Capacitors in DC Circuits

Electric Circuits Lab

Capacitors in DC Circuits

I. Objectives:

After completing this lab experiment, you should be able to:

· Measure the resistance and capacitance.

· Familiarize with Oscilloscope and Function generator.

· Measure the RC time constant using VR and VC.

· Understand the effect of series and parallel capacitors on RC time constant.

II. Parts List:

· Resistor (1) 1 kΩ

· Capacitors (2) 0.22 µF

III. Procedures:

Part I:

1. Construct the circuit shown in Figure 1 in Mutism.

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Figure 1: Series RC Circuit

2. Connect Channel A of the oscilloscope across the voltage source and Channel B across the capacitor.

3. Set the function generator to 5Vpp; 100 Hz, Square Wave 50% duty cycle with 2.5 DC offset if using a function generator . If using clock voltage, set it to 5Vpp, 100 Hz. The DC offset can be modeled by using DC mode on the oscilloscope.

4. Observe the signals on the scope screen. See Figure 2(a) below. (Use Volts/Div and Time/DIV settings to adjust the signal)

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Figure 2(a): Voltage across the Voltage Source and the capacitor

5. Disable Channel A, by setting it to 0, while observing Channel B. You should be able to see the waveform as shown below. Use time base and Channel A scale to adjust the signal.

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Figure 2(b): Voltage across the capacitor

6. Change the time base (Sec/Div) until you have a clear waveform on the scope as shown in Figure 2(c).

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Figure 2(c): Voltage across the capacitor

7. Calculate the time constant of the RC circuit using the circuit parameter values. Record the result in Table 1 under calculated value.

 = R*C

Calculated value

Measured value using VC

Measured value using VR

Time constant ()

220.049us

220.015us

Table 1: Calculated and measured values

8. Measuring the time constant with VC:

i. Measure the peak value of the signal, by placing one of the cursors (T1) at the peak point ___5V______.

ii. Calculate the 63% of the above value _____3.15 V____.

iii. Place the second cursor (T2) at the step (ii) value above and T1 at zero just before the capacitor voltage starts rising as shown in Figure 3.

iv. Observe the value of T2-T1 on the scope, which is the one time constant, as shown below.

v. Record the result in Table 1 above under measured value using VC.

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Figure 5: Measuring RC time constant using VC

9. Connect Channel B of the oscilloscope across the resistor.

10. You should be able to see the waveform as shown below. (Use Volts/Div and Time/DIV knobs to adjust the signal)

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Figure 6(a): Voltage across the resistor

11. Measuring the time constant with VR:

i. Measure the peak value of the signal, by placing one of the cursors (T1) at the peak point ___5 v______.

ii. Calculate the 37% of the above value ___1.85 V______.

iii. Place the second cursor (T2) at the step (ii) value above.

iv. Observe the T2-T1 value on the scope, which is the one time constant.

v. Record the result in Table 1under measured value using VR.

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Figure 6(b): Measuring RC time-constant using VR

Part II:

12. Place two capacitors in series as shown in Figure 7 below.

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Figure 7: Series Capacitors

13. Calculate the total capacitance value and record the results in Table 2 below.

Calculated Value

Measured Value

Capacitance

11µF

10.2µF

Table 2: Series Capacitors

14. Measure the total capacitance value. Use the following procedure to measure the capacitance in Multisim.

i. Connect the impedance Meter (Simulate>>Instruments>>LabView Instruments>>Impedance Meter) as shown in Figure 8.

ii. Measure the capacitive reactance, XC, as shown in Figure 8.

iii. Calculate the capacitance using the equation, and record the value in Table 2.

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Figure 8: Impedance Meter in Multisim

15. Modify the circuit as shown below, by placing two 0.22µF capacitors in series as in Figure 8.

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Figure 8: RC circuit with two series capacitors

16. Calculate the new RC time constant using measured values. Record the result in Table 3 below.

Calculated value

Measured value using VR

Time constant ()

114.653 us

Table 3: Calculated and measured values

17. Connect Channel A of the oscilloscope across the resistor

18. Adjust the trigger if needed, and you should be able to see the waveform as shown in Figure 9 below.

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Figure 9: Voltage Across the Resistor

19. Repeat step 11. Record the measured time constant in Table 3 above.

Part III:

20. Place two capacitors in parallel as shown in Figure 10 below. ( Note: The 0.001 Ω resistor is ONLY required for simulation in Multisim. Without the resistor, the mathematical model will not converge).

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Figure 10: Parallel Capacitors

21. Calculate the total capacitance value and record the results in Table 4 below.

Calculated Value

Measured Value

Capacitance

0.44µF

0.442µF

Table 4: Parallel Capacitors

22. Measure the total capacitance value. Use the following procedure to measure the capacitance in Multisim.

i. Connect the impedance Meter (Simulate>>Instruments>>LabView Instruments>>Impedance Meter).

ii. Measure the capacitive reactance.

iii. Calculate the capacitance using the equation, and record the value in Table 4.

23. Modify the circuit by placing two 0.22µF capacitors in parallel as in Figure 11.

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Figure 11: RC Circuit with Parallel Capacitors

24. Calculate the new RC time constant using measured values. Record the result in Table 5 below.

25. Connect Channel A of the oscilloscope across the resistor.

26. You should be able to see the waveform as in Figure 12 below. (Use Volts/Div and Time/DIV knobs to adjust the signal)

27. Use the cursors on the oscilloscope to measure the time constant (refer to step 11). Record the result in Table 5 below under measured value.

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Figure 12: Voltage across the resistor

Calculated value

Measured value using VR

Time constant ()

458.611us