ECE 201 Lab 7 Assignment

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Drexel University *

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Dec 6, 2023

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ECE 201 – Foundations of Electric Circuits Laboratory Assignment 7 Written by Keith Zuckerman Switched RC Circuits Assignment Goals At the end of this assignment, you will be able to:  Determine the rise and fall times for an RC circuit  Simulate a square wave voltage source using the function generator in Multisim  Operate the oscilloscope cursors in Multisim  Configure and operate the lab function generator and oscilloscope  Use the oscilloscope MATH feature to determine differential voltages Materials Required 8.2 kΩ, 1/4 Watt, 5% tolerance, carbon film resistor 10 nF capacitor Breadboard 2 BNC-to-alligator clip cables 1 BNC-to-BNC cable 1 BNC Splitter Procedure Part 1: Theoretical Analysis of a Switched RC Circuit Fig. 1. RC circuit. V src = 5 V , C = 10 nF, R = 8.2 k Ω . Consider the circuit shown in Fig. 1. Initially, the switch S 1 is open for a long time. At time t 1 , S 1 closes. The switch then remains closed for a long time. At t 2 , the switch opens. 1

Assume that the rise time is defined as the duration of time it takes for a signal to change from 10% to 90% of its steady-state value, and the fall time is the duration of time it takes to change from 90% to 10%. Determine the rise time of the voltage across the capacitor, V C , after the switch is closed. Assume the circuit then reaches steady-state. Determine the fall time of V C after the switch is opened. Assume the switch has been open for a long time. At time t = 0 , the switch closes. Determine the voltage across the capacitor at t = 100 μs. Show all work and enter the values in Table 1 on the worksheet. Part 2: Capture and Simulate a Switched RC Circuit in Multisim Fig. 2. RC circuit schematic with function generator input. 1. Build the circuit from Fig. 1 in Multisim, using a function generator to act as both the voltage source and switch, as shown in Fig. 2. 2. Add an oscilloscope to the capture. 3. Connect the positive terminal of Channel A of the oscilloscope to the positive output of the function generator. Connect the negative terminal of Channel A to ground. This will measure the output of the function generator. 4. Connect the positive terminal of Channel B of the oscilloscope to the positive output of the function generator. Connect the negative terminal of Channel B to the negative terminal of the capacitor. This will measure the voltage across the capacitor. 5. Right-click on the wire entering the positive terminal of Channel B and select “Segment color” and change the color to blue. 6. Set up a square wave output to oscillate from 0 V to 5 V at a frequency of 1 kHz by double- clicking on the function generator. Set the: 2

a. Waveform to square wave b. Frequency to 1 kHz c. Amplitude to 2.5 Vp d. Offset to 2.5 V 7. In the Simulation Toolbar, select Interactive 8. Under Active Analysis, select Interactive Simulation 9. Set End time (TSTOP) to 0.002 seconds and click save 10. Save your file with filename format LastName_FirstName_Assign7_Schematic. 11. Add a course title block to the lower right corner of the schematic. Add your name in the “Designed by:” field. 12. Run the simulation. 13. Open the oscilloscope by double-clicking. 14. Under Timebase, set the Scale to 200 uS/Div. 15. Right click on the oscilloscope display and click “Select a trace.” Set Trace to Channel B and click OK. 16. Select Cursor 1 and move it to 0.000 ms (Note: The cursor that is selected will have a larger flag at the top of the screen) 17. Right click on Cursor 1 and select “Set Y value =>” 18. Enter the value for 10% of the source voltage 19. Repeat the previous steps for Cursor 2, setting the value to 90% of the source voltage 20. Confirm that the T1 and T2 values for Channel B correspond to 10% and 90%, respectively 21. Record the rise time for the voltage across the capacitor in Table 1 on your worksheet. The rise time is the value shown in the Time column and the T2-T1 row. 22. Move both cursors slightly to the right and repeat the previous steps to determine the fall time for the voltage across the capacitor. Record the fall time in Table 1. Deliverable: A one-page Multisim schematic with function generator and oscilloscope. All component values and circuit designer’s name are visible. Part 2. Build and Test a Switched RC Circuit on Breadboard 1. Build the circuit from Fig. 1 on your breadboard, using a function generator to act as both the voltage source and switch, as in the simulation. 2. Attach the BNC T splitter to the Output of the Function Generator. 3

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3. Wire the BNC-to-BNC cable from the output of the Function Generator to Channel 1 of the oscilloscope. 4. Using the BNC splitter and cables, connect the output of the function generator to the circuit. The red alligator clip should be connected to the positive terminal of the capacitor, and the black clip connected to ground. 5. Using the second BNC cable, connect Channel 2 of the oscilloscope to the circuit. The red alligator clip should be connected to the positive terminal of the resistor, and the black clip connected to ground. 6. Turn on the function generator and set the Output Termination to “High Z” a. Once the function generator is on, press <Utility>. b. Press the Output Setup key. c. Press Load, and then press Load again and ensure that “High Z” is selected. 7. Set up a square wave output to oscillate from 0 V to 5 V at a frequency of 1 kHz. a. Press the Square key. b. Press the “Freq” softkey and enter the value 1. c. Press the softkey that corresponds to the desired units (kHz). d. Press the “Ampl” softkey and enter the value 5. e. Press the softkey that corresponds to the desired units (Vpp). f. Press the “Offset” softkey and enter the value 2.5. g. Press the softkey that corresponds to the desired units (V). h. Press the “Duty Cycle” softkey and set the value to 50%. 8. Press the Output key to turn on the output. 9. Turn on the oscilloscope and press auto-scale. Confirm that signal on Channel 1 is a square wave that oscillates between 0 V and 5 V. 10. Note that the test leads for Channel 2 are configured to measure the voltage across the resistor, not the capacitor. To measure the voltage across the capacitor, we will use the MATH function on the oscilloscope. a. Press the [Math] key on the front panel to display the Waveform Math Menu. b. Use the Operator key to select the subtraction operator. c. Use the Source 1 and Source 2 softkeys to subtract the resistor voltage waveform from the input voltage waveform. The resulting purple waveform is the differential voltage across the capacitor. 11. Use the cursors to determine the rise and fall times of the capacitor voltage. a. Press the [Cursors] key b. In the Cursor menu, press Mode and select Track Waveform 4

c. Press Source, and set the source to be the math waveform d. Select the cursor to be adjusted by pressing the Cursors knob, then turning the knob e. Set the cursors to 10% and 90% of the source voltage value on the math waveform f. Use the ΔX reading to determine the rise time. g. Repeat the previous step using 90% and 10% on the falling edge of the math waveform to determine the fall time. 12. Record the rise and fall times in Table 1. At the end of the lab session: 1. Upload a completed switched RC circuit schematic page from Multisim (Part 2), converted to PDF format, to the course Learn page. All component values, and circuit designer’s name should be visible. 2. Upload your completed worksheet. Uploads To complete this assignment, convert your Multisim schematic from Part 2 to PDF format. Upload your schematic and your completed Assignment 7 worksheet through the Assignment 7 upload links on the course Bb Learn site. The files should have filename formats of LastName_FirstName_Assign7_Schematic.pdf LastName_FirstName_Assign7_Worksheet.pdf 5