intrusion somewhere on the string.
Divide and test by section:
Segment the installation into manageable areas, such as by branch bases on a tree.
Use an extension cord to power each section individually and identify the faulty string.
Replace the faulty string.
2) Half the tree is out
This usually points to a failed rectifier on one string.
Identify which string is causing the outage and replace it.
Note: If the third rectifier at the end of a string has failed, that string may still light, but it will not pass power to the next string. The dark section may actually be caused by the previous string’s end rectifier.
3) Lights go out after heavy rain or snow
Moisture can create temporary shorts or trip fuses, especially near ground-level connections.
Raise connections 4 to 6 inches off the ground to reduce moisture exposure.
Inspect for cuts or insulation damage where water could enter.
Let components dry fully before testing again, then replace any string that repeatedly trips fuses.
Safety first
Always unplug lights before inspecting or replacing parts.
Use outdoor-rated extension cords, splitters, and adapters for exterior installs.
Do not operate strings with exposed copper or damaged insulation. Replace them promptly.
If a fuse repeatedly blows, stop testing and replace the faulty string to prevent further damage.
Quick checklist
Power source verified and working
Coaxial adapter fuse inspected and replaced if needed
Wires checked for cuts, staples, or exposed copper
Sections tested independently to isolate failures
Suspect rectifier string replaced
Ground-level connections elevated to reduce moisture