Lab#2

TEAM MEMBERS ______________________________________________________
I. Purpose
ECE 131 Laboratory Project 2
Analysis of an Automobile Lighting System
The purpose of this lab is to help familizarize you with the concepts of voltage, current,
power, series circuits, and parallel circuits. The lab will show you practical applications for
series and parallel circuits and you will verify Kirchhoff’s Current Law, Kirchoff’s Voltage Law,
and Ohm’s Law.
II.
Lab Preparation (must be completed before going to your lab period)
The following steps will help prepare you for the lab by showing you how the theory we
learned in class is related to the lighting system in an automobile and to the equipment we will
be using in the lab.
A. A light bulb can be modeled as a resistor. Examine Figure 1 and mark where the two
terminals of a car battery should be connected to illuminate the light bulb.
Filament (acts like resistor)
Wires holding filament
One wire soldered to brass case
Insulating material
Other wire soldered to contact at bottom of bulb
Figure 1. Single filament light bulb.
B. Using the symbols we’ve learned in class, draw a schematic showing the circuit of a 12-V
car battery (represented as a voltage source) connected to a light bulb (represented as a
resistor).
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If the filament has a resistance of R,
(1) How much current flows through the light bulb
(2) How much power is dissipated by the light bulb
(3) How much power must be provided by the battery
C. Redraw your schematic, adding a switch in series with the light bulb so that you can turn
the light on and off. The symbol for a switch is shown in Figure 2.
Figure 2. Schematic representation of a switch.
D. In the lab we will use a special “break-out box” to make it simpler for you to create your
circuits. This box, shown in Figure 3, has five light bulbs, a potentiometer (variable
resistor), a switch, and various terminals. Examine Figure 3 carefully.
The red and black terminals in the lower left corner are for you to connect the 12-V power
supply representing the car battery. The positive (+) terminal of the power supply will be
connected to the red terminal on the box and the negative (–) terminal of the power supply
will be connected to the black terminal on the box. This convention (using red (R) to
represent + and black (B) to represent –) is used by virtually all electrical and computer
engineers. Draw the symbol for a 12-V source on the upper left side of Figure 3, then show
how the source should be connected to the red and black terminals.
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E. Find Single Filament Bulb #1 on the Figure 3 breakout box. The lower terminal, which is
gray, is connected to the brass case of a single filament bulb. The upper terminal, which is
white, is connected to the contact at the bottom of the bulb (see Figure 1). In Part D you
showed how the power supply is connected to the red and black terminals on the breakout
box. Using Figure 3, show the rest of the connections necessary to build the circuit you
designed in Part C.
Potentiometer
(Variable Resistor)
Dashboard Light
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
Terminals for
Power Supply
Single Filament Bulb #1
Figure 3. Breakout box for automotive lighting system.
Single Filament Bulb #2
F. Suppose the single filament bulb is used for the brake light on the left rear fender of your
car. You also need a brake light bulb for the right rear fender.
(1) Draw a circuit showing the two bulbs connected in series to a 12-V car battery.
(2) Draw a circuit showing the two bulbs connected in parallel to a 12-V car battery.
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(3) When a light bulb fails, it almost always becomes an open circuit (because the
filament burns out). With this in mind, which do you think is a safer design for your
automobile brake lights, the series circuit shown in part (1) or the parallel circuit
shown in part (2). Justify your answer.
G. Using the breakout box redrawn in Figure 4, show how you would connect the elements to
produce the circuit in Part F (3). Draw a 12-V source to represent the car battery and use
the red and black terminals, the switch, single filament bulb #1, and single filament bulb
#2.
W
W
Bl
G
Potentiometer
(Variable Resistor)
G
Dashboard Light
W
Y
W
Y
G
G
R
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
W
W
B
Terminals for
Power Supply
G
Single Filament Bulb #1
G
Single Filament Bulb #2
Figure 4. Breakout box for automotive lighting system (redrawn).
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H. If each of the single filament bulbs has a resistance of R, for the circuit in Part F (3),
(1) How much current flows through single filament bulb #1
(2) How much current flows through single filament bulb #2
(3) How much power must be provided by the battery
I. Often times a bulb will contain more than one filament. For example, an automotive light bulb
may contain two filaments, one for a tail light and one for a turn signal. Figure 5 shows a dual
filament automotive light bulb. Using resistors R 1 and R 2 to represent filament 1 and filament
2, respectively, draw a circuit showing the two filaments connected in parallel across a 12-V
source.
Filament #1
Filament #2
One wire soldered to
brass case
Insulating material
Contact for filament #2
Contact for filament #1
Figure 5. Dual filament bulb.
