7 Lesson Computer-Aided Analysis (Section 2-7)(CLO 2-7) Practicing engineers routinely use these computer tools to analyze and design circuits, and so it is important to learn how to use them effectively. The purpose of doing computer examples is to help the student develop an analysis style that includes the intelligent use of computer tools. As they develop their style, always keep in mind that computer tools are not problem solvers. The student is the problem solver. Computer tools can be very useful, even essential once the problem has been defined. However, they do not substitute for an understanding of the fundamentals needed to formulate the problem, identify a practical approach, and interpret analysis results. Have the students follow the solution on their laptops. Use OrCAD PSPICE to find all of the voltages and currents for the circuit below.
VA 30Vdc R3 300
R1 100 R2 200
0
The main steps required to solve this problem using OrCAD are as follows: 1. Create the circuit schematic using OrCAD Capture. 2. Set the simulation profile parameters. 3. Simulate the circuit and record the results. Create the circuit schematic by placing the appropriate parts (resistors, a dc voltage source, and a ground the 0/source) onto the workspace. Adjust the values of the resistors and the voltage source to match the problem statement. This is done by clicking on the component values and opening the Display Properties window. Connect the parts using the wiring tool. Under the PSpice menu option, create a new simulation profile and select the bias point analysis, as shown in the figure below. The bias point analysis will determine all of the dc voltages, currents, and powers associated with the circuit. Select Run PSpice the blue triangle to run the simulation. OrCAD allows you to display directly the results of a bias point simulation on the schematic by selecting the V and I buttons on the analysis bar. Select these two buttons and then simulate the circuit to produce the results shown in the following figure.
Interpreting the results from the simulation, we have v1 = 10 V v3 = 30 V i1 = 100 mA v2 = 20 V vA = 30 V i2 = 100 mA
i3 = 100 mA
iA = 200 mA
We now consider how a math solver MATLAB can be used to solve the same problem.
Use MATLAB to find all of the voltages and currents for the circuit above. We start by writing the equations describing the element and connection constraints. v1 = 100 i1 v2 = 200 i2 v3 = 300 i3 vA = 30 iA i1 i3 = 0 i1 i2 = 0 vA + v3 = 0 v3 + v1 + v2 = 0
We must now define a vector of parameter values, x = [v1 v2 v3 vA i1 i2 i3 iA]T, where the T superscript is the transpose, making x an [8 1] column vector. Re-write the equations so that they can easily be expressed in matrix form. The extended spacing in the equations below is intentional. 10 0 i1 v2 20 0 i2 v3 vA i1 i1 v3 v1 + v2 v3 vA i2 i3 iA 30 0 i2
v1
=
0
=
0
= = = = = =
0 3 0 0 0 0 0
We can now write these equations using standard matrix notation, as A x = b, where A is an [8 8] matrix and b is an [8 1] vector.
1 0 0 0 0 0 0 1 0 1 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 0 0 1 0 0 1 0 100 0 0 0 200 0 0 0 300 0 0 0 1 1 0 0 0 1 0 0 1 0 0 0 0 0 0 0 1 0 0 0 v1 0 v 0 2 v3 0 v A = 30 i1 0 i2 0 i 0 3 iA 0
We can enter this information into MATLAB as follows: A = [1 0 0 0 -100 0 0 0; 0 1 0 0 0 -200 0 0; 0 0 1 0 0 0 -300 0; 0 0 0 1 0 0 0 0; 0 0 0 0 -1 0 -1 -1; 0 0 0 0 1 -1 0 0; 0 0 1 -1 0 0 0 0; 1 1 -1 0 0 0 0 0];
b = [0; 0; 0; 30; 0; 0; 0; 0]; We can now solve these eight equations for the eight variables with the single MATLAB command: x = A\b The results are: x= 10.0000 20.0000 30.0000 30.0000 0.1000 0.1000 0.1000 -0.2000 With MATLAB, we were able to simultaneously solve for all of the unknown variables: v1 10 v 20 2 v3 30 vA = 30 i1 0.1 i2 0.1 i 0.1 3 iA 0.2 All of the voltages and currents agree with the solution found using circuit simulation software in above. Comparison of Computer-Aided Techniques We applied two computer-based tools to solve the same circuit analysis problem. Both tools efficiently arrived at a complete and correct solution and may offer some advantages over the manual approaches presented in the remainder of the chapter. Students may wonder why two tools which is the better one to use? The answer is not intuitively obvious. As they explore these tools, they will develop the experience and judgment to know when each tool is appropriate to assist in solving a problem. The circuit simulation tools allow for visual representations of the circuits and efficiently compute numerical solutions when all of the parameter values are known. A math solver quickly manipulates a set of equations to find a solution, but they must relate the numerical results back to the original schematic to get a full representation of the answer. Math-solver software can offer distinct advantages over circuit simulation software when the circuit contains parameter values expressed as variables. The circuit simulation software cannot directly handle this type of the problem, but the math-solver software accommodates it with only minor changes, as we will explore later. As we develop circuit analysis and design techniques throughout the course, we will continue to emphasize the advantages and disadvantages of the various computer-based tools so that the student can expand their judgment, and efficiently apply the appropriate tools for each problem. No matter what technique one uses, good engineering sense must predominate. Results that just do not seem right should be challenged and verified. As the students grow in knowledge and experience, they will be developing and enhancing this important engineering quality. The following Computer Exercise can be done in class or given for homework.
