ECE 3310

Electrical Circuits: Laboratory

Multisim (Student Version)

©2008 by Professor Mohamad H. Hassoun

 

Multisim is an electronic circuit simulator supplied by National Instruments (NI). It runs as an application on your PC. The student version of NI Multisim is required (buy it on-line) for this course. A 30 day free trial version can be downloaded from the NI website.

 

You may think of Multisim as a virtual laboratory. You will mainly use this simulator in pre-lab and post-lab activities. The results of your simulations represent the ideal circuit response. In your lab reports, you will use those results to benchmark the experimental results obtained from the actual physical circuit.

 

Resistive Circuit Analysis

Run the software by executing Multisim. The following user interface window will display. The blank region inside is the Schematics window.

 

 

 

Multisim software integrates two programs: A circuit schematics editor, and a circuit simulator. The schematics editor allows the user to build a virtual circuit by connecting various electronic components; basically, the user “draws” the schematic of the circuit to be analyzed. Let us assume that the circuit to be analyzed is the following simple voltage divider circuit. We are interested in all component voltages and currents.

1.    The above circuit has three components: Two resistors and one DC voltage supply. In addition, we must specify a reference (ground) node. From the Place Menu, select Component. This opens the Component Selection Menu. Alternatively, click (i.e., left click) on the   icon in the Components Toolbar  (if you do not see this graphic menu, select “Toolbarsà Components” from the View Menu).

 

     

 

2.    In the Component Selection Menu, set “Database” to “Master database” and “Group” to “Sources.” Next, select “Power_Sources.” This will list a number of power source components under the “Component” list; click on DC_Power.  You should see the symbol for a DC voltage source under the “Symbol” area. Click “OK” to place the symbol in the Schematics window. Now, as you move the mouse cursor over the Schematics window, you will see the symbol of a DC voltage source tracking the mouse. Click the left button of the mouse to insert the component. The last inserted component can be removed if you select Undo from the Edit Menu. Alternatively, a part can be removed if you select it by clicking its symbol (a dashed box is placed around the symbol) and then selecting Delete from the Edit Menu, or by pressing delete key on the keyboard. The default name for the first inserted DC source is V1. The second inserted source name would be V2, and so on. The default value of the source is 12V. The position of the name label (and value label) can be relocated by clicking and holding and then dragging the label. Similarly, the whole symbol can be repositioned by clicking and holding the left button of the mouse (at the center of the symbol) and then dragging.

 

 

3.    You may now edit the voltage value of the inserted DC source by double clicking its symbol. This will open the POWER_SOURCES window. Type the desired voltage value (5V in this case) and click “OK.” The name of the supply can also be edited under the “Label” tag in this menu.

 

 

 

4.    Repeat Steps 1 and 2 for the two resistors (here, select the “Basic” Group and the “Resistor” Family of components). The filter parameters in the Component Selection window should be set to W and 5% for the 470W resistor, and kW and 5% for the 1KW resistor. The filter allows for faster location of the specific component. The inserted resistors names will automatically be set to R1 and R2, respectively.

 

 

The Schematic Window should now look as shown below.

 

 

Note: Multisim supports the following (case sensitive) scale factors for component values: T (tera) = 1012, G (giga) = 109, M (mega) = 106, k (kilo) = 103, m (milli) = 10-3, m (micro) = 10-6, n (nano) = 10-9 and p (pico) = 10-12.

 

5.    The displayed symbol for an inserted component can be rotated. Select R2 (by left clicking the mouse on the resistor symbol), then select 90 Clockwise from the Edit Menu. Alternatively, right click the resistor and select 90 Clockwise from the automatically displayed menu.

 

 

 

6.    Every simulated circuit must have a reference (ground) node. Open the Place Menu and select the “GROUND” component from the “POWER_SOURCES” Family, under the “Sources” Group (refer to Steps 1 and 2). Click OK to close the menu. Click the mouse inside the Schematics window to insert the ground symbol.

 

 

 

7.    The last step in drawing the circuit schematic is to interconnect the components. Select Wire from the Place Menu. The mouse cursor changes to a “+” cursor. Point the cursor to the upper tip of the DC source and click. Then, click the mouse at the left terminal of R1. This inserts a wire connecting the DC source to R1. Repeat this process to finish connecting the remaining components together, according to the given circuit topology. Right click the mouse upon completing the circuit wiring; this, deactivates the Wire Tool. Now, try dragging a component and observe how the position of the connecting wires change. A wire segment can be repositioned by dragging (left click, hold and move) it with the mouse.

 

 

8.    The final schematic editing step is to define a voltage reference node for the circuit. This can be accomplished by dragging the ground symbol towards the bottom wire. As soon as the upper tip of the ground symbol touches the wire, it automatically gets connected to the circuit. The circuit position can be changed by using the mouse to select the whole circuit (all components and wires will be highlighted); then click and hold the center of any component and drag it. This will reposition the whole circuit.

