Christmas Mini-Lights FAQ

A Ciphers By Ritter Page

Terry Ritter

2007 May 20

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

How many strings can I chain together?

Most mini-light sets have an AC socket at the end to power a few other mini-light strings. The instructions usually say that at most 3 strings can be chained together. In practice, the 3A fuses in the plugs should handle loads up to 360 watts. So if the loads are 100-bulb 40-watt strings, one might think the fuses would handle 9 strings. But that would keep the fuses in the first plug pretty hot, and hot fuses are likely to fail. Limiting the load to 1A instead of 3A will keep the fuses cool, which takes us back to a 3 string limit.

LED strings can create longer chains, since they use about 1/10 the power of incandescent strings. But if incandescent strings are plugged into LED strings, the chain will be limited because of the higher power load.

One alternative to an endless chain of light strings is a tree of extension cords, each supporting a single short chain of mini-light strings.

Do I need to replace the fuses?

Both fuses in the AC plug are needed send power into the string. If any part of a string lights, we know both fuses are good and do not need to be replaced. Both fuses are also good if there is power at the AC socket at the end of the string.

My fuses keep blowing; can I replace the fused plug with an unfused type?

Running mini-lights without 3 amp fuses is extremely risky. These strings require 3A fuses because they use lamps designed to short out when they fail. And if all the bulbs short out, that would be a direct short across the AC line. The little fuses protect the wires, and that protects us.

Why bother fixing strings when they are so cheap?

Removing a bad string and re-stringing a new one can be more difficult than just fixing the string. And even a new string can fail.

Why bother replacing burnt-out bulbs?

Every time a bulb burns out and activates the internal shunt, the voltage increases across the other bulbs. The remaining good bulbs run hotter and fail sooner, with each failure further increasing the voltage on the other bulbs. The bulbs will fail increasingly quickly, especially the last 20, until they are all gone and finally blow a fuse. Allowing all the lamps to go will mean that it will take more just than a few replacement lamps to get the string going again.

Why are half my lights blown out?

Without some continuing maintenance, the natural process of bulb failure leads inevitably to blowing out every bulb in a circuit and one or both AC plug fuses as well. We can postpone the inevitable by replacing burnt-out bulbs. If we can keep 40 or 45 bulbs good in each circuit of 50, the worst case need never happen.

Why is half my string dark?

Mini-lights generally have 2 separate circuits. One circuit can go out leaving the other lit.

Why does half my string blink?

Regular blinking normally means a blinker bulb, or perhaps an electronic controller. Normally, mini-light strings have 2 or 3 independent circuits. A blinker bulb positioned anywhere in any of these circuits will cause only that circuit to blink. Blinker bulbs normally have a red tip, but apparently some do not, so we may not realize a blinker is in the circuit or where it may be. Close examination of the bulbs will reveal a blinker, since the inside of a blinker bulb differs from normal bulbs. Note that a string that has been blinking may have accumulated several weakened bulbs which may fail if allowed to cool to room temperature.

Why does a circuit remain lit when a bulb burns out?

In a series circuit, a burnt-out bulb would normally darken the whole circuit. To minimize that, manufacturers include a "shunt" in each bulb, across the filament support leads. When the bulb burns out, a higher voltage appears across the filament leads, which causes the shunt to weld itself into place, which allows current to flow.

Why do some burnt-out bulbs darken the whole string?

The shunt technology sometimes does not work. So when the bulb burns out, no current flows, and that circuit goes dark.

How can I find the burnt-out light in a dark circuit?

