CMPE 306. Lab III: Network Laws, Current and Voltage Measurements

Transcription

1 CMPE 306 Lab III: Network Laws, Current and Voltage Measurements Created by: E.F.C. LaBerge based on previous unattributed lab description July 2013 Revised Fall 2016 E. F. C. LaBerge and Aksel Thomas

2 1. Purpose and Introduction The purpose of this second lab exercise for CMPE306 1 is to expose the student to the physical manifestations of the basic network laws: Ohm s Law, Kirchoff s Current Law (KCL) and Kirchoff s Voltage Law. This lab includes a pre-lab exercise, discussed in Section 2. This lab requires the basic equipment skills presented and practiced in Lab I. Students should review those skills especially the procedure for non-destructive measurement of current before starting this lab. In this lab, you will be required to build various circuits using the breadboards and parts from your lab kit. By the successful conclusion of this lab, students will have observed the following features of linear DC circuits: 1. Ohm s Law 2. Kirchoff s Current Law 3. Kirchoff s Voltage Law 4. The fact that a series combination of resistors operates as a voltage divider. 5. The fact that a parallel combination of resistors operates as a current divider. 6. The reciprocal sum characteristic of parallel resistors. 7. The computation of the virtual mesh currents used in mesh analysis. The lab report in standard format is due at the start of next week s lab session. Spelling, grammar, coherence, content and formatting all count. 2. Pre-lab The pre-lab for this lab exercise consists of the following pieces: LTSPICE (or equivalent) simulation of the circuits in Figure 1 through Figure 5 of this lab. Some of these circuits are simple modifications of the circuits simulated during Lab II, so if you saved your circuits from last week, you should be nearly complete with this portion of the pre-lab. During this simulation exercise, pay particular attention to the signs of the currents! What happens to the sign if you rotate the component (with <CTRL> R) by 180? Keep this in mind as you create the circuits. You should spot check the lab computations with your circuit simulations before you get to class. Have your lab assistant certify your simulation results prior to starting to construct your circuit. 1 Wherever CMPE306 is used, the content applies to ENEE302, as well. 1

3 3. Equipment 1. Agilent triple power supply. 2. Digital multimeter 3. Two sets of banana-alligator patch cords (two each red and black) 4. Breadboard 5. Resistors from lab kit: 510Ω, 1kΩ, two 2kΩ 6. Miscellaneous wires from lab kit. 7. You may want to record data directly into Excel or Word or their equivalents. If so, you will need a laptop or the lab computer. Don t forget to save your work to a USB stick if you are using the lab computer so that you have the data to complete your report. 4. Procedure There are five parts to today s lab, so you need to move smartly. If you get hung up ask for help so that you don t fall behind. Initially, use the multimeter to measure and record the actual values of the all four resistors Ohm s Law and Voltage Division Figure 1 Circuit for Ohm s Law and Voltage Division 1. Using a 1kΩ and a 2kΩ resistor, wire together the circuit shown in Figure 1. You will need to use the +25V output of the Agilent power supply to create the circuit. 2. Be careful! Follow the process for measuring current defined in Lab I. Current must always be measured in series! Following the procedure from Lab I, measure and record the current flowing from the power supply to the breadboard, and the current flowing between and. 3. How are the resistors connected in this circuit? Using the measured values of the resistors, verify a) Ohm s Law, V = IR, and b) KCL for the series circuit. 2

4 4. Compute the ratios + and. Multiply these ratios by the supply voltage and + compare the results with the voltage measured across and, respectively. 5. Add the 510Ω resistor in series with the 2kΩ resistor and call this R3. Measure and record the current flowing between the 2kΩ resistor and the 510Ω resistor. Why is this value different from that recorded in Step 3? Using the measured values of the resistors, reverify Ohm s Law and KCL. 6. Be careful, change the multimeter setting to the appropriate voltage setting before this step! Measure the voltage drop across all three of the resistors. Use these measured values to verify KVL for this circuit. 7. Compute the ratios + + R3, + + R3 and R3. Multiply these ratios by the + + R3 supply voltage and compare the results with the voltage measured across, and R3, respectively. 8. Change the voltage to 15V and repeat steps 6 through 8. Be careful when you go back to measuring current. Be sure to use the right current measuring procedure. 9. Turn off the power supply output and construct the circuit for 4.2, below 4.2. Ohm s Law and Current Division 1. Using all four resistors, wire together the circuit shown Figure 2. As in the previous section, you will need to use the +25V output of the Agilent power supply to create the circuit. i C R3 510ohm i B R4 i D i E Figure 2 Circuit for Current Division 2. Measure and record the current flowing through points A, B, C, D, and E as noted in Figure 2. Be sure to use the right current measuring procedure. 3

