How to Find a Bad Christmas Light Bulb

There are many reasons for burned-out Christmas lights, but one of the most common reasons is a bad bulb. Yes, a single bad bulb can keep all the bulbs on Christmas light strings from coming on. Since all the other bulbs on the string will go off alongside the broken one, picking out the burnt one may not be so easy. But do not fret – your holiday season is not ruined. With a few simple tools and in a few steps, you can readily find the bad Christmas light bulb.

Finding a Bad String Light Bulb

Materials Needed:

• Christmas light tester (a hum tracer)
• Replacement bulbs of the same rating. (For longevity, do not use a blinking bulb as a replacement. It will shorten the lifespan of the non-blinking bulbs in the string).

Step 1: Confirm That Everything Else Is Okay

Before you start testing out your broken string lights, confirm that the issue is not caused by something else. Do the following:

• Ensure you do not have a malfunctioning outlet. If the power outlet is faulty, the lights will not come on. So, test the lights with a different outlet that is functional.
• Confirm that the electrical plugs are not faulty and the fuse is not blown. If the plugs are faulty or the fuse is blown, replace them and test the lights again.
• Inspect the string for burnt, frayed, or damaged wires. If you see any sign of wire damage, you should work on fixing those wires, as that may just be the only problem.
• Check for shaky bulbs. If there’s any, get a firm replacement in its place.

If none of the above seems to be the problem, proceed to the next step.

Step 2: Test the Lights

 

As you can see in the schematic diagram above, Christmas string lights feature 3 types of wires: hot wire, neutral wire, and socket-to-socket wire.

When trying to detect a bad bulb using a Christmas string light tester, you are only going to test the socket-to-socket wires.

To test the socket-to-socket wires, do the following:

• Connect the string light to a functioning power outlet.
• Switch your hum tracer/light tester on.
• Place the hum tracer on the first socket-to-socket wire and look out for the signal from the hum tracer.
• If the tracer gives a signal, it means the current is flowing through the piece of wire you are testing. So, move to the next socket-to-socket wire and repeat the hum test.
• If the two socket-to-socket wires on both sides of a bulb pass the hum tracer test, then the bulb is functional. But if the first wire passes and the second doesn’t, then the bulb just before the non-humming wire is most likely the problem.

Step 3: Replace the Bad Bulb

Once you find the bad bulb, disconnect the string lights from the power outlet, then remove the bulb and put the replacement bulb in its place. After replacing it, the whole string light should come on.

However, if all the other bulbs do not come on after installing a replacement, there may be more broken bulbs. So, find the other ones and replace them to get your Christmas string lights working again.

Why Finding Bad Christmas Light Bulbs Is Not So Easy

When the bulbs of Christmas lights are connected in series, finding a broken bulb is not so straightforward. In a series circuit, when one bulb goes bad, all the bulbs in the series will go off. Of course, with all the bulbs going off alongside the burned-out bulb, you cannot readily find faulty bulbs.

The same thing applies to Christmas lights that connect multiple series circuits in parallel. When one of the bulbs in any of the multiple series blows, every other one within the same series will go off.

Incandescent Christmas Lights vs. LED Christmas Lights

Incandescent Christmas lights are typically connected in series (larger light strings may have multiple series circuits connected in parallel). So, when a single bulb breaks/burns, finding the faulty bulb is usually an issue.

Unlike Christmas lights that use incandescent bulbs, LED Christmas tree lights are more reliable. LED Christmas lights have longer operational lives and are more efficient.

Also, when LED bulbs blow, they typically do not affect other bulbs within their circuit and you may find that only half of the string is out. LED Christmas tree lights use removable bulbs with built-in shunts. So, the current still flows to others when an individual bulb stops working.