Christmas Lights and How to Fix Them

Introduction

Christmas lights are the strings of little bulbs often used inside onChristmas trees or outside to decorate homes or yards. A wide rangeof bulbs and strings are available, but this article is about "mini"bulb strings, because they are surprisingly complicated andcommonly misunderstood. The ideal from the retail point of viewmight be for consumers to buy a few strings, use them for a fewweeks, then throw them out. But some consumers want to keep thestrings longer. And keeping Christmas light strings running formonths 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 metalfilament so hot that it glows. The heat itself makes the filament emitlight, and the hotter the filament, the greater and whiter the lightand the shorter the lamp life.

String Faults

The most common problem with these strings is that a bulb can burnout. Most strings come with a couple of replacement bulbs whichmay be enough for a few weeks of operation. But if we want tokeep a string running for years, we are well advised to purchase anadditional 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

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year. Probably we could remove the bad bulb from its base, andre-use that base with a new bulb. But using a bulb from a differentset often has sad consequences.

These lights are designed to be wired in series, and so drop the120V line voltage to typically 2.4V (120V/50bulbs=2.4V/bulb)without using an expensive transformer. As a result, any single bulbfailure 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, getsslightly brighter, and lasts less long. So it is important to replace badlights promptly.

Beyond bad bulbs, some socket connections may become intermittent (especially outside). Even lesscommonly, 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 ACplug. The fuses are intended to melt or "blow" before the copper wires in the string could melt or cause a firefrom overload. No fault in any one lamp is going to do that, but somebody who staples the wiring (andaccidentally 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 thestring itself), flows through the little fuses in the plug. The AC outlet is not a general extension cord, but just aconvenience 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 notconnect too many strings together. How many is too many? Sometimes the instructions with the lights say, forexample: "DO NOT OVERLOAD. CONNECT END TO END A MAXIMUM OF THREE (3) LIGHTINGSTRINGS 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 thatthe AC outlet could support a lot of light strings, but that is deceptive. Wattage figures consider only the fusesand not the wires or connections, which may be some sort of squeeze-terminal or pressure contact whichsometimes does not make good connection. For example, if poor contact inside the AC plug causes internalparts to heat, the plastic plug could distort or melt, which could lead to a very serious situation. I would notconnect more than 3 strings together end-to-end.

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


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Actually replacing a fuse in these strings can be frustrating. Sometimes a little drawer slides out from betweenthe tines of the AC plug, and that holds the fuses. Sometimes a cover can be pulled up on the side of the plugto 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 bediscarded 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 usefulsomeday. 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 ofspace. Sometimes I remove each string from its box, put it in a separate zip-bag, and mark that bag. Mosttimes 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 sometimesreplacement 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, sothey will not get tangled or dusty.

Maintenance

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

Bad sockets can be cut out of the operating string, and a replacement cut from the replacement string. Thewires are stripped of insulation, twisted to make good contact, and insulated with tape. Ordinary people cando 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 andlamps to the other side. A normal string has two such circuits. All the metal wires and contacts are coveredwith insulation so the electricity does not short out or hurt anybody. Unfortunately, that insulation also makesit awkward to measure the voltage and track down problems. So devices can be useful which register thepresence 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 reallyis 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

Ideally, the badbulb will be darkwhile the rest ofthe stringremains lit. Butoften that doesnot happen.


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The lights in these strings are basically in series, and any open connection, whether in a wire, at a socket, oreven inside a bulb, will turn off that whole circuit. Normally the bulbs have a "shunt" which shorts the bulbwhen it experiences full line voltage. That leaves the bad bulb OFF, and the rest of that circuit ON, whichshows 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 testsgood. But not only is that a heck of a lot of work, it has the potential to make things much worse: When thestring 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. Butwhen we put a bulb in, we may have to try several times before the socket makes contact. Only when thestring 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 arenot making good contact.

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

Hum-tracingdevices workbecause the ACline has a "hot"side and a "cold"side. The coldside is connected to ground. The hot side has 120VAC on it, and so has a "hum" which some instruments candetect 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 theothers will be going to the far end AC socket and one will be hot and one will be cold all the time. So if wewant to follow the hum through the bulbs, we need to test the bulb wire alone. We can find a bulb wire fromthe 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 thatcondition, we can replace the bulb, or re-seat it, or possibly replace the socket.

Sometimesthings are morecomplicated:Incandescentlight bulbs aremost likely tofail at the instant they are turned on. And if two different lights in the same circuit happen to fail at the sameturn-on instant, neither light may see enough voltage or current to operate their shunt. (Or perhaps one ormore shunts are simply faulty, since of course they cannot be tested before they operate.) A circuit with evenone bad connection is going to have all the lights out, so finding the location of each one of multiple problemsis 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 theway remaining depending whether hum is present or not. But that can be deceptive if two bulbs are out at thesame time, since at best that will mean "no hum" for the whole substring between them. When we try to tracethe hum in a circuit with two (or more) bad shunts, we eventually come to a line which is "floating," neither


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hot nor cold. The hum detector has to be pretty good to show the difference between a hot wire and onewhich acts an antenna to pick up the hum normally around us.

