How Christmas Lights Work

By: Marshall Brain  | 

Christmas lights, house, night
This homeowner shows off their Christmas decorations. Rob Lewine/Getty Images

Christmas lights are a big part of the holiday season. As November and December roll around, you might see strands of lights everywhere — on Christmas trees, houses, shrubs, bushes and even the occasional car! Have you ever wondered how these lights work? Why is it that if you pull out or break one of the bulbs, the whole strand of lights goes out? And how do they create the lights that sequence in different color patterns?

­If you were to go back in time 30 or 40 years and look at how people decorated their houses and trees with lights, you would find that most people used small 120-volt incandescent bulbs. Each bulb was a 5- or 10-watt bulb like the bulb you find in a night-light. You can still find strands of these bulbs today, but they aren't very common anymore for three reasons:

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  1. They consumed a lot of power. If you had a strand of 50 5-watt bulbs, the strand consumed 250 watts! Consider that most people needed two or three strands to do a tree and five or 10 strands to do a house and you are talking about a lot of power!
  2. Because the bulbs consumed so much power, they generated a lot of heat. When used indoors, three strands at 250 watts per strand were generating as much heat as a 750-watt space heater! The heat from the individual bulbs could also melt things.
  3. They were expensive. You could buy a 10-pack of miniature bulbs for about a dollar. The large bulbs might cost five to 10 times more.
Parallel lights
Parallel light structure
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The one advantage of this arrangement is that a bulb failure has absolutely no impact on the rest of the bulbs. That's because a 120-volt bulb system places the bulbs in parallel, like this diagram on the left.

You can have two, 20 or 200 bulbs in a strand that is wired in parallel. The only limit is the amount of current that the two wires can carry.

In this article, we'll look at Christmas lights so you can understand everything about them, starting with energy-saving Christmas mini-lights.

Christmas Mini-lights

close-up Christmas light
Mini-lights have been around for decades. Teerapat Seedafong/EyeEm/Getty Images

The 1970s saw a revolution in decorative lighting: Mini-lights were introduced. They now dominate the market when it comes to strands of lights. A mini-light is a small, 2.5-volt incandescent bulb. These bulbs are not much different from any incandescent flashlight bulb (see below for details).

Given that you are plugging these 2.5-volt mini-lights into a 120-volt outlet, the obvious question is, "How can that work?"

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The key to using these small, low-voltage bulbs with normal house current is to connect them in series. If you multiply 2.5 volts by 48, you get 120 volts, and originally, that's how many bulbs the strands had. A typical strand today adds two more bulbs so that there are 50 lights in the strand — a nice round number. Adding the two extras dims the set imperceptibly, so it doesn't matter. The lights in a 50-bulb strand are wired as a "series" of lights along one electrical path. So electricity must pass from one light to the next for the entire strand to light up. When a filament in one bulb blows out, it creates an open circuit in the wiring, so electricity is not passed along.

This explains why mini-light strands are so sensitive to the removal of one bulb. It breaks the circuit, so none of the bulbs can light. When mini-lights were first introduced, any bulb burning out would darken the entire strand. Today, the bulbs can burn out, and the strand will stay lit, but if you pop one of the bulbs out of its socket, the whole strand will go dark. This difference in behavior occurs because the new bulbs contain an internal shunt.

If you look closely at a bulb, you can see the shunt wire wrapped around the two posts inside the bulb. The shunt wire contains a coating that gives it fairly high resistance until the filament fails. At that point, heat caused by current flowing through the shunt burns off the coating and reduces the shunt's resistance. (A typical bulb has a resistance of 7 to 8 ohms through the filament and 2 to 3 ohms through the shunt once the coating burns off.)

Several series or groups of lights can be wired to just one plug. This is known as being wired in parallel. Although you can buy simple 50-bulb strands, it is more common to see 100- or 150-bulb strands. These strands are simply two or three 50-bulb stands in parallel. If you remove one of the bulbs, its 50-bulb strand will go out, but the remaining strands will be unaffected. If you look at a strand wired, you will see that there is a third wire running along the strand, either from the plug or from the first bulb. This wire provides the parallel connection down the line.

The big advantages of mini-bulb strands are the low wattage (about 25 watts per 50-bulb strand) and the low cost (the bulbs, sockets and wire are all much less expensive than a 120-volt parallel system). The big disadvantage is the problem of loose bulbs. Unless there is a shunt in the socket, a loose bulb will cause the whole 50-bulb strand to fail. It's not hard to have a loose bulb because the sockets are pretty flimsy. There are inexpensive testers on the market that can help find loose bulbs faster. You point the tester at each bulb and it tells you which bulb is loose.