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J. In addition to single filament bulbs, the breakout box has two dual filament bulbs. As shown
in Figure 6, the lower terminal, which is gray (G), is connected to the brass case of the dual
filament bulb while the upper terminal, which is white (W), is connected to the other end of
one of the two filaments. The center terminal, which is yellow (Y), is connected to the other
end of the second of the two filaments.
(1) Using the switch and Figure 6, show how the 12-V supply representing the car battery
can be connected to one of the two filaments on dual filament bulb #1.
(2) Using the switch and Figure 7, show how the 12-V supply representing the car battery
can be connected to the other of the two filaments on dual filament bulb #1.
(3) Using the switch and Figure 8, show how the 12-V supply representing the car battery
can be connected in parallel to both filaments on dual filament bulb #1.
W
W
Bl
G
Potentiometer
(Variable Resistor)
G
Dashboard Light
W
Y
W
Y
G
G
R
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
W
W
B
Terminals for
Power Supply
G
Single Filament Bulb #1
G
Single Filament Bulb #2
Figure 6. Breakout box for automotive lighting system (redrawn).
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W
W
Bl
G
Potentiometer
(Variable Resistor)
G
Dashboard Light
W
Y
W
Y
G
G
R
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
W
W
B
Terminals for
Power Supply
G
Single Filament Bulb #1
G
Single Filament Bulb #2
Figure 7. Breakout box for automotive lighting system (redrawn).
W
W
Bl
G
Potentiometer
(Variable Resistor)
G
Dashboard Light
W
Y
W
Y
G
G
R
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
W
W
B
Terminals for
Power Supply
G
Single Filament Bulb #1
G
Single Filament Bulb #2
Figure 8. Breakout box for automotive lighting system (redrawn).
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K. Potentiometers (also known as “pots”) are variable resistors. These resistors contain three
terminals, as shown in Figure 9 below. The potentiometer is made up of a wire winding between
terminals a and b. The total resistance of the winding is a fixed value, say R ohms. Terminal c is
connected to another wire that can move up and down along the winding, making contact with a
single point (this wire is known as a wiper arm). The resistance between a and c (and the
resistance between b and c) is dependent on the point on the winding touched by the wiper arm.
Let R ac symbolize the resistance between terminals a and c, and let R bc symbolize the resistance
between terminals b and c. The more winding between a and c, the larger the resistance between a
and c. Note that the sum of the resistances between a and c and between b and c is R (i.e., R ac +
R bc = R).
a
c
b
Figure 9. A variable resistor (or potentiometer).
If the voltage across terminals a and b is V ab , write an expression for the voltage V cb (the
voltage across terminals c and b) in terms of V ab , R, and R bc ).
L. Based on your analysis in Part K, discuss how a potentiometer can be used as a voltage divider.
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M. Draw a circuit with a 12-V source connected across terminals a and b of the potentiometer and
with a single element dashboard light connected across terminals b and c of the potentiometer.
Discuss how moving the wiper arm of the potentiometer causes the dashboard light to dim or
brighten.
N. As shown in Figure 10, the breakout box contains a potentiometer and a dashboard light. The
white (W), gray (G), and blue (Bl) potentiometer terminals on the breakout box correspond to
terminals a, b, and c of the Figure 9 schematic, respectively. Using the switch and a 12-V
source, show the connections on Figure 10 necessary to create a dashboard light with
adjustable intensity.
W
W
Bl
G
Potentiometer
(Variable Resistor)
G
Dashboard Light
W
Y
W
Y
G
G
R
Dual Filament Bulb #1
Dual Filament Bulb #2
Switch
W
W
B
Terminals for
Power Supply
G
Single Filament Bulb #1
G
Single Filament Bulb #2
Figure 10. Breakout box for automotive lighting system (redrawn).
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III. Lab
A. Inspect your breakout box. Is it what you expected after performing the prelab
B. Inspect a single filament bulb and a dual filament bulb (the bulbs will be provided by your
lab instructor – please do NOT remove the bulbs from your breakout box). Are the bulbs
what you expected after performing the prelab
C. With the switch open-circuited and the power supply turned off (using the power strip),
wire the breakout box for a switch and single filament bulb, as shown in the wiring
diagram you developed in Prelab Part E. After you complete your wiring, turn on the
power supply and close the switch.
(1) Measure and record the voltage across the terminals of the single-element bulb.
(2) Turn off the power supply, open the switch, and remove the power supply from the
circuit. You will do this by removing the wires from the power supply’s terminals,
not from the red and black terminals on the breakout box (why).