A hum tracer can be confusing when the lights are lit.When a circuit is working, the hum voltage starts out at120VAC at one side of the AC line, then declines at2.4V 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 abad shunt, things are completely different. When nocurrent flows, there is no voltage drop across the bulbsuntil we get to the circuit break. From the hot end ofthe light circuit, we should have the same strong humuntil we reach the bad lamp or socket. From the otherend of the light circuit, there will be no hum until wereach the bad lamp or socket. It can be helpful toreverse the AC plug and put the hum on the mostconvenient end of the string.

Some hum tracers are sensitive enough to detect humfrom the filament and support wires inside the glassenvelope of a bulb. With such a device, as we check adark circuit, we can quickly move from bulb to bulb,and find where the signal changes. With only one badbulb, there will be full hum on one filament supportwire, and none on the other, which can confuse thehum 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 otherwill have none.

A different sort of device (e.g., the LIGHTkeeper PRO) puts a tiny high-voltage pulse into the dark stringwhile still connected to the AC line, thus triggering perhaps multiple shunts into operation. Presumably thehigh-voltage breaks down the insulation on the shunts, which then weld themselves in place as usual whencurrent starts to flow. Then the good lights glow, the bad lights are out, and we know which to replace. Whenit 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 unitwould just glow an LED a little brighter when it found hum. That was surprisingly hard to interpret, andespecially bad outside in sunlight. But the LIGHTkeeper PRO gives an audible beep when it detects hum, andseems far more sensitive. It is only necessary to wave the unit at a bulb. And that is much, much faster thanpulling 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 aremore visible than felt-tip marker.

Bulbs seem most likely to go out when power comes on. The simple operation of removing a bulb turns thepower off in that circuit. Then, when we plug a new bulb in, power goes on, and weak bulbs can go out at thattime. But if we already know which other bulbs need to be replaced, we can do that and hopefully the string


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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 anotherbulb to blow. It is not at all uncommon to start out to replace one bulb and end up replacing three. The sameeffect can happen when the house power goes out from a storm, or if a string is unplugged to use the ACsocket.

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-onor sometime in between. The process of replacing bulbs turned the power off and on again and again, whichcaused even more bulbs to fail. In the end, I replaced no fewer than 8 burnt-out bulbs (first 3 thenimmediately another, then 2, then 1 and 1 and 1) from one 50-light circuit. No bulbs at all were lost in theother 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 bulbsbefore turning the power on again. My guess is that some other bulbs would fail anyway, although perhapsfewer than might fail with one-by-one replacement. But bulb failure is part of incandescent light operation.

Removing a Bulb

Simply getting the bulb out of the socket can be aproblem. Modern hum-tracers (and the LIGHTkeeperPRO) have little tin cutouts which are supposed to helplever the bulb out of the socket. But those seemawkward and may cause dropped bulbs.

Another option is to use a cheap pair of electronic wirecutters (often called "diagonal cutters" or "dikes"). Thesecan be gently squeezed into the slot between bulb andsocket, and then used to lever the bulb out. Do notover-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 desirableoption.

Many strings have two separate circuits of bulbs. Within each circuit, all the bulbs are wired in series. Theyeach see the same small fraction of the line voltage, but only as long as the bulbs operate identically. Differentstrings can have bulbs of significantly different resistance, yet all bulbs will operate at the same voltage, aslong as the bulbs are similar within a circuit. But if we place a high-resistance (typically dimmer) bulb in agenerally low-resistance (typically brighter) circuit, that bulb can see far more voltage than it was designed tohandle. 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-warmingbulb is placed in a slow-warming string, it can quickly see much more voltage than expected, and may blowout.

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


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For existing strings, there may be an option: If two or more strings were bought at the same time, we can takeone 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 stringand a 50-bulb dark replacement string. That can be as simple as taking bulbs as needed from one half of thestring and using them in the other half. Of course, then we end up with half a string that looks like itdesperately needs work.

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

Bulb Life

Everybody likes brightlights, but brightincandescent lights operatehotter and burn out faster.This same issue occurs withordinary light bulbs, wherewe can get "long life" bulbswhich use the exact sameamount of power, butoperate dimmer. Or we canadd an electronic dimmerto a light circuit, and havebulbs last generally longer.

Incandescent bulb life is astatistical thing: We cannotpredict how long anyparticular bulb will last, butwe can keep records to help predict how many bulbs will burn out per unit time. New bulbs are notnecessarily 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 tofilter to blue or green or purple, we have to run the bulbs hotter. If we want more light, perhaps to glow insidea 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.

A Homemade Dimmer

This past year we bought a very cheapstring with a "snowball" diffusingcoating that started blowing 4 or 5bulbs a week. Even our usual full


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replacement string would have been consumed in 5 or 6 months. Eventually, I took the plug and socket from areplacement 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 canbe oriented in either direction and placed in either line. In my versions, clear heat-shrink tubing covers theconnections 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 withother strings, with similar results. Somewhat surprisingly, in no case was the dimmer light a problem.

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


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Christmas Lights and How to Fix Them