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How Do Christmas Lights Work? BY LAUREN DONOHOE Figure 1: Strand of Incandescent Christmas Lights1 The typical white or colored strands of miniature light bulbs which are used to decorate around the holiday season are called incandescent Christmas lights and are shown in Figure 1. As a decoration enthusiast, you will have a basic understanding of the mechanisms that allow these strands of lights to shine and blink after reading this article. After first comprehending how incandescent light bulbs work, you will be able to understand how tiny bulbs are strung together to make strands of Christmas lights. With a firm understanding of how the strands of lights work, you will next be able to grasp the concept of how the lights consistently blink on and off. How Do Incandescent Light Bulbs Work? The incandescent light bulb today still uses the same concept as the light bulb that Thomas Edison is credited with having invented in the 1870s. An incandescent light bulb, as shown in Figure 2, contains a filament held in place by two metal supports and surrounded by a mixture of inert gasses, all of which is contained inside a glass casing. Two contact wires connect the filament to the electrical contact which is located below the metal base of the bulb. The threads in the metal base allow for the bulb to be screwed into a socket where the electrical contact will be connected to the source of electricity. When you flip the light switch on, an electrical current comes into the bulb through the electrical contact and the contact wires, through the filament, 1 and back out through the other contact wire and the electrical contact, as shown by the green path on Figure 2. The reason that light this process produces light is that the filament is made of a material, often tungsten, which glows when it is heated to high temperatures by the current passing through. The inert gas surrounding the filament inside the bulb is a stable gas that serves two functions. First, it does not allow any unwanted chemical reactions, for example a flame inside the bulb. Second, it keeps the filament from evaporating quickly which makes the light bulb last much longer. Figure 2: The Incandescent Light Bulb3 Understanding these few key concepts about incandescent light bulbs allows us to draw a few conclusions. Have you have ever noticed that these bulbs sometimes take a second to turn on after you flip the switch or that the light is brighter after the bulb has been on for a minute or so? This is because depending on the material makeup of the filament, it can take a couple seconds to reach a high enough temperature to begin glowing. Have you even been told to check if a light bulb is “burnt out” by checking to make sure the filament is still intact? This is because when the filament has broken, there is no path for current to flow and the bulb can no longer produce light. How Do Incandescent Mini-Bulbs Work? Now that we have a basic understanding of how an incandescent light bulb works we have taken our first step toward understanding strands of incandescent Christmas lights. Each individual bulb in a strand of these Christmas lights is an incandescent light bulb. One of these incandescent mini-bulbs from a strand of lights is shown in Figure 3. Each of these mini-bulbs produces light using the exact same process as a large incandescent bulb - the current flows through the filament heating to a high temperature and causing it to glow. 2 The only difference between the large bulb and these mini-bulbs, other than the size, is that the two supports that you see holding up the filament in Figure 3 are actually the contact wires. As the bulb is much smaller, the filament is also much smaller and therefore no longer needs the extra metal supports. Now that you know how each bulb works we will shift our focus to understanding how an entire strand of these bulbs works. Figure 3: The Incandescent Mini-Bulb1 How Do Series and Parallel Connections Work? In order to understand how a strand of mini-bulbs works we need to first cover the concepts of series and parallel connections. When multiple items such as light bulbs are each connected in a line as shown in Figure 4, they are said to be “in series”. When an electrical current runs through a series of light bulbs, another component of electricity called voltage needs to be considered. A specific amount of voltage is used up in each bulb. (The amount of voltage depends on the type and brightness of the bulb.) An easy way to imagine what is happening is to think of each bulb as a toll where your car, as the current, must pay a certain amount of voltage to pass. As the current (car) goes through each bulb (toll) it loses some more of its voltage (money). Another very important distinction is that current can only flow around a closed loop. The current comes from its origin through any light bulbs (tolls) and back to its origin only loosing voltage (money) along the way. Figure 4: A Series Connection When multiple bulbs are each connected to the next bulb on both sides as shown in Figure 5, they are said to be connected “in parallel.” Now, when the electrical current enters the circuit, it must split up between the different paths. You can imagine if four cars (current) entered the circuit, each would choose a different toll (light bulb) as to not clog up one lane. Since each car (current) would only go through one toll (bulb), each will spend only one toll’s worth of money (one bulb’s worth of voltage) before returning to its origin. 