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ECE 220 PSpice 9.2 Tutorial and Assignment
Voltage & Current Divider Circuits, Bridge Circuits
A. VOLTAGE & CURRENT DIVIDERS
Ohm’s Law and Series Resistance Background : Ohm’s Law says that voltage V, resistance R and current I are related according to V=IR. Read Nilsson p. 31.
Combining Series Resistors In a series circuit all components (resistors, sources) carry the same current. Hence, in Fig. 1A all the voltage drops are the same, because all the resistors have the same value (1kΩ). If we put a box around the top three resistors and pretend we don’t know what is inside, and likewise put a box around the bottom two resistors, we obtain a circuit with two black boxes accessible to measurement only through the voltage v (^) DC=5V and the voltages at the nodes OUT 1 & O UT2. The circuit of Fig. 1A becomes that of Fig. 1B. Using only current and voltage information, we cannot distinguish between the circuits of Fig. 1B and Fig. 2. In other words, the combined 1Ω resistors of R5, R6, and R7 are equivalent to the 3Ω resistor R10, or R10 = R5+R6+R7. That is, the equivalent resistance of resistors in series is the sum of the individual resistors. The result is a consequence of Ohm’s law.
5V
Figure 1B
5V
Figure 1A
5V
Figure 2
Voltage Division From Fig. 1A it is clear that the voltage at node O UT2 will be twice that of one resistor, because all the resistors are the same. Likewise the drop between nodes OUT2 and O UT1 will be three times that of one resistor, and the total drop at O UT 1 will that of five resistors. Putting the black boxes around the resistors as in Fig. 1B doesn’t change these results. Thus, we see that in Fig. 2 V(OUT4)=V(O UT3) [R11/(R10+R11)]. In words, the voltages between pairs of nodes in a series circuit are in proportion to the resistance values between the nodes. This rule of voltage division again is a consequence of Ohm’s Law.
PSpice We now will use PSpice to generate circuits like those of Figs. 1 and 2. Starting a New Project To open a new project using PSpice 9.2 go to the START menu, go to PROGRAMS, go to ORCAD FAMILY RELEASE 9.2, and select CAPTURE CIS. This will bring up a blank text editing document and near the top of the screen it says “SESSION LOG.” In this screen go to the FILE menu, go to NEW and select PROJECT…
Figure 3: File menu in session log
This brings up another screen containing most of the information about the file. You must specify a name, a location for the file and you must select the type of simulation you want to do. ALWAYS select ANALOG OR MIXED A/D (This ensures that PSpice will function properly).
File Name
Figure 4: New project window
Location of File
Once you have selected a name a location to save your file click OK. A small screen will appear asking you if you want to create a project based on a hierarchal design or if you wish to create a blank project. Select CREATE A BLANK PROJECT and click OK. Now you are in PSpice and you can start drawing your circuit.
Placing Parts In PSpice 9.2 there are usually at least three ways to do everything: you can use the pull-down menus, you can click on the icons on the top and the right hand side of the screen, or you can use keyboard shortcuts. To see what function any of the icons perform, you can slowly move the mouse pointer over a particular icon without clicking the mouse. A small box will appear describing that icon’s function or text will appear in the dialog box at the bottom of the screen. I will describe how to draw circuits using the drop down menus. Many of the shortcut keys are listed in the menu next to the command. To start drawing the circuit, go to the PLACE menu and select PART. This will bring up the following dialog box.
and NAME ONLY will display DC. Play around with the settings to adjust the display to one that suits you. You can double-click on V1 to change the name (called the part reference) of the voltage source in the same way you selected the value for DC. Note that if you choose to display NAME AND VALUE, Part Reference =V1 will appear in the schematic. Please notice that PSpice has some standard abbreviations for the values of components: Unit PSpice femto f pico p nano n micro u milli m kilo k mega meg giga g tera t The units that are going to be trouble are the use of “u” for “μ” and the use of “meg” for “mega”. PSpice is not case sensitive, so if you type “M” for “mega” instead of “meg”, the value you will get is a “milli”, not a “mega”. Also, PSpice doesn’t care what you type after the unit designation, so you can type mA for millampere, or GF for gigafarad. However, if you type 1F for the value of a capacitor, you will get a 1 femto Farad capacitor!