Computer Exercise #1
An Introduction to Circuit Simulation Objective This computer exercise introduces you to how circuit simulation is performed using commercially available software. You will learn how to create circuit schematics, run DC simulations, and display results. You will simulate two circuits, and compare your results with those you have calculated by hand. NOTE: Instructions for the installation of OrCAD 10.0 Demo, the software simulation package you will use for this exercise, are available at the course website. Instructions for getting started using OrCAD are on pages 2 through 4 of this handout. Some helpful (time saving) schematic editing hints explain how to place multiple circuit schematics on a single schematic page (a requirement for this exercise). Circuit Simulations for this Exercise
Rs
Vdc
RL
Id c
Rp
RL
0
0
Figure 1.
Circuit #1
Figure 2.
Circuit #2
Circuit Simulation #1: In Figure 1 above, Vdc = 6 V, RS = 13.3 k, and RL = 6.8 k. On a separate sheet of paper, calculate the voltage drop across and the current through RL; also, determine the power supplied by Vdc and the power dissipated in RS. Then, using OrCAD 10.0, draw the circuit schematic shown in Figure 1. Simulate your circuit and compare the results to your hand calculations. Circuit Simulation #2: On a separate sheet of paper, use a source transformation to change Vdc and RS into a current source, Idc, in parallel with RP. Calculate the voltage across and the current through RL; also, determine the power supplied by Idc and the power dissipated in RP. Then, using OrCAD, simulate the circuit you designed. Compare the results to your hand calculations.
Basic Instructions for the Use of OrCAD Capture 10.0 for Analysis of Resistive Circuits with DC Sources Using the Simulation Software OrCAD Capture is based on a public domain software engine called PSpice. In days of old, users of the software had to create a file that explains to the computer how the circuit is connected. This file was called a netlist. OrCAD Capture still computes this file prior to running the simulation. It will be useful to check this file before running a simulation to see if the circuit was wired correctly.
Start a new project
o From Windows go to Start, Programs, OrCAD 10.0 Demo and launch the Capture CIS
Demo program. (You may want to create a shortcut on your desktop for this program.) Place o In the OrCAD Capture window that opens, choose menu selection Parts File/New/Project, type in a name for your project, enter a location for your project file (e.g. C:\OrCAD\OrCAD_10.0_Demo\EE231\), Draw Wire and select Analog or Mixed A/D. o To create a new project, in the Create PSpice Project window that opens, select Create a Blank Project and click OK. o A schematic window with a grid will appear clicking anywhere on that grid will generate drawing icons on the right of the OrCAD Capture window. GND Icon
Create a schematic o To analyze DC, resistive you circuits need access to a DC voltage source (VDC), DC current source (IDC), and resistors (R). To make those parts available you must first load the libraries that contain those parts. Click on the OR-gate icon in the list of drawing icons (second icon from the top on the right side of the OrCAD Capture window) to open the Place Part window. In the Place Part window, click on Add Library, select ANALOG_P (contains R) Drawing icons and SOURCE (contains VDC and IDC) libraries (hold CTRL key for multiple selections), and Open them. Note: if the standard ANALOG library is not shown, add it as well. o To select and place a part, you can go to the Place menu at the top of the page and select Part or simply click on the OR-gate icon on the right side of your screen. The window that opens contains available libraries in which models of parts are stored. Check out each library for what they contain. Select the library(s) that contains the part(s) you want, select the part, click OK, then drag-&-drop the part onto the Schematic window grid. You may place as many of the same kind of part as you like by left-clicking where you want the part. Right-click and select End Mode to stop placing a selected part (which also gives you the opportunity to select a different part). You can orient parts by using Rightclick menu options.
