Christmas Lights

and How to Fix Them

A Ciphers By Ritter Page

Terry Ritter

2005 November 27

Also see: Troubleshooting Christmas Mini-Lights
Also see: LED Christmas Lights and How to Fix Them


[A Bulb]

Christmas lights are the strings of little bulbs often used inside on Christmas trees or outside to decorate homes or yards. A wide range of bulbs and strings are available, but this article is about "mini" bulb strings, because they are surprisingly complicated and commonly misunderstood. The ideal from the retail point of view might be for consumers to buy a few strings, use them for a few weeks, then throw them out. But some consumers want to keep the strings longer. And keeping Christmas light strings running for months or years can get surprisingly tricky.

Most light sets are based on the classic Edison incandescent bulb. As in an ordinary light bulb, electrical energy heats up a tiny metal filament so hot that it glows. The heat itself makes the filament emit light, and the hotter the filament, the greater and whiter the light and the shorter the lamp life.

String Faults

The most common problem with these strings is that a bulb can burn out. Most strings come with a couple of replacement bulbs which may be enough for a few weeks of operation. But if we want to keep a string running for years, we are well advised to purchase an additional identical string simply to provide replacement bulbs.

It is interesting to note that each year the bulb bases seem to differ, and typically it is impossible to get direct replacements another year. Probably we could remove the bad bulb from its base, and re-use that base with a new bulb. But using a bulb from a different set often has sad consequences.

These lights are designed to be wired in series, and so drop the 120V line voltage to typically 2.4V (120V/50bulbs=2.4V/bulb) without using an expensive transformer. As a result, any single bulb failure could darken an entire circuit. To avoid this, each bulb has a "shunt" of several turns of tiny wire inside the bulb near the bead. The shunt is intended to conduct current when the filament fails, but the shunts often do not work.

When a shunt does operate, it takes on a low resistance. As a result, each bulb in the rest of that circuit takes slightly more voltage, gets slightly brighter, and lasts less long. So it is important to replace bad lights promptly.

Beyond bad bulbs, some socket connections may become intermittent (especially outside). Even less commonly, a fuse in the plug may also become intermittent. But it is very common to overload the fuses.

Fuse Faults

Typically, these strings of lights have two tiny fuses, one in each side of the AC line, located inside the AC plug. The fuses are intended to melt or "blow" before the copper wires in the string could melt or cause a fire from overload. No fault in any one lamp is going to do that, but somebody who staples the wiring (and accidentally connects from one wire to another) could.

Most strings have an AC outlet at the end, but all the power taken there (plus that used by the lights in the string itself), flows through the little fuses in the plug. The AC outlet is not a general extension cord, but just a convenience link to one or two more low-power strings of lights. If we connect too much of a load, fuses (especially in the first string) may blow immediately, or perhaps even periodically.

Most strings include one or two spare fuses, which can be replaced in the plug. But the real solution is to not connect too many strings together. How many is too many? Sometimes the instructions with the lights say, for example: "DO NOT OVERLOAD. CONNECT END TO END A MAXIMUM OF THREE (3) LIGHTING STRINGS OR DECORATIVE OUTFITS OF THE SAME TYPE," and that is the answer.

100-bulb light strings typically use "3A" fuses. Since 3 amps at 120V is 360 watts of power, it might seem that the AC outlet could support a lot of light strings, but that is deceptive. Wattage figures consider only the fuses and not the wires or connections, which may be some sort of squeeze-terminal or pressure contact which sometimes does not make good connection. For example, if poor contact inside the AC plug causes internal parts to heat, the plastic plug could distort or melt, which could lead to a very serious situation. I would not connect more than 3 strings together end-to-end.

Fuses are necessary specifically because these strings use bulbs with internal shunts. Each activated shunt reduces the string resistance and causes the string current to increase, which shortens the life of the remaining bulbs. If bulb failure is allowed to continue, the last dozen or so bulbs will go in a rush, leaving a circuit consisting only of low-resistance shunts. With low-value fuses, we can hope the fuses will blow under the high current. But if a plug without fuses is used, the wiring will get hot, the wires in a bundle could melt through their insulation and form a copper-to-copper hot arc that could ignite a fire. Never operate shunt bulbs off the AC line without low-value fuses in the circuit! Fuses are absolutely required to protect against the worst-case situation where all bulbs have burnt out and the circuit consists only of shunts.