• The obvious approach is to remove each bulb and check it, in another string or a tester. But trying to check each bulb and put it back ignores the fact that the sockets are not so good. If each bulb does not make contact when it is re-inserted into the socket, that can make things worse than they were. When a circuit is dark, what are the chances that 25 bulbs can be pulled out and replaced and all will make good connection?
• It is possible to use what I call a "hum tracer," to identify the "hot" side of the AC line as it progresses through the dark string. It is important to pull each socket away from the wire bundle to get a good reading on the socket wires. A single bad bulb will be "hot" on one side, and cold on the other, which is fairly easy to detect. But if multiple bulbs are out, the wires between bad bulbs will be neither hot nor cold, which can be tricky to diagnose. A good approach is to find one problem, fix that, and move on. Often it is easiest to find the first problem on the hot line. Often one can reverse the AC plug to work from either end of a circuit.
• When working on incandescent mini-lights the best tool I know is a Lightkeeper Pro: Take one of the bulbs out of the dark circuit, plug the newly-open socket onto the front of the gun, then pull the trigger a few times. Often, that will light up the dark circuit, then just replace the dark bulbs. Typically, job done in a few minutes, much easier than stringing a new set of lights.

How are mini-lights wired?

The common 100-lamp string consisting of two independent series circuits of 50 lamps, each taking 2.4V.

Normally, these strings have 2 power wires stretching from the fuses on the plug end, to the AC socket at the other. In essence, this is a fused mini extension cord, with 2 little wires good for only 3 amps.

Each lamp circuit consists of a continuous metallic connection from one of the power wires through sockets and lamps to the other power wire. One lamp circuit connects near the AC plug to one of the power wires, and then winds with the power wires to near the middle where it connects with the other power wire. The other lamp circuit starts near the middle and ends near the AC socket.

Most of the string has 3 wires: 2 AC wires and 1 lamp wire series-connecting 50 lamps at a time. But at the AC plug, the AC socket, and in the middle, there will only be the 2 AC wires.

Can I cut a string in half?

Normally, yes. The common string has a 2-wire section in the middle between the two lamp circuits. If you cut it there (with the power off!), then insulate the ends, you can use the first part alone. If you can get a 3 amp fused AC plug from another string, you can attach that to the second half and use that as well. Never use even half a string without 3 amp fuses!

Can I get rid of some of the wires?

Sure, if you are willing to rewire the system. A single circuit typically has 50 bulbs in series, each connected to the next with a single wire. If we connect one end to one side of the AC power line through a 3 amp fuse, and connect the other end to the other side of the AC line, we can eliminate the 2 wires which extend AC power. In this way we can light a circuit of lights while eliminating 2/3 of the wiring.

Why are these lights hard to understand?

The mini-light series connections seem hard because they have very different consequences than the parallel connections used in normal house lights. The series connection allows little 2.4V lamps to run off 120VAC lines without an expensive transformer. But the series connection to some extent also makes every lamp dependent on every other.

In the series connection, exactly the same current flows through every lamp in the circuit. But the voltage across an individual lamps divides according to the resistance of each. When all bulbs in a circuit are similar, they all get the same voltage, even with wide manufacturing variations. But if some of the bulbs are manufactured differently, they may get too much voltage and blow out. The idea that bulbs need to be matched is much different than our familiar use of house lighting.

When replacing bulbs in a series circuit, even knowing the intended operating voltage is not sufficient. Bulbs operating at a particular voltage can produce a wide range of light depending on operating current and resulting power. Bulbs are designed for a particular current at a particular voltage. But the operating current may not be listed for replacement bulbs, or may not be understood if it is listed, because it is not needed for ordinary house or flashlight bulbs.

I have an old decoration that uses 10 bulbs; can I use modern replacements?

In these series circuits, the 120V line divides equally across each light. So in 50-bulb circuits, the bulbs run at 120V / 50 = 2.4V. Similarly, 35-bulb circuits use 3.4V bulbs, and a 10-bulb circuit uses 12V bulbs. We cannot expect to replace 12V bulbs with 2.4V bulbs. However, modern, fairly-cheap 10-light, 20-light and 30-light sets and replacement bulbs have been available on the net. My approach would be to get 2 or even 3 new strings each having the same number of bulbs as in the decoration. Then I would replace all the old bulbs with new ones, and keep the remaining string(s) for future bulb replacements.