(3) Now insert the ammeter in the circuit. Be sure that you are using the 10A input and
ask your lab instructor to inspect your circuit before reconnecting the power supply
and closing the switch.
(4) After the inspection but while your lab instructor is still watching, reconnect the
power supply and turn it on, close the switch, and measure and record the current
flowing through the bulb.
(5) Open the switch, turn off the power supply, and remove the power supply as you
did in Step (2).
(6) Remove the ammeter from the circuit.
(7) Calculate the resistance of the bulb and the power absorbed by the single filament
bulb. Record your calculations.
(8) How much power must the power supply generate to illuminate the single filament
bulb
D. With the switch open-circuited and the power supply turned off, wire the breakout box for
a switch and two brake lights using the wiring diagram you developed in Prelab Part F (3).
(1) Are the bulbs in series or in parallel
(2) Turn on the power supply, close the switch, and measure and record the voltage
across the power supply, the first brake light, and the second brake light.
(3) Turn off the power supply, open the switch, and remove the power supply from the
circuit (remember to do this by removing the wires from the power supply’s
terminals, not from the red and black terminals on the breakout box).
(4) Insert the ammeter in the circuit to measure the current flowing out of the power
supply. Be sure that you are using the 10A input, and ask your lab instructor to
inspect your circuit before reconnecting the power supply and closing the switch.
(5) Reconnect the power supply and turn it on, close the switch, and measure and
record the current flowing out of the power supply.
(6) Open the switch, turn the power supply off, and remove the power supply as you
did in Step (3).
(7) Calculate and record the power provided by the power supply.
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(8) Insert the ammeter in the circuit to measure the current flowing through the second
brake light. Be sure that you are using the 10A input, and ask your lab instructor to
inspect your circuit before reconnecting the power supply and closing the switch.
(9) Reconnect the power supply and turn it on, close the switch, and measure and
record the current flowing through the second brake light.
(10) Open the switch, turn the power supply off, and remove the power supply as you
did in Step (3).
(11) Calculate and record the power absorbed by the second brake light.
(12) Repeat Steps (8) – (11) for the first brake light. You may need to significantly
rewire your circuit to ensure that the ammeter is only measuring the current through
the first brake light.
E. Describe how you can use the power measurements from Part D to show conservation of
power.
F. Decribe how you can use the current measurements from Part D to show Kirchhoff’s
Current Law.
G. Using the wiring diagram from Prelab Part J (1),
(1) Measure and record the voltage across the first filament of the dual filament bulb.
(2) Measure and record the current through the first filament of the dual filament bulb.
(3) Calculate the resistance of the filament and the power absorbed by the filament.
Record your calculations
H. Using the wiring diagram from Prelab Part J (2),
(1) Measure and record the voltage across the second filament of the dual filament
bulb.
(2) Measure and record the current through the second filament of the dual filament
bulb.
(3) Calculate the resistance of the filament and the power absorbed by the filament.
Record your calculations
(4) Which of the two filaments in the bulb was brighter Is your observation consistent
with your voltage and current measurements Justify your answer.
I. Using your measurements in Parts G and H, calculate how much current would flow from
the power supply if both filaments were connected in parallel. Also calculate the power
that the power supply would have to provide. Record your calculations
J. Using your wiring diagram from Prelab Part J (3), verify your calculations from Part I.
Be sure to record any discrepancies in your measurements.
K. Using the ohmmeter,
(1) Measure and record the resistance between terminals a and b of the potentiometer.
(2) Turn the potentiometer knob as far as possible counterclockwise and measure and
record the resistance between terminals c and b.
(3) Repeat Step (2) with the potentiometer knob turned clockwise as far as possible.
(4) Repeat Step (2) with the potentiometer knob turned to three various positions
between the two stops. Lightly mark each position in pencil on the breakout box.
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L. Connect the power supply across terminals a and b of the potentiometer. Switch the Fluke
45 multimeter back to “volts”. With the potentiometer knob in each of the five positions
described in Part K, measure and record the voltage between terminals c and b.
M. Are your voltage measurements in Part L consistent with the resistance measurements you
made in Part K Justify your answer.
N. Construct the circuit designed in Prelab Part N. Turn the potentiometer knob and observe
the dashboard bulb growing brighter or dimmer. Are your observations consistent with
the theory of voltage division
IV. Equipment List
1 12-volt power supply capable of providing at least 60 W power (i.e., 5A current)
1 Breakout box containing switch, two 1073 single-element automotive light bulbs, two
1157 dual-filament automotive light bulbs, one 1003 automotive light bulb, and one
100Ω, 2W potentiometer.
1 Fluke 45 digital multimeter (or equivalent)
Assorted banana plug leads
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