Actually replacing a fuse in these strings can be frustrating. Sometimes a little drawer slides out from between the tines of the AC plug, and that holds the fuses. Sometimes a cover can be pulled up on the side of the plug to reveal fuses. But often the plastic parts are very tight or stuck, which can make replacement tough.

Replacement Storage

Be sure to mark the replacement container with the location of the operating string and the install date. Eventually, the replacement bulbs will be consumed, or, more likely, the operating bulbs will lose their color. Eventually, the operating string will be replaced, which of course means that the replacement string can be discarded or recycled. But we may not notice that until much later, and then we will need the marked date.

When a string is recycled, the bulbs can be placed in small bags against the (faint) possibility of being useful someday. The wire can be placed in bags to be used for speaker leads or other connections.

The simplest way to store lights is to just keep them in their original box, but that can end up taking a lot of space. Sometimes I remove each string from its box, put it in a separate zip-bag, and mark that bag. Most times I cut out and save the sockets with their bulbs, and I also save the wire for use in speaker connections. Another alternative is to save only the bulbs, which really cuts down on storage space, but sometimes replacement sockets are needed.

The best way to save lights for another year seems to be to put each string into a separate plastic zip bag, so they will not get tangled or dusty.


If a bulb is burnt out, fixing it means replacement. The same goes for bad fuses, bad light sockets or even bad wires.

Bad sockets can be cut out of the operating string, and a replacement cut from the replacement string. The wires are stripped of insulation, twisted to make good contact, and insulated with tape. Ordinary people can do this. It is helpful for the socket wires to have different lengths so the connections are not adjacent.

The goal is to have continuous metal-to-metal contact from one side of the AC plug, through the wiring and lamps to the other side. A normal string has two such circuits. All the metal wires and contacts are covered with insulation so the electricity does not short out or hurt anybody. Unfortunately, that insulation also makes it awkward to measure the voltage and track down problems. So devices can be useful which register the presence of line voltage even under the insulation.

Before touching the metal of any wiring, be sure the power is off. You may bet your life that the power really is off when you actually just think it is. Pull the plug! The fuses are there to protect the wires, not people. Even a small current can be dangerous if it goes through a human chest.

Finding the Bad Bulb

[Overall Schematic]

Ideally, the bad bulb will be dark while the rest of the string remains lit. But often that does not happen.

The lights in these strings are basically in series, and any open connection, whether in a wire, at a socket, or even inside a bulb, will turn off that whole circuit. Normally the bulbs have a "shunt" which shorts the bulb when it experiences full line voltage. That leaves the bad bulb OFF, and the rest of that circuit ON, which shows which bulb to replace. Unfortunately, the shunt often fails to operate, and then an entire circuit is off.

One possibility is to go down the string light-by-light and remove a bulb, test it, then put it back if it tests good. But not only is that a heck of a lot of work, it has the potential to make things much worse: When the string does not light up we cannot know that we have seated a bulb properly.

These strings generally have sockets that work well enough as long as we leave the bulbs in the socket. But when we put a bulb in, we may have to try several times before the socket makes contact. Only when the string lights up do we know we have been successful in seating a bulb. So if we are working on a dark string, we might find the bad bulb, and yet have the string still not light because several of the bulbs we re-seated are not making good contact.

Devices are sold to help with the issue of finding the bad bulb without first removing it. One sort of device picks up the 60Hz hum from a wire which is connected to the "hot" side of the AC line. Then one can follow that hum from the plug, into and out of each successive light socket, until at some place the hum fails. Then we have a bad bulb, or a bad connection, maybe just a poorly-seated bulb. The device I got a decade or so ago was helpful, but also relatively insensitive and tricky to use.
[One Bad Shunt]

Hum-tracing devices work because the AC line has a "hot" side and a "cold" side. The cold side is connected to ground. The hot side has 120VAC on it, and so has a "hum" which some instruments can detect at a short distance, even through insulation. But it is important to realize that typically three similar-looking wires are wrapped together between sockets. Of these, only one is the bulb wire we want. Both of the others will be going to the far end AC socket and one will be hot and one will be cold all the time. So if we want to follow the hum through the bulbs, we need to test the bulb wire alone. We can find a bulb wire from the base of a bulb and pull it away from other wires. Then we can check that wire for hum. At the bad bulb, one side will have 120VAC and hum, while the other side will be at ground with no hum. When we find that condition, we can replace the bulb, or re-seat it, or possibly replace the socket.
[Two Bad Shunts]

Sometimes things are more complicated: Incandescent light bulbs are most likely to fail at the instant they are turned on. And if two different lights in the same circuit happen to fail at the same turn-on instant, neither light may see enough voltage or current to operate their shunt. (Or perhaps one or more shunts are simply faulty, since of course they cannot be tested before they operate.) A circuit with even one bad connection is going to have all the lights out, so finding the location of each one of multiple problems is going to be an issue.

