1 University of Essex School of Computer Science and Electronic Engineering Introduction to Computer Aided Design Multisim 8 A 'GETTING-STARTED'TUTORIAL for Analogue Circuits Nikolaos Thomos Contents 1 Introduction 2 Simulation exercise: halfwave rectifier Appendix2 1. Introduction 1.1 Overview MultiSIM is a CAD software used for electronic design and simulation. It is a complete design tool, which offers a large component database, schematic entry, and full analogue/digital SPICE simulations. MultiSIM offers a user-friendly feel to its graphical interface and, with a little bit of practice, it makes the design and simulation of circuits a very rewarding experience. This tutorial offers only a brief introduction to the basics of MultiSIM schematic capture and simulation capabilities – many issues will not be addressed and will be left to investigate through self-study. Online help is available in two places; in PDF format and within the MultiSIM environment. To access the PDF file, Start - Programs - Multisim 8 User Guide Within the MultiSIM environment, click on Help - MultisimHelp.The MultiSIM help option provides information about using the software, the MultiSIM reference option provides detailed definitions of the electrical components found within MultiSIM. Much of the information contained in this brief document is also found in the online documentation and many references to these sources will be made. You are strongly encouraged to experiment yourself on the workings of the simulation package - this is by far the best way to learn how it works and how best to use it. Appendix 1 contains advice and suggestions from Multisim users.3 2. Exercise: Getting started with Multisim In this first exercise, MultiSIM is introduced using an example of time domain analysis of a half- wave rectifier circuit. 2.1 Starting MultiSIM First create a folder, called multisim, on your M:\ drive to save your captured circuits. Then, to run the program go to the relevant directory area for the executable, or click on the relevant desktop icon. Certain toolbars may or may not be displayed at this stage. Figure 1. Typical MultiSIM workspace view4 Click on View - Toolbars to see which are already ticked. Methodically check and place each toolbar around the editing window. Move the toolbars so that they correspond to the layout shown above. In order to facilitate with the placement of components, it is wise to display a grid and to display the sheet size so that we can place our circuit centrally within the page limits. Click on Options –Sheet Preferences –Workspace. The "Sheet Size" screen appears asbelow, click OK. Figure 2. Properties setup screen. To display the page bounds we will show the title block and border as though we were creating a true engineering drawing. To do this click View - Show Page Bounds. This option should nowbe checked. (If we want to remove the extraneous border and title block, we can do this prior to printing the circuit) To display or remove a grid click View – Show Grid. A grid should appear or disappear in the editing window. To choose a symbol set (either ANSI or DIN) click Options –Global Preferences – Parts Bin and check the DIN option for now. When the editing window has a similar layout to that shown in Figure 1, you are ready to begin selecting and placing components.5 2.2 The Toolbars To use the MultiSIM interface efficiently, we need to know how each icon in the toolbars is used. To find out, move the cursor over top of any icon to see a basic description. The following pages, reproduced from the documentation, list most ofthe toolbars and their functions.67 Instruments Toolbars8 2.3 Choosing a Component - Real or Virtual Many components in the MultiSIM library come in two types - real and virtual - the difference being that a real component exists and could be purchased from a supplier, whereas virtual or 'generic' components enable arbitrary values and properties to be specified. Virtual components will be coloured differently to their real counterparts on the schematics. In this section we want to select a 10k resistor, which can be done in two ways. The first method uses the toolbars; the second method uses the application menus. Method 1 Move the cursor over the resistor icon in the Component toolbar: a new set of icons willbe displayed. Left click once when over the resistor icon and the Component Browser screen will appear, (as shown on the next page). Method 2 Place Component - Scroll through Component Family until resistor is highlighted andthe Component Browser screen will appear (as shown on the next page).9 Component Browser To choose a component, scroll through the Component List until the desired component is highlighted or, to speed things up, place the cursor in the Component List box, left click with the mouse and type in the desired component value. If the component exists it will be highlighted, otherwise MultiSIM will attempt to make a match. This match is not guaranteed to be the closest value that exists in the MultiSIM library and may be ignored. in the Model section of the browser, MultiSIM provides limited data for the highlightedcomponent. click on Detailed Report for a more comprehensive description of the highlighted component. click on Search to find a specific component parameter within the MultiSIM library. to choose a 10kΩresistor use one of the two methods described above. click OK to accept the highlighted component which can now be placed in the schematic. Figure 3. Component Browser10 2.4 Placing a Component This section will look at placing, moving, rotating, and changing a component. We have just selected our resistor and we now must place it on the schematic in the correct orientation. move the component in its skeleton form and place it by clicking the left mousebutton. to rotate the component 90° clockwise, place the cursor over it, right click – 90 Clockwise. We could have also chosen an anti-clockwise rotation or a horizontal or vertical flip. to move an existing component, place the cursor over it, left click once and hold, move the component in its skeleton form and place by releasing the button. Place a 10 kΩ resistor and a virtual diode as shownbelow. Figure 4. Diode and resistor placement11 2.5 Changing a Component's Value We already know how to choose a component's value using the component browser, but not all components have a browser screen. These components are selected in the same manner, but the skeleton form appears without giving you the choice of selecting a value. Place a 10 V, 10 kHz AC Voltage Source on the schematic. to change the default value, place the cursor over top, double left click - change the necessary values - OK. Place a GROUND symbol on the schematic. Figure 5. Signal source and Ground added12 2.6 Placing Input/Output Connectors Both input and output pins are placed in the same manner. To install an input/output, place the cursor in the editing window, right click - Place schematicHB/SC connector - left click to place a skeleton form. A default name and orientation will be given to the new pin. To change these characteristics follow the steps outlined in the previous section. Add connectors labelled Vin and Vout to the schematic as shown below. 