o
o
o o
o o o o
o
Note that once a part is placed on the schematic it is entered in the rectangular window located in the center of the menu bar at the top of the screen. To reuse a part, you can just click the windows drop down arrow and select the desired part instead of using the Place Part window. Set part values by double-clicking on the parts value box, then entering the desired value in the Display Properties dialog-box value field. Do not place units like VDC in the parts value box, e.g. 5 not 5VDC. Leave no spaces between numerical values and multipliers, e.g. "5k", not "5 k". Create wires using the wire icon (the icon right below the OR gate icon in the OrCAD Capture window) to interconnect your circuit components. Place the cursor where you want the wire to start, click the left mouse button, move the mouse to draw the wire, left click again to change direction or to end the wire. You know you have made a connection when a red dot appears at the connection point. Simply repeat this left clicking procedure to draw multiple wires. To exit the wire tool, right click and select End Wire. CHECK TO ENSURE THAT YOU DID NOT DRAW WIRES THROUGH THE DEVICE. If you accidentally did, the part would be shorted out. You can verify a device is connected by selecting that device with your mouse and dragging it. If the wires remain connected, you have a connection. To delete a wire, left click the wire with the arrow cursor (arrow icon above the OR gate) and hit the Delete key. Before you can run a simulation, you MUST add a ground (the circuit will not simulate without one). Click on the GND drawing icon on the toolbar on the right in the OrCAD Capture window. In the Place Ground window select the 0/Source ground then drag, drop, and connect it to your circuit schematic. If the 0/Source ground is not available, choose Add Library, open the PSpice folder, select the source library, and click Open. Be sure to highlight all libraries when you return to the parts library. Place the ground in your circuit to reflect the node you wish to be your reference. Put the projects title and your name in the Title block located at the extreme lower-right corner of the schematic. Double-click on <Title> to edit. Use the Place Text drawing icon (bottom icon on the list of drawing icons in the OrCAD Capture window) to add descriptive text anywhere on a schematic. If necessary, relocate part values and names so they are not obscured by their associated schematic symbol. (Simply grab-and-drag.) Do not leave any component hanging. OrCAD does not recognize an open circuit as an element. If you ever need to consider an open circuit, simulate the open circuit using a very large resistor like 100MEG across the desired open circuit. To simulate a short circuit, use a small resistor like 0.01. You are now ready to check to see if you have built the circuit correctly. Click on the PSpice menu and select Create Netlist. The software checks that you have no major errors. If it flags that you have netlisting errors, something is not correctly connected. If you forget a ground it will issue a warning. You should ask to View netlist (also under the PSpice menu) and see if you can detect the error.
Prepare to run your simulation. o Click PSpice on the top menu on the screen. Select New Simulation Profile. o In the New Simulation dialog box that opens, give your profile a name. (It can be the same as your project name.) o Click on Create. In the Simulation Settings window that appears, choose the Analysis tab. Under Analysis Type select Bias Point. (This option calculates DC voltages and currents in the circuit during simulation.) Click OK. Run your simulation and display the results o Click on the blue play (sideways triangle) icon located in the 2nd row of icons at the top of the OrCAD Capture window. (A graphing window will open that is of no interest for DC analysis, -- close it.) o Click on the V and I and P icons in the 2nd row of icons at the top of the OrCAD Capture window to display simulation results (voltage, current, and power values) directly on the schematic. Note that all voltages are measured from the node of interest to the GROUND node. (If necessary, reposition the resulting values so they are not obscured by other items on the schematic -- simply graband-drag.) o Do not forget to periodically save your work (Disk icon or Save under the File menu). Opening an existing project that was saved
o From Windows go to Start, Programs, OrCAD 10.0 Demo and launch the Capture CIS
Demo program.
o In the OrCAD Capture window that opens, choose menu selection File/Open/Project, enter
the location for your project file, select the project file to edit, and click OK. o In the window that opens, select the File tab, then follow the directory tree that begins with folder Design Resources until you get to the schematic page of the project you want to edit. Double-click on the appropriate page to open it. Some helpful (time saving) schematic editing hints o You can duplicate circuits by using the select icon (arrow icon at the top of the drawing icons in the OrCAD Capture window) to drag a selection box around a circuit (or portion of a circuit) you want to duplicate. Use the Copy and Paste selections from the right-click menu to complete the duplication. NOTE: If you duplicate a circuit on the same page of a schematic as the original, you must change all the component names (ex: R1 => R1a, V1 => V1a, etc) so you can run a simulation. Simply double-click on any components name to open the dialog box that will allow editing.
Computer Exercise #1
An Introduction to Circuit Simulation Using PSpice Turn-in Requirements 1. Attach a printout with both circuits on a single schematic with the simulation resultssimulated voltages and currents and powers should be displayed where appropriate. NEVER handwrite on a schematic or printout. Use the label function to write or draw on computer outputs. 2. Answers to the following questions. Include any sheets with your hand calculations (be neat!) 1. Refer to Circuit #1. Write in the results of both your hand calculations and simulation: Hand Calculation #1: VRL = ___________ Hand Calculation #1: IRL = ___________ Simulation #1: VRL = ___________ Simulation #1: IRL = ____________
2. Refer to Circuit #2. Write in the results of both your hand calculations and simulation: Hand Calculation #2: VRL = ___________ Hand Calculation #2: IRL = ___________ Simulation #2: VRL = ___________ Simulation #2: IRL = ____________
3. Are there any differences between the calculated values of VRL when using the Circuit #1 versus the Circuit #2? Why or why not?
4. Refer to circuit #1 and circuit #2. What is the power supplied by each source and the power dissipated in RS or RP? Circuit #1: Psource = __________ PRS = __________ 5. Are the powers the same for both circuits? Why or why not? Circuit #2: Psource = __________ PRP = __________
Grading Criteria (Schematic correct, values/names/results not obscured, descriptive title for each circuit, your name and exercise title in title block, correctly solved hand calculations, and reasonable explanations)
Item Title Block Filled In (Your Name and Title) Hand Calculations Circuit #1 Schematic Labeled and Simulation Values Circuit #1 Hand Calculation Circuit #2 Schematic Labeled and Simulation Values Circuit #2 Answers to Questions 3 and 5 TOTAL
# Points 10 20 15 20 15 20 100
Points Earned