The optimal way to trace a single open connection in a series string is to start in the middle, then go half the way remaining depending whether hum is present or not. But that can be deceptive if two bulbs are out at the same time, since at best that will mean "no hum" for the whole substring between them. When we try to trace the hum in a circuit with two (or more) bad shunts, we eventually come to a line which is "floating," neither hot nor cold. The hum detector has to be pretty good to show the difference between a hot wire and one which acts an antenna to pick up the hum normally around us.
[LIGHTkeeper PRO Retail Package]

A hum tracer can be confusing when the lights are lit. When a circuit is working, the hum voltage starts out at 120VAC at one side of the AC line, then declines at 2.4V per bulb until ground is reached on the other side. As we move the hum tracer along a working string, there simply is progressively less hum to find. With a bad shunt, things are completely different. When no current flows, there is no voltage drop across the bulbs until we get to the circuit break. From the hot end of the light circuit, we should have the same strong hum until we reach the bad lamp or socket. From the other end of the light circuit, there will be no hum until we reach the bad lamp or socket. It can be helpful to reverse the AC plug and put the hum on the most convenient end of the string.

Some hum tracers are sensitive enough to detect hum from the filament and support wires inside the glass envelope of a bulb. With such a device, as we check a dark circuit, we can quickly move from bulb to bulb, and find where the signal changes. With only one bad bulb, there will be full hum on one filament support wire, and none on the other, which can confuse the hum tracer. So we either find no hum at the bad bulb, or at the next one. But at the socket of the bad bulb, one of the bulb wires will have full hum, and the other will have none.

A different sort of device (e.g., the LIGHTkeeper PRO) puts a tiny high-voltage pulse into the dark string while still connected to the AC line, thus triggering perhaps multiple shunts into operation. Presumably the high-voltage breaks down the insulation on the shunts, which then weld themselves in place as usual when current starts to flow. Then the good lights glow, the bad lights are out, and we know which to replace. When it works, it is magical, and can save hours of effort.

When the trigger pulse does not work, the LIGHTkeeper PRO also has an improved hum-tracer. My old unit would just glow an LED a little brighter when it found hum. That was surprisingly hard to interpret, and especially bad outside in sunlight. But the LIGHTkeeper PRO gives an audible beep when it detects hum, and seems far more sensitive. It is only necessary to wave the unit at a bulb. And that is much, much faster than pulling each bulb wire away from the bundle.

Marking the Bad Bulbs

If more than one bulb is out, first mark the bad bulbs before replacing any. Bits of masking tape generally are more visible than felt-tip marker.

Bulbs seem most likely to go out when power comes on. The simple operation of removing a bulb turns the power off in that circuit. Then, when we plug a new bulb in, power goes on, and weak bulbs can go out at that time. But if we already know which other bulbs need to be replaced, we can do that and hopefully the string will light except for the bulbs which have just blown. That often works even when several lights have failed.

If a string has been operating for a while without being turned off, simply replacing a bulb may cause another bulb to blow. It is not at all uncommon to start out to replace one bulb and end up replacing three. The same effect can happen when the house power goes out from a storm, or if a string is unplugged to use the AC socket.

Recently, a breaker trip caused our light power to go off for a few minutes. When the power came back on, half of one string was dark. I now know that 3 bulbs in the same circuit failed either at power-off or power-on or sometime in between. The process of replacing bulbs turned the power off and on again and again, which caused even more bulbs to fail. In the end, I replaced no fewer than 8 burnt-out bulbs (first 3 then immediately another, then 2, then 1 and 1 and 1) from one 50-light circuit. No bulbs at all were lost in the other circuit of that same string.