3 Figure 5: A Parallel Connection When you plug something into a wall outlet in the United States, it begins to receive a current which carries a voltage of about 120 Volts. The flat prongs that go into the wall provide a closed loop as there are two paths, one for current to enter and one to exit the strand of lights. (The round prong, which can be found on some plugs, is simply a safety measure.) How Do Strands Incandescent Christmas Lights Work? The typical incandescent bulbs used in strands of Christmas lights today have been designed to each consume a little less than 2.5 Volts. This way, a strand of 50 bulbs will use about 120 Volts, which is the amount of voltage that comes out of the wall. The way these strands are designed is that 50 mini incandescent bulbs like the one from Figure 3 are connected in series as shown in Figure 6. This design works well because each of the bulbs consumes about 2.5 Volts. Therefore, after passing through the 50 bulbs (tolls) all 120 Volts (all your dollars) have been used up and the current (your car) must return to where it came from. The voltage consumed by each bulb corresponds to the brightness of that bulb. This is why it does not make a huge difference whether exactly 2.5 Volts or a little less than 2.5 Volts can be contributed to each bulb. With only slightly less voltage being consumed by each bulb, each bulb is only faintly less bright, making the strand almost unnoticeably dimmer. Figure 6: Series Connection of 50 Mini-Bulbs1 While the series connection of 50 bulbs would work on its own, sometimes we want more than just 50 bulbs on one strand. The way that this works is that sets of 50 bulbs are connected to each other in parallel. For example, a strand of 150 mini-bulbs actually consists of three 50 bulb strands in parallel, as shown in Figure 7. This means that 4 the current (cars) splits up and takes the different paths. No matter which path (set of tolls) is taken, the full 120 Volts (same total of money) is consumed by the 50 bulbs in that path. These three sets of bulbs connected in parallel explain why a strand of 150 lights contains three wires twisted together but each bulb is only connected to one of those three wires. The three wires are three individual sets of 50 bulbs that have been twisted together so that the strand now contains 150 bulbs. Figure 7: Three 50 Bulb Strands Connected in Parallel1 There are many different ways for the strands to be designed before they are twisted together. The two most common ways are represented in Figure 8. The top left of figure 8 shows the design where a sequence of bulbs is close together and extra wire is added to be twisted. The top right shows a different design where the bulbs in each set of 50 are spaced out such that when they are twisted together, there will be an alternating pattern of which bulbs are attached to each wire. In either of these designs, when the three wires are twisted together to create the strand the result will look the same, as shown at the bottom of Figure 8. These different designs will be important when we examine a pitfall of this type of Christmas lights and when we learn how the strands of lights blink. Figure 8: Ways for Strands of Christmas Lights to be Twisted Together Why Do So Many Lights Stop Working When One Burns Out? With this basic background we can explain a common pitfall of this method of combining series connections of 50 bulbs in parallel with each other. Have you ever noticed that if just one bulb has burnt out either the entire strand or some segment of 5 the strand will not light? This can be explained by the closed loop rule. When the filament has burnt out in one bulb, there is no longer a closed loop for current to flow. This means that if the strand was only a series of 50 bulbs the entire strand will be burnt out. If the strand had multiple series of 50 bulbs connected in parallel, all the bulbs that are connected in series to the one that has burnt out will no longer work. The two different designs for twisting the bulbs together explain why sometimes a portion of the strand stops working and sometimes alternating bulbs throughout the strand stop working. As a more recent attempt to fix this problem, an extra piece of metal has been added to each bulb. The extra piece is a metal shunt that is coated with a material which as a very large resistance. Figure 9 shows a mini-bulb with the shunt circled in red. As a result of this added shunt, when the filament is intact, the current will continue to travel through the filament because the filament is the path of least resistance. When the filament has burned out, the current will