 

 

9.    A virtual Multimeter instrument is available in Multisim. Double click the  icon, located in the Instruments Toolbar on the right hand side of the Multisim interface (if you do not see the Instrument Toolbar, you can display it by selecting “Instruments” from the ViewàToolbars Menu). Toolbars can be relocated from the side to the top Menu (and vice versa) by dragging the Toolbar at its base (i.e., at the Toolbar “” or “” shaped region). Double clicking the  icon pastes the multimeter symbol () in the circuit area. Double clicking this symbol opens the Multimeter window. The user can set the multimeter mode to Voltage, Current or Ohm measurement and to DC or AC signals. Select DC Volts as shown below. The multimeter is now a voltmeter which is set to measure DC voltages across a circuit component.

 

 

     Connect the Voltmeter across R2 as shown below.

Insert a second Multimeter by clicking on  and then clicking a location next to the circuit where you would like to insert the Multimeter. Configure the Multimeter for DC current measurement (Ammeter). Insert the Ammeter in series with R1 to measure the loop current. To do that, the wire connecting the DC source and R1 must be deleted first (click on it and then press delete). Use the Wire Tool to connect the left (+) terminal of the Ammeter to the DC source. Next, connect the (-) terminal of the Ammeter to the left terminal of R1. The resulting circuit should look like the one below.

 

 

10.Now, you may simulate the circuit. Choose Run from the Simulate Menu or click on    the “1” on the switch symbol . The simulation results are shown in the figure below.

 

 

The voltage across R2 is 3.401V (which can be confirmed using the voltage division formula: VR2 = V1*R2/(R1+R2) = 5*1000/(1470) = 3.401V. The voltage across R1 can be computed as the difference between its node voltages (i.e., 5 – 3.401 = 1.599V, with “+” polarity to the left of R1). The Ammeter displays the value of the loop current. Since all components are connected in series, they all share the same current of 3.401mA. We can verify the value of this current by applying Ohm’s law (V = R*I) for resistor R2. Here, I = VR2/R2 = 3.401V/1000W = 3.401mA.

 

Note: The simulation must be terminated (press the “0”on ) before you can make changes to the existing circuit.

 

Exercise

Employ Multisim to find and display all resistor voltages and branch currents, in the following resistive DC circuit.

 

 

Transient Analysis

Consider the following RC circuit. Employ Multisim to find and plot the transient response of the capacitor voltage. Assume that the initial capacitor voltage is zero.

First, draw the circuit schematic and set component values. The capacitor can be selected by clicking  and then selecting “CAPACITOR” (and selecting its value). The switch component can be selected by clicking  and then selecting “SWITCH (SPDT).” Choose “Flip Horizontal” from the Edit Menu in order to change the orientation of the switch from  to . The circuit should look like the one in the following figure.

The SPDT (single pole double throw) switch can be toggled each time the user presses the space bar on the keyboard. When the switch in the lower position, it will connect the 5V source to the rest of the circuit. This  charges the capacitor. On the other hand, when the switch is in the upper position, it will result in a parallel RC circuit that discharges the capacitor.

 

The capacitor voltage waveform will be displayed on the (virtual) Agilent Oscilloscope. Double click the  icon in the Instruments Toolbar. This will insert the scope symbol in the schematics editor window.

Connect the scope’s Channel 1 to the upper terminal of the capacitor. There is no need to connect the scope’s ground to the circuit ground; the simulator sets that connection by default.

 

 

Now, double click the scope symbol to open and display the Agilent scope front panel. Press the ON switch to turn on the scope. Set the scope to display Channel 1 in 2V/div resolution, and set its time resolution to 2ms/div. Consult the Oscilloscope Unit for a tutorial on using the Agilent Scope.

 

Click on the switch symbol to select it. Toggle the space bar on the keyboard and observe how the switch position is changed. Leave the switch in the upper position (this, connects R1 to ground). Now, run the simulation. Watch the scope trace. The displayed waveform of the capacitor voltage is zero, as expected. As soon as the trace refreshes on the left hand side of the screen, press the space bar. This will connect the 5V source to the circuit and the waveform of a charging capacitor voltage shows on the display. As soon as the capacitor becomes fully charged, press the space bar again. This will disconnect the source and connect the resistor to ground. The resulting circuit is a parallel RC circuit, which will discharge the capacitor into R1. Here, the scope will display an exponentially decaying waveform. You can repeat the experiment by toggling the switch. Once you have a complete waveform of a charging/discharging capacitor voltage (as in the figure below), press the pause button () in the main menu.

 

Exercise

Employ Multisim to find and display (on the Agilent virtual scope) the transient response of the capacitor voltage in a series RC circuit with R = 1kW and C = 1mF. Assume that the input source is a square signal with minimum voltage of 0V, maximum voltage of 5V, 50% duty cycle and frequency of 60 Hz.

 

Use Multisim’s virtual Agilent function generator (AFG) to generate the square wave. The virtual AFG can be activated by simply double clicking on the AFG icon in the right hand side menu (refer to the figure below). This will insert the AFG instrument symbol (labeled XFG1) in the work area. Next, double click the inserted symbol to display the front panel of the virtual AFG. Refer to the Function Generator Unit for a tutorial on how to use the Agilent function generator.

 

     

 

 

Multisim also supports some generic menu driven instruments such as a generic function generator and a generic scope. The following is a screen capture of the solution to the above RC circuit driven by a square wave. The figure shows Multisim’s generic function generator and generic scope and their settings.