Now try double-clicking on the source itself. This should bring up the PROPERTY EDITOR SPREADSHEET (this can also be accomplished by selecting the source, clicking the EDIT menu and selecting PROPERTIES).
Figure 7: Property Editor Spreadsheet
This shows a spreadsheet of all of the properties of the voltage source. Here you can change the color, the value (located under the heading DC), and the name (located under the heading Part Reference). There is a lot of other information listed (you can scroll right to see more) that deals with the graphic display and different simulation parameters. Leave these alone. This is a way to edit multiple properties at once and it can be useful when using parts that have many properties (like pulsed voltage sources). To close this window click on the lower X in the top right corner. To place resistors on the schematic you can go to the PLACE menu and select PARTS again or you can just press P on the keyboard. In the parts list a resistor is called R. It is useful to know that you can zoom in or out on the schematic by pressing I or O on the keyboard (you can do this while placing parts). You can also go to the VIEW menu and go down to ZOOM. There are several zooming options: IN and OUT are self explanatory, SCALE will scale the area on the screen by a specified percentage, AREA will allow you to click and drag a box over the area you wish to see on the screen, ALL will fit the entire page onto the screen, and REDRAW redraws the screen and cleans up additional marks on the screen that are not intended to be there. You can also rotate and flip parts as you place them or when they are selected. If a part or part parameter/value is selected, you can always click and drag it to anywhere on the screen. To rotate or flip a part you can go to the EDIT menu and select MIRROR or ROTATE (you can also use R for rotate, H for horizontal mirroring, and V for vertical mirroring). Also notice that all parts in PSpice have an indicated voltage direction. PSpice obeys the passive sign convention in that current flows from the + terminal of a device to the – terminal. This is important because DC current values are displayed at the pins of devices. If a current of 1.2mA is displayed at the + terminal of a resistor, 1.2mA is flowing into the positive pin of that resistor. If the value is – 1.2mA, that means 1.2mA is flowing out of the positive pin.
R 1k Positive current is assumed to be flowing in this direction. It is important that you orient all of the parts in your schematic in the correct direction so that your results conform to the expected directions of current flow.
Wiring Circuits To wire your circuit you can go to the PLACE menu and select WIRE, type W, or click the icon on the right. In wiring mode you click on a part pin to start a wire. You can move the mouse (without dragging) to the point at which you want the wire to end and click again. If the wire connects two or more parts a red dot will appear where the connection will take place. To exit wiring mode press ESC or right click and press END WIRE.
Labeling Nodes Another useful function of PSpice is the ability to label nodes, or particular points of your circuit. Several parts can be used to name a node. One way to name a node is to place a net alias. Go to the PLACE menu and select NET ALIAS. Name the net alias and press OK. A small box will appear on the cursor. Move the mouse until the box touches the point in your circuit that you want to name. Click the mouse to place the label. Another way of naming nodes is to use the power symbols. Go to the PLACE menu and select POWER. You can choose any part that starts with VCC. Connect this part to your circuit at the node and name it as you would any other part. The third way to name a node is to use an off-page connector. Go to PLACE and select OFFPAGE CONNECTOR. Place this and name it as you would the power symbol. An important thing to know about naming nodes is that two nodes with the same name are considered to be the same node. This is useful for connecting nodes on separate pages (usually done with offpage connectors), and for placing voltage sources outside of a circuit (usually done with the power connectors). As an example I have shown a circuit with 3 voltage sources, one connected with a power connector, one with an offpage connector and one with a net alias.
VCC
0
Vg DC = 10V
Isource
0 VCC
I
Vin
Vin
R
1k
DC = 1mA
R 1k
R
1k
0
R
1k
Vx DC = 3V
R 1k
R
1k
0
Power
Same voltage
Isource
Off-page connector
Net Alias
Figure 8: Example of Node Labeling Ground symbol
All of the labeled nodes in figure 8 are at the same potential. PSpice also treats this as one circuit when it does its analysis. In the figure 8, the node labeled with the net alias Vin has a voltage that is determined not only by the small loop it is contained in but also by the circuit on the right that it is connected to through the use of node naming. Naming nodes also helps for identification purposes when you do graphical simulations.
