UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE. Department of Electrical and Computer Engineering

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1 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.

2 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 R 5, R 6 and R 7 related to each other? How are there currents related? 2

3 How are there voltage drops related to the voltage across branch B and branch A? b. In branch B, how are R 3 and R 4 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 R 1? 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? R 3 is in branch B and R 5 is in branch C. Is the voltage across R 3 and R 5 the same? Why or why not? (INSTRUCTOR S SIGNATURE DATE ) 3

4 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. V R1 = V R2 = V R3 = 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. I R1 = I R2 = I R3 = I total = Figure 4: Lab circuit setup part 2 4

5 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 R 1, R 2 and R 3. Record each value below. R 1 = R 2 = R 3 = 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. I 1 = I 2 = I 3 = Figure 6: lab circuit setup part 4 I t = 5

6 INSTRUCTOR'S INITIALS DATE: 3. Change the value of R 2 to 5.6kΩ. Again, measure and record the total current and the current through each resistor. I 1 = I 2 = I 3 = I t = 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 R 1 and R 2 are equal. In succeeding procedures, R 1 will always be 3.3kΩ but R 2 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. I 1 = I 2 = I t = Figure 7: lab circuit setup part 5 R t = 3. Change R 2 to 330Ω. Repeat procedure 2 and record. I 1 = I 2 = I t = R t = 6

7 INSTRUCTOR'S INITIALS DATE: 4. Change R 2 to 33kΩ. Repeat procedure 2 and record. I 1 = I 2 = I t = R t = 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. E source = 3. Using the DMM, measure each resistor s voltage drop. Record in Table 1. Table 1: Resistors voltage drop Resistor R 1 R 2 R 3 R 4 R 5 R 6 R 7 Voltage Current 7

8 INSTRUCTOR'S INITIALS DATE: 4. Using the milliammeter, measure the current through each resistor. Record in Table Disconnect the circuit from the DC source terminals. Measure with the DMM the total resistance. Record. R t = (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 (V AB ) 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. V AB = c. Using the milliammeter, measure the current through branch C. I branch C = 8

9 INSTRUCTOR'S INITIALS DATE: Post Lab Parallel and Series Part One Circuits Parallel 1. What conclusion can be made from the procedures 1 and 2 in part one? Part Two 2. Add the measured currents through R 1, R 2, and R 3 together and compare with the measured total current. Record the sum of measured currents. I total = 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 9

10 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 R total, I total, and the current through R 1. 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 R 1 R 2 R 3 R 4 R 5 R 6 R 7 Voltage Current 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 R 1, 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 I branch C. 10