2.7 Wiring the Circuit After all the components and the required input/output pins have been placed we are ready to wire the circuit to make junctions. Care must be taken when wiring the circuit because wiring faults are difficult to find in complex circuits and will lead to incorrect simulation results. MultiSIM will not catch missing wires or 'floating' components when the circuit is saved. For additional information refer to the Appendix, Wiring theCircuit. Wire the half-wave rectifier circuit as shown below and Save the circuit in your multisim folderon the network drive. Figure 6. Circuit wiring13 2.8 Analysing the Circuit In this section you will simulate the time domain behaviour of the half-wave rectifier - this is called Transient Analysis. To begin a simulation either, click Simulate - Analyses - Transient Analysis or use the Design toolbar and click on the appropriate icon followed by Transient Analysis Either method results in the TransientAnalysis screen where we must set certain parameters. Select Analysis Parameters and enter the values as below. Setting the End time to 0.3 ms means that the analysis will run for three cycles of the 10 kHz test frequency. Figure 7. Setting the Analysis Parameters14 Next we select Output Variables on the left-hand side there is a list of variables in the circuit, highlight the variables Vin and Vout and select Plot During Simulation To start the Transient Analysis click the Simulate button. Figure 8. Setting the Output variables15 Analysing the Graph - a TransientAnalysis On completion of simulation, a plot of the waveform will be shown in the Analysis Graphs window similar to the one shown below. Figure 9. Simulation waveforms If you want to keep the display, e.g. for a written report, use the ALT and Print Screen keys to capture the window and paste an image into a convenient image manipulation application, for example GIMP, then save in a suitable format: you can annotate the image as well at this stageif necessary. Locate and click the appropriate icon in the Grapher view to bring up the Transient Analysis window. This box details the values of both waveforms. Cursors on the left hand side of the Grapher view can be used, for example, to measure the peak value of Vout (the graphswindow can be resized if required to facilitate this measurement).16 Instruments - Placing Instruments in the Circuit MultiSIM is unique among simulation packages because it allows the user toconnect instruments into the circuit. This brings the simulation environment closer to the 'real' hardware environment and provides practice in using the equipment that is used in hardware laboratory work. We have just seen the results of a transient analysis. To carry out a similar analysis in the hardware laboratory a function generator would be used to apply a signal to the circuit with an oscilloscope to monitor the input/output waveforms. Adding the Instruments Save another version of your circuit as we will be making changes. For the half-wave rectifier circuit, delete the AC voltage source and instead place a function generator and an oscilloscope from the Instruments menu on the right hand side of thecircuit screen. When you hover over the icons the names are displayed. Connect them into the circuit and connect channel A to Vin and channel B to Vout. Ground the oscilloscope.17 Figure 10. Half-Wave Rectifier Circuit withInstruments. Figure 10 shows the rectifier (note that a real diode, 1N916, is now being used rather than the generic version) with the addition of models of actual manufacturers' instruments together with their display windows (click on the skeleton versions in the circuit to activate). The panel controls function in exactly the same way as they would on the physical boxes: you will even need to operate the on/off switches to make them work. To start the simulation, operate the rocker switch icon at top left of the display window. The easiest way to initialise the oscilloscope is to use its 'Autoset' button. Finally, do not be afraid to experiment: modifications to the half wave rectifier might include adding a capacitor across the load resistor, and varying its value. Summary You are strongly encouraged to use the Multisim package as a support tool for circuitsimulations generally, both analogue and digital (a tutorial introduction for digital circuits is alsoavailable). Note, however, that the simulations cannot be a perfect representation of what will happen withthe physical hardware, where factors like power supply decoupling (or lack of it) can have a critical effect on performance. Multisim is available on all open-access machines in the Department. Good Luck!18 APPENDIX Tips when Using MultiSIM 1. Wiring the Circuit Ensure that where two wires intersect that a junction (red dot) can be clearly seen, otherwise there is not a physical connection at this point, only a crossover of the two wires. When connecting intersecting wires, you must stop and make each connection in sequence. Overlaying a junction will not necessarily result in a connection being made even through a junction is visible at the cross-point of the two wires. These errors are difficult to find in complex circuits and will not be caught by MultiSIM when the circuit is saved. Care must be made to wire a complex circuit logically; place all the componentsand then start on one side and work outwards connecting each component stage by stage. Remember to place junctions as necessary to ensure the physical connection of the wires. When removing or replacing a component, delete the surrounding wires and redraw themodified portion of the circuit. Do frequent 'saves' of your circuit, or make backups, especially before doing major alterations to a design. 2. Simulation Run/Stop Switch When using the simulation switch to start and stop simulation, allow time for the switch to operate before clicking again: as circuit complexity increases, the response of the switch can be quite slow. Make sure the switch is OFF before attempting to make changes to the circuit. Remember to switch off simulation as soon as the results have been obtained,otherwise memory will fill with continuing simulation data.