Grounding Another peculiarity of PSpice is that every circuit requires a ground connection. Go to the PLACE MENU and select GROUND. Choose the part called 0 (it may look like 0/Source ).^1 This is the only part that will work as a ground. Pick one node to be ground in your circuit. You can see in figure 8 that there are actually four nodes labeled with ground. All four are connected in the same way the labeled nodes are connected. This is necessary to connect both ends of the circuit elements to the resistor loop on the right. Consider the independent current source I1 on the bottom. The top of this source is connected to the off-page connector Isource between R and R4 and the bottom of this source is connected to ground (the wire at the bottom of the resistor loop). Note: I could have just as easily put a power connector called VCC at the bottom of the Current source, which would have connected it to the node between R2 and R3. PSpice will allow you to do this just as long as there are no floating nodes (nodes not connected to anything), and every independent circuit on the diagram has one ground node (if two resistor loops had no common nodes they would be independent and hence each need a ground).
(^1) If this is missing, click on ADD LIBRARY, and go to c:\Program Files\ OrcadLite\Capture\Library\PSpice and
add the "SOURCE" library.
To display currents and voltages on the schematic click on the V and I icons near the top of the screen. It’s best to do these one at a time because they tend to get cluttered. To remove a particular voltage or current label (like the 0V label at the ground node), click on the label so it is selected and click on the icon directly to the right of the I or V icon.
Select a current label and click here to remove it
Click here to display all curents
Select a voltage label and click here to remove it
Click here to display all node voltages
The two icons used to remove selected voltage labels and current labels can also be used to display current and voltage labels once they have been removed. For example, if you have removed the current label for the current flowing into the positive pin of a resistor you can click on that positive pin (or the resistor itself) to select it and then click on the small I icon to display the current again. Note that you can only display the current going into the positive pin of a device.
Common Errors It is easy to make mistakes when first learning PSpice 9.2. If you make an error drawing your circuit the simulation won’t run. If green washers (circles) appear on your schematic then you have made a drawing mistake. Double-click on the washer to see what the mistake is. Other mistakes might be indicated in the probe window. To avoid common mistakes always double check your circuit to make sure that it has a ground, make sure there are no floating nodes (both pins of every device are connected to something), check to make sure you don’t have two parts on top of each other, and make sure you haven’t drawn a wire through a part.
Exercise 1: With this background
- Select the parts for Fig. 2 and place them as shown in Fig. 2. Out 1 and Out 2 are just node labels that can be made using the power connector VCC and changing the name.
- Set all component values.
- Next, wire the circuit.
- Create a new simulation profile, run PSpice, correct any errors you find, and display the voltages shown in figure 2.
- You are not required to submit anything on paper for this exercise.
Exercise 2: Write a description of the use of Ohm’s Law in determining the behavior of parallel resistors and
current division that is an exact analog of the description of series resistors and voltage division at the start of this handout. Use PSpice to draw circuits that are analogs of Figs. 1 and 2. As a hint, the circuit analogous to Fig. 1B is shown below. The branch labels B1 and B2 are made by using net aliases for the wires below L1 and L2.
1.000V
R 1k
1.000mA
L 10uH
3.000mA
I 5.000mADC = 5mA 1.000V
0
R 1k
1.000mA
L 10uH
2.000mA
R 1k
1.000mA
0V
R 1k
1.000mA
1.000V
Figure 11: Current Analog of Figure 1B
You will notice that there are two new parts in this picture: L1 and L2. These are inductors, which can be found in the place part menu under the name L. For DC current, inductors are the same as plain wires. (The values chosen for L1 and L2 have no effect on the circuit.) The only reason L1 and L2 are in this circuit is to inform us of the current going through B1 and B2. Pspice will not display currents that travel through wires but it does display currents that go into devices. Since an inductor is a device that acts like a wire for DC currents, I used it to “trick” PSpice into giving me the current through wires B1 and B2. In the real circuit there is no need to use inductors, this is only used here to fool PSpice.
Submit one sheet of paper with: Description (see first sentence of this exercise). PSpice simulation for the current divider analogous to Fig. 2, with appropriate currents and voltages labeled (similar to Fig. 2). You probably won’t need the inductor trick.