try to flow through the shunt raising the temperature so high that the coating burns off. Now, the current can flow through the shunt of the burnt out bulb so that the circuit is still a closed loop and only this burnt out bulb will not light. Figure 9: Mini-Bulb with Shunt Circled in Red Figure 10: Colored Incandescent Christmas Lights4 How Do Incandescent Christmas Lights Appear Colorful and Blink? Many strands of incandescent Christmas lights feature colored bulbs, as shown in Figure 10. This is accomplished by coating each glass bulb with different colors of translucent paint. Since the light shines through the paint which covers the bulb, the light appears to shine with color. Some Christmas lights also blink. The special bulb which creates this blinking effect is called a blinker bulb. An example of a blinker bulb, which is often marked by a red tip, is shown in Figure 11. Comparing this blinker bulb with the regular bulb in Figure 8, we can see there is a different mechanism inside the glass. 6 Figure 11: Blinker Bulb1 In the blinker bulb in Figure 10, we are again looking at a filament which will glow when it has current running through it. The difference between this bulb and a normal bulb is that in this blinker bulb the filament is not connected between two electrical contacts that carry the current to and from the bulb. While one of the supports for the filament is an electrical contact the other, the taller middle support, is a conducting wire that is not electrically connected outside the bulb. The extra part that causes the blinking is the third metal piece labeled bi-metallic strip on the blinker bulb in Figure 11. A bi-metallic strip contains two attached metals with different material properties. One of the metallic strips is resistant to change in size while the other shrinks and expands relatively easily with temperature changes. Since the two materials are attached, when one of the metals expands or shrinks more than the other metal, the bimetallic strip bends. This shrinking and expanding and the corresponding bending directions are shown in Figure 12. In Figure 12, the green metal changes size with temperature more than the red metal does, causing the strip to bend in different directions depending on whether the temperature is increased or decreased. Figure 12: The Bending of the Bi-Metallic Strip5 7 As we can see from Figure 11, the blinker bulb, when the bulb is off the bimetallic strip is in contact with the support that holds the filament. This contact means that the current has a closed loop around which it can flow. When the bulb turns on and the filament gets hot and glows, the bi-metallic strip also gets hot and bends. As it bends away from the support which holds the filament, the bi-metallic strip no longer makes contact with the support. The result of this bending is that now there is no closed path and the current stops flowing causing the bulbs (the blinker bulb and every bulb in series with it) to shut off. As it cools, the strip bends back to its original position and again creates contact closing the loop and lighting the bulbs. This cycle repeats and as a result the lights blink. The material labeled in green in Figure 12 can be different metals that expand at different temperatures. If the green metal is varied between two different blinker bulbs while the red metal is kept the same, this would produce two blinker bulbs that blink at different speeds. Blinking Christmas lights is where the different twisting designs of the strands come into play. We know that every bulb in series with the blinker bulb will blink the same as it does since it is opening and closing the loop for current to flow. If we are working with a strand of three sets of 50 bulbs as we explored before, there are different ways that this blinking can work. If the twisting design was such that the bulbs on each wire are grouped together, each segment of the strand will blink with its corresponding blinker bulb. If the twisting design was such that the bulbs on each wire alternated, then the blinking bulbs will alternate throughout the strand. Using three different speed blinker bulbs, one on each of the three wires of an alternating twisting pattern causes the more complicated twinkling effect that we often see in Christmas lights. We have completed the journey through first understanding that the current causes the filament in an incandescent light bulb to glow, to the concepts of current and voltage in series and parallel connections, and ended with the mechanics of a bimetallic strip. Putting together each of these components, you now have a basic understanding of how incandescent Christmas lights blink in unison. Sources: 1. <http://people.howstuffworks.com/culture-traditions/holidays/christmaslights1.htm> 16 March 2013. 2. <http://www.bulbs.com/learning/incandescent.aspx> 19 March 2013. 3. <http://green.lasvegasnow.com/tag/incandescent/> 19 March 2013. 4. <http://fivestar-windows.com/christmas.php> 18 March 2013. 5. <http://www.equipmentexplained.com/physics/agent_delivery/vaporizer/vapori zers.html> 19 March 2013. 8