RC CIRCUITS LAB 5
Part I – Series RC Circuit
OBJECTIVES
After completing this exercise, you should be able to:
• Determine the impedance of a series RC circuit.
• Demonstrate the effect that a change in frequency has on a series RC circuit.
• Demonstrate that the basic rules of series circuits are fulfilled by a series RC circuit.
DISCUSSION
In Lab 3, you looked at the characteristics of series RL circuits. In this exercise, you will examine the
characteristics of series RC circuits. Like series RL circuits, series RC circuits comply with all the basic
rules of series circuit operation:



Current is the same at any point in the circuit.
The sum of the component voltages must equal the source voltage.
The total opposition to current is equal to the sum of the component oppositions.
For the RC series circit, current is the 0o reference for phase angle measurements, since current is equal
at all points in a series circuit. Resistor voltage VR is in phase with the circuit current, so we could use VR
as the 0o reference. At specific points in this exercise, however, you will be required to swap the
capacitor and resistor positions. Since this affects our ability to visually compare the various circuit
waveforms to the resistor voltage on the oscilloscope screen, VS will be used as the reference for our
circuit phase angles. As a result, the phasor diagram is referenced to VS as shown in Figure 1. Note that
the 90o phase angle between VR and VS is represented in the figure. Once all the phase angles are
established, you can then alter them to match the form discussed in the text (VR-IT is the 0o phase
reference) .
Analog Circuits I – Lab 5
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Figure 1
PROCEDURE
1. Construct the circuit shown in Figure 2 in ISIS. Channel 1 (Cl) is connected across the entire
circuit and displays VS. Channel 2 (C2) is connected to display VR. Set the function generator for
an 8 VPP sine wave at a frequency of 1 kHz.
Figure 2
2.
Measure the peak-to-peak value VR and record your measurement in Table 1. Measure the phase angle of
VR relative to VS. Record this value (θR) in Table 1.
Table 1
Frequency
VR
θR
VC
θC
1KHz
2KHz
5KHz
10KHz
20KHz
Analog Circuits I – Lab 5
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3. Repeat Step 2 for the other frequencies listed in Table 1. Make certain that VS remains constant
at 8 VPP, as you change frequencies.
Figure 3
4. Reverse the components as shown in Figure 3. Repeat Steps 1 through 3 for this circuit,
measuring VC and its phase angle θC relative to VS. Enter your results in the two right-hand
columns of Table 1.
5. Use the values for VR and VC in Table 1 to calculate the magnitude and phase angle of VS. Enter
your results in Table 2.
Table 2
Frequency
VS
θ
1KHz
2KHz
5KHz
10KHz
20KHz
6. Calculate XC for each frequency shown in Table 3. Use these values and your results from Table 1
to calculate IC. Enter these values in Table 3. (Remember: XC has a phase angle of -90°.)
7. Use your results from Table 1 to calculate IR for each frequency shown in Table 3. Enter your
results in the table. (Remember: R has a phase angle of 0o.)
Analog Circuits I – Lab 5
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Table 3
Frequency
1 KHz
2KHz
5 KHz
10 KHz
20 KHz
XC
IC
R
330
330
330
330
330
IR
QUESTIONS & PROBLEMS
1. Refer to your results in Table 1. As frequency increased, did the circuit become more or less
capacitive? Explain your answer.
2. Refer to your results in Table 2. Explain how your results demonstrate that Kirchhoff’s voltage law
applies to series RC circuits.
3. Refer to your current calculations in Table 3. What basic rule of series circuits do these results
support? Explain your answer.
Part II – Series RC Circuit
OBJECTIVES
After completing this exercise, you should be able to:
 Determine the impedance of a parallel RC circuit.
 Demonstrate the effect that a change in frequency has on a parallel RC circuit.
 Demonstrate that a parallel RC circuit complies with the basic rules of parallel circuits.
DISCUSSION
Parallel RC circuits comply with all the basic rules of parallel circuits:


The branch voltages are equal, regardless of whether a resistor or a capacitor is in the branch.
The sum of the branch currents equals the total circuit current.
However, since the resistive and capacitive currents are out of phase, you must consider the
phase angles when calculating the total circuit current. In Lab 4, you observed that capacitor current
leads capacitor voltage by 900. Since the voltage across the capacitive and resistive branches in a parallel
RC circuit are equal, the current through the capacitive branch leads the current through the resistive
branch by 900. Thus, when you add the two branch currents, you must add them geometrically.
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PROCEDURE
1. Construct the circuit shown in Figure 4 in Proteus ISIS . Channel 1 (Cl) is connected to display the
VS waveform. Channel 2 (C2) is connected to display VRI and VC. Note the sensing resistor (Rs).
Later, you will use the voltage across this resistor (VRS) to calculate IT and to observe its phase
angle relative to VS.
Figure 4
2. Set the function generator for a 10 VPP sine wave at a frequency of 1 kHz. Measure the peak-topeak voltage across the parallel network VNET, and record your measurement in Table 4. Measure
the phase angle of the voltage across the parallel network relative to VS and record this value
θNET in Table 4.
3. Repeat Step 2 for each frequency listed in Table 4. Make certain that VS remains constant at 10
VPP as you change the frequency.
Table 4
Frequency
1KHz
2KHz
5KHz
10KHz
20KHz
50KHz
100KHz
VNET
θNET
VRS
θRS
4. Reverse the components as shown in Figure 5. Repeat Steps 2 and 3 for the new circuit.
Measure the magnitude of VRS and its phase angle relative to VS.
Enter your results in Table 4.
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Figure 5
5. Calculate Xc for each frequency in Table 4, and record these values in Table 5
6. Use VNET and XC to calculate the magnitude and phase angle of IC at each frequency. Enter your
results in Table 5.
7. Use VNET and R1 to calculate the magnitude and phase angle of IR at each frequency. Enter your
results in Table 5.
8. Use VRS and RS to calculate the magnitude and phase angle of IT at each frequency. Enter your
results in Table 5.
9. Use IC and IR1 to calculate the magnitude and phase angle of IT. Enter your results in Table 5.
10. Use your results from Table 5 and Ohm’s law to calculate the magnitude and phase angle of ZT
at each frequency shown in Table 6. Enter your results in the table and graph them.
Table 5
Frequency
XC
IC
IR1
IT
1KHz
2KHz
5KHz
10KHz
20KHz
50KHz
100KHz
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Table 6
Frequency
1KHz
2KHz
5KHz
10KHz
20KHz
50KHz
100KHz
ZT <θ
QUESTIONS & PROBLEMS
1. Refer to Table 4. As the frequency increased, did the total circuit current become more or less
capacitive? Explain your answer.
2. Refer to Table 5. Explain how these results demonstrate that parallel RC circuits comply with
Kirchhoff’s current law.
3. Refer to Tables 5 and 6. Explain why the phase angle of current becomes more positive as frequency
increases, while the phase angle for ZT gets more negative.
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