One could argue that it might be best to mark the bad bulbs, then turn the power off and replace all bad bulbs before turning the power on again. My guess is that some other bulbs would fail anyway, although perhaps fewer than might fail with one-by-one replacement. But bulb failure is part of incandescent light operation.
[Two Diagonal Wire Cutters]

Removing a Bulb

Simply getting the bulb out of the socket can be a problem. Modern hum-tracers (and the LIGHTkeeper PRO) have little tin cutouts which are supposed to help lever the bulb out of the socket. But those seem awkward and may cause dropped bulbs.

Another option is to use a cheap pair of electronic wire cutters (often called "diagonal cutters" or "dikes"). These can be gently squeezed into the slot between bulb and socket, and then used to lever the bulb out. Do not over-do or you will chop the bulb in half.

Replacing a Bulb

The ideal replacement is one from an identical "replacement" string bought at the same time as the "operating" string. Anything else is a much less desirable option.

Many strings have two separate circuits of bulbs. Within each circuit, all the bulbs are wired in series. They each see the same small fraction of the line voltage, but only as long as the bulbs operate identically. Different strings can have bulbs of significantly different resistance, yet all bulbs will operate at the same voltage, as long as the bulbs are similar within a circuit. But if we place a high-resistance (typically dimmer) bulb in a generally low-resistance (typically brighter) circuit, that bulb can see far more voltage than it was designed to handle. In that situation, the bulb may simply burn out in a fraction of a second.

Another issue seems to be warm-up time: All incandescent bulbs increase their resistance as they warm up. But even bulbs with the same ultimate operating voltage can warm up at different rates. And if a fast-warming bulb is placed in a slow-warming string, it can quickly see much more voltage than expected, and may blow out.

It is much, much better to buy two strings to operate one string, and then use the other simply for replacement bulbs. But that requires prior planning.

For existing strings, there may be an option: If two or more strings were bought at the same time, we can take one out of service, put it in a zip bag and use it for replacement bulbs.

For existing lone strings, there still may be an option: Convert a 100-bulb string to a 50-bulb operating string and a 50-bulb dark replacement string. That can be as simple as taking bulbs as needed from one half of the string and using them in the other half. Of course, then we end up with half a string that looks like it desperately needs work.

An alternative is to cut the string in the middle and end up with one fully-working short string plus a short string of replacement bulbs that we can keep in a bag. Typically, three wires are twisted together across most of a string, but only two are twisted at the start, middle, and end. So if we look for the two-wire section in the middle, we can cut there (after first turning the power off). On the working half, we can fold each of the two just-cut wires back upon itself and wrap each separately with tape, or separately insulate with electrical heat-shrink tubing.
[Homemade String Dimmer]

Bulb Life

Everybody likes bright lights, but bright incandescent lights operate hotter and burn out faster. This same issue occurs with ordinary light bulbs, where we can get "long life" bulbs which use the exact same amount of power, but operate dimmer. Or we can add an electronic dimmer to a light circuit, and have bulbs last generally longer.

Incandescent bulb life is a statistical thing: We cannot predict how long any particular bulb will last, but we can keep records to help predict how many bulbs will burn out per unit time. New bulbs are not necessarily an advantage: Sometimes old, used bulbs are likely to last just as far into the future as new bulbs.

Dim lights operate at lower temperature and are less white and more red. If we want more white, perhaps to filter to blue or green or purple, we have to run the bulbs hotter. If we want more light, perhaps to glow inside a diffusing coating, we have to run the bulbs hotter. And hotter bulbs die sooner.

Most strings include a couple of bulbs with red tips called "blinker bulbs." These alternately turn on and off, and take the rest of the circuit with them. They also reduce bulb life across the entire blinking circuit.
[Homemade Dimmer Schematic]

A Homemade Dimmer

This past year we bought a very cheap string with a "snowball" diffusing coating that started blowing 4 or 5 bulbs a week. Even our usual full replacement string would have been consumed in 5 or 6 months. Eventually, I took the plug and socket from a replacement string, and put a silicon power diode in the circuit. Any diode over, say, 200V (peak-inverse-voltage) and 1A current should work (and if it fails, the lights just go back to full brightness). The diode can be oriented in either direction and placed in either line. In my versions, clear heat-shrink tubing covers the connections and the diode itself.

The diode puts the string at half intensity, and we now lose about a bulb a month. I did the same thing with other strings, with similar results. Somewhat surprisingly, in no case was the dimmer light a problem.

Terry Ritter, his current address, and his top page.