UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE
Department of Electrical and Computer Engineering
Experiment No. 5 -Parallel and Series-Parallel Circuit Characteristics
Overview:
This experiment will have the student investigate the characteristics of parallel circuits. After that
the student will investigate the voltage and current relationships in a combination series-parallel
circuit.
Parallel circuits:
A parallel circuit is one that has two or more paths for the electricity to flow. In other words, the
loads are parallel to each other. If the loads in this circuit were light bulbs and one blew out there
would still be current flowing to the others as they are still in a direct path from the negative to
positive terminals of the battery.
Parallel circuits have the following rules:
a. The voltage is the same across each branch of a parallel circuit.
b. The sum of the individual branch currents equals the total current in a parallel
circuit.
c. The reciprocal of the total resistance equals the sum of the reciprocals of the
individual branch resistances.
The student will verify these rules experimentally using measured and calculated values.
Combination circuits:
A combination circuit is one that has a “combination” of series and parallel paths for the
electricity to flow. Its properties are a synthesis of the two. In this example, the parallel section
of the circuit is like a sub-circuit and actually is part of an over-all series circuit.
Pre – Lab Parallel and Series Circuits
1. Using one of the relationships for resistors connected in parallel solve for the total
resistance in Figure 1. Show Calculations.
Figure 1: Parallel Resistive circuit
2. Series – Parallel circuit analysis: Refer to Figure 2.
Figure 2: A complete series-parallel circuit
a. In branch C, how are R5, R6 and R7 related to each other?
____________________________________________
How are there currents related? ______________________
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How are there voltage drops related to the voltage across branch B and branch A?
_________________________________________________________
b. In branch B, how are R3 and R4 related to each other?
_________________________________________________________
How are their voltage drops related to the voltage across branch B and
branch A?______________________________________________
c. What is the relationship between the sum of the currents of branches A, B and
C to the current through R1?
__________________________________________________________
Why? ______________________________________________________
d. Make a statement that compares the amount of current through each branch A,
B and C with the amount of resistance on A, B and C.
___________________________________________________________
___________________________________________________________
e. Branch B and branch C are in parallel. Correct? _________________
Voltages across parallel branches are the same. Correct? __________
R3 is in branch B and R5 is in branch C. Is the voltage across R3 and R5 the
same?
_________________________________________________________
Why or why not? ___________________________________________
(INSTRUCTOR’S SIGNATURE_____________________________DATE
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Lab Session – Parallel and Series Parallel Circuits
Part One – Voltage characteristic in a parallel circuit.
1. Show your pre-lab work to the instructor at the beginning of the lab session.
2. Connect the circuit in Figure 3. Adjust the voltage source to a value of 12 volts (with the
circuit connected).
3. Using the DMM, measure the voltage across each resistor. Record below.
VR1 = __________
VR2 = __________
VR3 = __________
Figure 3: Lab Circuit setup part 1
Part Two – Current relationships in a parallel circuit.
1. Connect the circuit in Figure 4. Make sure that the source voltage is properly set to 12
volts with the circuit connected.
2. Using a current meter, measure the current through each resistor and the total current.
Record below.
__________
__________
__________
Itotal = __________
Figure 4: Lab circuit setup part 2
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IR1 =
IR2 =
IR3 =
INSTRUCTOR'
S INITIALS
DATE:
Part Three – Resistance
relationship in a parallel circuit.
1. Connect the circuit in Figure 5. Note that there is no source voltage connected.
2. Using a DMM, measure the total resistance. Record.
Figure 5: Lab circuit setup part 3
3. Remove each resistor from the circuit. Using the DMM, individually measure R1, R2 and
R3. Record each value below.
R1 = __________
R2 = __________
R3 = __________
4. Using one of the relationships for resistors connected in parallel solve for the total
resistance in Figure 5. Show calculations(s).
Part Four – Effect of changing a resistor in one branch of a parallel circuit.