5 3. Compute the ratios, i B, i C, i D,and i E, and compare those ratios to the ratios + R3, R3 + R3, + R4, and R4 + R4, respectively. 4. Be careful, change the multimeter setting to the appropriate voltage setting before this step! Compute the equivalent resistance of the parallel combination of and R3, and the parallel combination of and R4. Measure the voltage across the parallel combination of and R3 and the parallel combination of and R4. Show that the voltage division property demonstrated in Section 4.1 applies to the equivalent resistances, as well. 5. Turn off the power supply output and construct the circuit for 4.3, below 4.3. Kirchoff s Voltage Law (KVL) 1. Construct the circuit of Figure 3. Note that you need both the +25V and the +6V sources. 2 B" C" R3 510ohm A" V2 D" 6V E" Figure 3 Circuit for KVL 2. Use the multimeter to measure the following voltages V AB,V BC,V CD,V DE,V EA. (careful! Check your settings!), which progress clockwise around the circuit. Keep careful track of the signs! Use a consistent method of measurement based on the order of subscripts. Record your voltages. 2 You could use the -25V supply. In this case, the red wire in the red banana jack of the Agilent would be at the minus side of the circuit indicated by the minus side of the voltage source, and the black wire in the black banana jack of the Agilent would be at the plus side of the circuit. 4

6 3. Change your multimeter settings and measure the current at point A. Why do you only need to measure this single current for this circuit? 4. Use your voltages to verify KVL around the loop. Verify Ohm s Law for the series combination of the resistors. 5. Measure (careful! Check your settings!) and record voltages V BE,V CA,V BD. 6. Using the voltages recorded in step 4 to compute the each of voltages measured in Step 5 in two different ways. 7. Turn off the power supply output and construct the circuit for 4.4, below. 4.4 Kirchoff s Current Law (KCL) i C R4 A" i B 510ohm B" i D R3 i F i E C" V2 6V 1. Construct the circuit shown in Figure 4. Figure 4 Circuit for KCL 2. Measure and record the indicated currents. Be careful about proper current measuring procedure, and make sure that the currents are measure in the indicated direction. Hint: When measuring current, the current flow is from the red lead, through the meter, to the black lead and then back to the circuit. So the tail of the arrow is the red lead and the head of the arrow is the black lead. 3. Verify KCL at nodes A, B, and C, as indicated in Figure Turn off the power supply output. 4.5 Mesh Currents 5

7 The circuit analysis method known as meshes or the mesh method uses mathematical abstractions called mesh currents, which may or may not correspond to physical currents that can be measured with a multimeter. This exercise demonstrates cases where the mesh currents do and do not correspond to the physical current. 1. Construct the circuit shown in Figure 5. i 1 i 3 R3 510ohm I 1 I 2 i 2 V2 6V Figure 5 Circuit for Mesh Currents 2. Measure and record the currents i 1,i 2 and i 3. Be careful about proper current measuring procedure, and make sure that the currents are measure in the indicated direction. 3. According to mesh analysis (which we may not quite have covered in class, but I ll tell you anyway), I 1 and I 2 are the mesh currents in meshes 1 (left side) and 2 (right side), respectively. Observe that the only mesh current to flow in is I 1, and the only mesh current to flow in R3 is I 2. Using this information, find the relationship between the measured currents, i 1 and i 3 and the mesh currents. 4. Observe that both mesh currents flow in. Observe the direction of the mesh currents as the flow in. Verify that current i 2 = I 1 I Reset your multimeter to voltage measurements and measure the voltages (careful) V 12 and V 6. Using the mesh currents, write and verify the KVL equations around mesh 1 (left) and mesh 2 (right). 4.6 Preparation for Next Lab If you manage to complete everything with time to spare, you might take a look ahead at the next lab. Find the circuits that you will be building and simulate them in LTSPICE. It won t take long, as you should be gaining confidence in LTSPICE. In any case, you will have to complete this simulation activity as part of your pre-lab for next week. 6

8 The lab report in standard format is due at the start of next week s lab session. Spelling, grammar, coherence, content and formatting all count. You should have multiple pieces of measured data. Organize these in appropriate tables. This lab report should not contain any graphs. 5. Tear Down and Clean Up 1. Turn off the power supply and set the multimeter to the OFF position. Return the multimeter to your TA for storage. 2. Deconstruct your last circuit and save the parts and wires back to your lab kit. I suggest that you straighten the wires as best you can. Return your lab kit to the TA for storage. 3. Return the banana plugs and hang them neatly in their proper rack. 4. Police your lab area: leave it neat and clean. 5. If you re using your own laptop, there s nothing else to clean up. 6. If you re using the lab computer, save whatever work you want to your USB drive. Close LTSPICE. Eject your drive. 7