Experiment 2: Simulation of DC Resistive Circuits Objectives: Simulate DC Resistive circuits using Orcad PSpice Software. Verify experimental and theoretically calculated results for a given resistive network. Observe balanced and unbalanced Wheatstone bridge circuit. Investigate voltmeter loading effects. Pre Lab: Bring a 3.5 diskette to lab. You will need it to save your files. Read and understand Appendix D in the text. Do problem 2.18. Determine the current through the 12-Ω resistor, current passing through the 25-Ω resistor, and the voltage drop across the 30-Ω resistor. Bring your detailed solution to lab. Enter these values on Table 1 of the Data Sheet. Transfer your measured resistor values from Experiment 1 to Table 2 of the Data Sheet Transfer your measured voltage and current values from Experiment 1 for voltage source and resistors to Table 3 of the Data Sheet. Using your measured resistor values from Experiment 1, calculate theoretical voltages and currents for the circuit described by Figure 1. Enter these values on Table 3 of the Data Sheet. Components Used: Orcad Lite 9.2 Software for the PC supplied on one of the CDs with the text. Part A: Introduction to Capture/PSpice 1. Using the computer in the laboratory, start the Orcad software and follow the example described in Section D.1 of the text. Ask your Lab Instructor for help if necessary. 2. Save the completed circuit as a file to your diskette. This circuit can be printed to the network printer. You may also copy the circuit and paste it in a word document as a circuit schematic and include with your lab report. 3. Open the sample circuit provided in lab for Problem 2.18 of the text and record the current value through the 12 Ω resistor on Table 1 of the Data Sheet. 4. Click on the current (voltage) icon to display the element currents (node voltages). Record values on Table 1 of the Data Sheet. (You will need to calculate the difference between two node voltages to determine the voltage across the 30 Ω resistor.) 1. Does the simulation provide accurate values when compared with calculated values? Explain. Page 1 of 4
Part B: Parallel and Series Circuit 1. Figure 1 is the schematic for the final circuit constructed in Experiment 1. As a prelab activity, you should have transferred measured values for R1, R2, R3, and R4 to Table 2 of the Data Sheet. You should have also transferred measured and calculated values for voltages and currents to Table 3 of the Data Sheet. 2. Construct a schematic of the circuit shown in Figure 1 using Capture. Assign the measured values from experiment 1 to the components of this schematic. Change the values by double clicking on the component and editing the value cell. Components may be rotated or flipped by right clicking on a component and using the pop-up menu. Figure 1 3. Run the bias point simulation and record the values for current through and voltage across each component in Table 3 of the Data Sheet. 1. Does the simulation provide accurate values when compared with both measured values and calculated values? Explain. Page 2 of 4
Part C: Wheatstone bridge 1. Construct a schematic of the circuit shown in Figure 2 using Capture. Assign various values to R1, R2, R3 and R4 according to the Table 4 in the Data Sheet. Change the values by double clicking on the component and editing the value cell. 2. Figure 2 3. Record the values for current through XY for all these cases. 1. Comment on the effect of different resistance values on the current across the branch XY. Part D: Instrument Loading Voltmeters have very high resistances that typically exceed 1 MΩ. If the circuit uses relatively low resistance components (say, less than 10KΩ) the resistance of the voltmeter has a negligible effect on the circuit. However, this is not always the case. When a voltmeter changes the voltages in the circuit being measured, we say that the meter loads the circuit. In Part C we investigate loading caused by the internal resistance of a voltmeter and its effect on a circuit. 1. Construct the circuits shown below using Orcad Capture. Run a PSpice Bias Point Analysis and verify that the node voltages are the same in both circuits. Record the voltages in Table 5 of the data sheet. 2. Now suppose that we have these circuits actually set up in the lab and we want to measure the voltages at nodes A and B. Voltmeters have finite resistances. In other words, they draw some current from the circuit that they are measuring. A model for a typical voltmeter is a 1 MΩ resistance. Add 1 MΩ resistors from points A and B to ground in each circuit to model voltmeters used to measure the node voltages. Simulate the circuit and record the node voltages in Table 4. Page 3 of 4
2. Comment on the effect that the resistance of the voltmeter has on the voltages. When should we be suspicious that a voltmeter might load a circuit being measured? When is it likely to not be a problem? Page 4 of 4
Data Sheet Table 1: Problem 2.18 I12 I25 V30 Calculated (Prelab) Simulation Table 3: Parallel and Series Circuit Vs Is Measured Calculated (Prelab) Simulation (EWB) V470 Table 2: Measured Resistor Values Nominal Value (Ohms) R1 R2 R3 R4 Measured Value (From Experiment 1) I470 V1000 I1000 V1500 I1500 V220 I220 Table 4: Wheatstone Bridge R1 R2 R3 R4 I XY 100Ω 110Ω 200Ω 220Ω 100Ω 110 Ω 200Ω 400 Ω 100 Ω 110Ω 100Ω 220Ω 150Ω 100 Ω 75Ω 50Ω Table 5: Instrument Loading Original Circuits Circuits with voltmeters added VA VB Your Name Partner Name Lab Instructor Date Page 5 of 4