Final Notes: You can re-size the area seen on your monitor by drawing a box around an area you want to see in more detail and using ZOOM/AREA. Then the boxed area will fill the screen. The same trick works for printing: box the area you want to print and check the box on the FILE/PRINT menu for ONLY PRINT SELECTED A REA.
B. BRIDGE CIRCUIT
Background: To understand how a bridge is used, read Nilsson p. 77. In this session we will learn how to analyze the bridge circuit of Fig. 1. We already know how to set up this circuit, but the value of R has been chosen as {R}. The curly brackets indicate that the value of R4 is a variable “R”. When we choose to use a variable in PSpice, we also have to introduce the part PARAM found by typing this name into the PLACE/PART window. Once P ARAMETER is pasted on the schematic, we double click on it to get the property editor spreadsheet in Fig.
- Click on NEW COLUMN and enter the variable name R (without the curly brackets). You must enter a value for R (doesn’t matter what value). This value will be used in the analysis of this circuit when a sweep command (discussed next) is not used to change the value of R. Here a sweep will be used. Once you have selected a value for R click OK.
Figure 1: Bridge Circuit
Click here
You now have to display your new variable under the parameters icon. Do this by first clicking on the R heading to select it, click on DISPLAY, and then select NAME AND VALUE.
Figure 2: Property Editor
- Click here
- Click Here
- Click Here
Figure 3: Using Property Editor to Display New Parameter R
I’ve selected the markers called VOLTAGE DIFFERENTIAL to make PROBE plot the difference in voltage between nodes A and B. With these preparations concluded, we select PSPICE/RUN and the plotting engine of PSpice produces the graph shown in Fig. 7.
Figure 7: Probe Window for DC Sweep
Click this icon to make the graph full-screen
To make the graph larger click the icon at the far right on the menu bar or go to VIEW/ALTERNATE DISPLAY. Note that the graph is automatically printed on a log scale. We want to find the point at which this curve intersects 0 so we can first zoom in around that area to get a better look. I used the click and drag method to box the area shown below and then I went to VIEW/ZOOM/AREA to get the picture in Fig. 8.
Figure 8: Zoomed in View of DC Sweep Plot
To proceed further we use the TRACE/ ADD TRACE menu choice in PROBE to get the A DD TRACES menu below (Fig. 9). In the TRACE EXPRESSION box we enter “0” to get a trace y=0 (the x-axis). This trace makes it easier to see the zero crossing. Then we select TRACE/CURSOR /DISPLAY as shown in the menu of Fig. 10.
Figure 10: Displaying Cursor
Figure 9: Add Traces Menu
You can move the cursor by clicking on a point on the graph or using the arrow keys on the keyboard. The coordinates of the point where the cursor is are shown in the small PROBE CURSOR box. The top two points are the coordinates. Once you have positioned the cursor on the intersection go to the PLOT menu, select LABEL and then select MARK to mark the point the cursor is at on the graph. Also notice that you can use PLOT/LABEL/TEXT to add a text box anywhere on the screen for descriptive purposes.
Figure 11: Using the cursor and labeling points
Probe cursor Box
Cursor Coordinates
Labeling the point where the cursor is located
I found the intersection to be at 2.402K as shown in the probe cursor window above. To check our results, we can go back to the schematic and double-click on the value of R in the PARAMETER box. We change the value of R to 2.4k, the zero-crossing value. Now go to the PSPICE menu and select EDIT SIMULATION PROFILE. Under ANALYSIS TYPE change DC SWEEP to BIAS POINT. Click OK and then back in the main schematic go to PSPICE/RUN again. Now you can display the currents and voltages on your schematic like you did in Part A. The result is shown in Fig. 12.
R {R}
2.778mA
6.667V
R 3k
2.222mA
PARAMETERS: (^0) R = 2.4k
R 1.2k
2.222mA
0V
10.00V
V 10V 5.000mA
Figure 12: Circuit with new value of {R}
From Fig. 12 it can be seen that the voltages on nodes A and B are equal to the accuracy displayed on the screen. We also can read the currents in the two branches.