In the next section, we'll take a look at how blinking lights work.

Blinking Christmas Lights

Christmas ornaments, Christmas Lights, Christmas tree
Most pre-lit Christmas trees are programmable, allowing the user to change the color of the lights and other functions. Victoria Viola/EyeEm/Getty Images

There are two different techniques that are used to create blinking lights. One is crude and the other is sophisticated.

The crude method involves the installation of a special blinker bulb at any position in the strand. The bulb has an extra piece of metal at the top called a bi-metallic strip. The current runs from the strip to the post to light the filament. When the filament gets hot, it causes the strip to bend, breaking the current and extinguishing the bulb. As the strip cools, it bends back, reconnects the post and relights the filament so the cycle repeats. Whenever this blinker bulb is not lit, the rest of the strand is not getting power, so the entire strand blinks in unison. Obviously, these bulbs don't have a shunt (if they did, the rest of the strand would not blink), so when the blinker bulb burns out, the rest of the strand will not light until the blinker bulb is replaced.

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The more sophisticated light sets come with 16-function controllers that can run the lights in all sorts of interesting patterns. In these systems, you typically find a controller box that is driving four separate strands of mini-bulbs. The four strands are interleaved rather than being one after the other. If you ever take one of the controller boxes apart, you will find it is very simple. It contains an integrated circuit and four transistors or triacs — one to drive each strand. The integrated circuit simply turns on a triac to light one of the four strands. By sequencing the triacs appropriately, you can create all kinds of effects! Patent 4,215,277 is a good one to read if you want to learn more about sequencers.

Mini-light Flashlights

If you hook a mini-light bulb up to a normal AA battery, the bulb will light just like a flashlight bulb. It will be dim, however, because the bulb expects 2.5 volts rather than the 1.5 volts the battery is generating. You can put two batteries together to create 3 volts, or you can hook the bulb up to a 9-volt battery.

Because you are driving the bulb at a significantly higher voltage than it expects, it will burn extremely brightly and will not last very long (perhaps 30 minutes or an hour).

LED Christmas Lights and Projectors

Holiday projector, colored light
Holiday projectors are a quick and easy way to decorate the outside of your house for Christmas. These use LED lights. Marta Iwanek/Getty Images

Nowadays, it's more common for Christmas lights to use LED lights rather than incandescent lights.

Henry Joseph Round was credited with the discovery of the light-emitting diode (LED) in 1907, but it wasn't until the 1960s that LEDs were used commercially. LED lights don't have filaments that will burn out and they generate light without adding heat. LEDs are illuminated by electrons moving in a semiconductor material. Here's how a blog at the U.S. Dept. of Energy describes the process:

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A diode consists of something called a p-n junction — two semiconductor materials next to each other, one with a positive charge (p) and one with a negative charge (n). When current is applied to the system, electrons from the negative side move toward the positive side. On the positive side, particles called “electron holes” move toward the negative side. When an electron and an electron hole collide, a small amount of energy is released as a photon. The result is the visible light we see in an LED.

LEDs are wired in a parallel series like incandescents, but they don't use shunts. When an LED bulb fails, it typically short-circuits, so the rest of the string can work even in the absence of a shunt. The first LED Christmas lights were sold in 1998, and by the early 21st century, Christmas trees at both the U.S. Capitol and Rockefeller Center had been lit using only LED lights.

Even though they cost consumers more upfront, LED lights use 80 percent less energy and last much longer than traditional incandescent lights. For instance, 10 strings of incandescent mini-lights would burn 408 watts, but the same strings of LED lights would only burn 4 watts. An LED bulb may last 50,000-200,000 hours while the incandescent bulb will burn for 3,000 hours.

An added benefit is that these lights are often programmable, allowing users to change the color of the lights and select different blinking modes. They also come in a variety of shapes and styles.

LED lights have also allowed the use of laser projectors in holiday light décor. When these projectors are pointed at a house, they create special effects like falling snow or flying reindeer. Here's how they work: Light is passed through a lens, which magnifies and displays a picture. Unlike traditional projectors which use light bulbs, laser projectors use LEDs to produce the light. Holiday laser projectors are popular because they're easier to set up than string lights, especially when decorating rooftops.

However, these projectors can disrupt air traffic if they aren't used properly. In 2020, 6,852 laser incidents involving aircraft were reported to the FAA, compared with 385 in 2006. When lasers hit airplanes, the pilot and other crew members may experience temporary blindness, headaches and watery eyes. It's important that consumers point these projectors directly at their homes, not at the sky.

Other popular holiday light trends include ombre (lights moving from one color to another seamlessly), oversized Christmas lights and geometric shapes.

Originally Published: Apr 1, 2000

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