1. Connect the circuit of Figure 6. Set the source voltage to 10 volts with the circuit
connected.
2. Measure and record the total current and the current through each resistor.
I1 = __________
I2 = __________
I3 = __________
It = __________
Figure 6: lab circuit setup part 4
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INSTRUCTOR'
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3. Change the value of R2 to
5.6kΩ. Again, measure and record the total current and the current through each resistor.
I1 = __________
I2 = __________
I3 = __________
It = __________
Part Five - Measuring the effect on circuit parameters as the ratio of resistance values are
changed in a two branch parallel circuit.
1. Connect the circuit of Figure 7. Note that R1 and R2 are equal. In succeeding procedures,
R1 will always be 3.3kΩ but R2 will be changed.
2. Measure and record the total current and the current through each resistor. Then measure
the total circuit resistance with the voltage source removed. Record.
I1 = __________
I2 = __________
It = __________
Figure 7: lab circuit setup part 5
Rt = __________
3. Change R2 to 330Ω. Repeat procedure 2 and record.
I1 = __________
I2 = __________
It = __________
Rt = __________
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INSTRUCTOR'
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4. Change R2 to 33kΩ.
Repeat procedure 2 and record.
I1 = __________
I2 = __________
It = __________
Rt = __________
Part Six – Current characteristics in a series parallel circuit.
1. Connect the circuit in Figure 8. When wiring this circuit, allow for convenient means of
measuring each individual current and voltage drop.
Figure 8: lab circuit setup part 6
2. With the current meter in series with the entire circuit, set the supply voltage so that the
current meter reads exactly 6mA. Measure the source voltage and record.
Esource = __________
3. Using the DMM, measure each resistor’s voltage drop. Record in Table 1.
Table 1: Resistors voltage drop
Resistor
Voltage
Current
R1
R2
R3
R4
7
R5
R6
R7
INSTRUCTOR'
S INITIALS
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4. Using the milliammeter,
measure the current through each resistor. Record in Table 1.
5. Disconnect the circuit from the DC source terminals. Measure with the DMM the total
resistance. Record.
Rt = __________(measured)
Part Seven
1. Reconnect the circuit of Figure 2 to the source voltage. Make sure the voltage source is
set to the same value as in Procedure 2.
a. Write a Kirchoff’s voltage or current equation that will give the voltage at point A
in respect to point B. Show this equation below and then calculate this voltage
(VAB)
b. Using the DMM, measure the voltage at point A in respect to point B. Make sure
the probe is at A and the common of the meter is at B. Record.
VAB = __________
c. Using the milliammeter, measure the current through branch C.
Ibranch C = __________
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INSTRUCTOR'
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Post Lab – Parallel and Series
DATE:
Circuits
Parallel
Part One
1. What conclusion can be made from the procedures 1 and 2 in part one?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Part Two
2. Add the measured currents through R1, R2, and R3 together and compare with the
measured total current. Record the sum of measured currents.
Itotal = __________
3. What conclusions can be made from procedures 1 and 2 in part two?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Part Three
4. What conclusions can be made from procedures 1-3 in part three?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Part Four
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5. Compare the results of procedure 3 with procedure 2. What conclusions can be made?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Part Five
6. Compare the results of procedures 2, 3, and 4. What conclusions can be made concerning
the ratio of the two individual resistance values upon circuit parameters Rtotal, Itotal, and
the current through R1.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Part Six
7. Using the same amount of source voltage measured in Procedure 2, calculate the voltage
across and current through each of the seven 1.2kΩ resistors. Record in Table 2.
Resistor
Voltage
Current
R1
R2
R3
R4
R5
R6
R7
Table 2
8. Compare the measured voltages and currents from Table 1 with the calculated values
from Table 2. Are they in reasonably close agreement?
Part Seven
9. Using the measured current through R1, branch A and branch B, write a Kirchoff’s
current or voltage equation that will yield the current through branch C. Show this
equation in your writeup and then calculate Ibranch C.
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