Q: My strings of miniature Christmas lights often burn out partially. Why is that? I don’t hook more strings together than what’s recommended.
— Pam Wilson, Green
A: That aggravating problem is rooted in the way miniature incandescent lights are wired.
Unlike old-fashioned C7 and C9 bulbs, miniature bulbs are low voltage. Household current is too much for individual bulbs, so groups of bulbs are wired together in a series that collectively can handle the higher voltage. The electrical current has to pass through all the bulbs in the series to complete its circuit.
Usually a series contains 50 bulbs. A 100-light string has two series; a 150-light string has three series.
If one bulb in a series is defective, loose or missing, the circuit is broken and the series won’t light. The sockets in miniature light strings often aren’t very sturdy, so bulbs can loosen easily, especially outdoors.
In some light strings a burned-out bulb can also cause the series to fail, but better-quality bulbs usually have an extra wire called a shunt that serves as a backup. The shunt is designed to keep electricity flowing through the bulb even if the filament has burned out, so the circuit can be completed.
However, the shunt doesn’t always work right. It has a covering that’s supposed to burn off when the filament fails, but that doesn’t always happen. That’s where a repair device like the LightKeeper Pro comes in handy. It sends a surge of electricity through the circuit that can break down the coating and activate the shunt.
Replace burned-out bulbs even if the string keeps working. Otherwise the other bulbs have to handle the extra voltage, which can cause them to burn out faster.
Lights can also go out if a fuse burns out, but that will darken the whole string.
Have a question about home maintenance, decorating or gardening? Akron Beacon Journal home writer Mary Beth Breckenridge will find answers for the queries that are chosen to appear in the paper. To submit a question, call her at 330-996-3756, or send email to firstname.lastname@example.org. Be sure to include your full name, your town and your phone number or email address.