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,
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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.
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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.
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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
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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
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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.
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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
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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.
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