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ECE 201 Lab 6
Introduction to LTSpice for Circuit Simulation
Introduction
Spice (Simulation Program with Integrated Circuit Emphasis) is a widely-used computer simulation program. PSpice is the version developed for PC’s. Many different versions of PSpice are available, all having the same basic simulation code but with different user interfaces, device libraries, plotting programs and various bells and whistles. In an effort towards uniformity, so that you are not using three different versions of PSpice in your classes, we are going to use LTSpice for all our classes and labs. This lab will provide an overview of the basic operations. You will continue to learn more about the capabilities of LTSpice throughout further courses.
LTSpice is also called SwitcherCad III. It is a free program available from Linear Technology at http://www.linear.com/company/software.jsp. You can easily download it to your own computer. There is not a lot of documentation available from LT itself, but there is a lot of other web support. There is a Yahoo group for LTSpice at http://groups.yahoo.com/group/LTspice/. They have many files for download, including several tutorials and an extensive (290+ page) manual. This lab is largely based on a tutorial written by Nick Kennedy, an amateur radio enthusiast. His web page is http://www.cox-internet.com/wa5bdu/ltguide.doc.
The following sections provide instructions for drawing and simulating circuits in LTSpice. You can refer to these instructions to complete the exercises in the worksheet. There is no prelab this week.
Drawing the circuit
Double-click on the SwCAD III icon to open the program. Go to File – New Schematic to start a new drawing. To place circuit components on the schematic, you can use the keyboard, the Toolbar or the Edit menu:
- for a resistor: press ‘R’ or push the toolbar button with the resistor symbol
- for a capacitor: press ‘C’ or push the toolbar button with the capacitor symbol
- for an inductor: press ‘L’ or push the toolbar button with the inductor symbol
- for a ground : press ‘G’ or push the toolbar button with the ground (triangle) symbol. You must have a ground in your circuit!
- for a diode: Press ‘D’ or push the toolbar button with the diode symbol.
For other components , press F2 or the component button (the AND gate symbol) and a menu comes up. Find your component and double-click. On the left are other sub-menus of parts you may need, for example op amps. Note that the same voltage and current sources are used for AC, DC or other types.
In each case, the component appears when you move the mouse. Move it to the desired location and click. Press ctrl-r to rotate before placing. After placing, you are ready to place another of the same type. Right-click, press a different key or button or press ESC to exit placing that component type.
You can connect components by aligning their terminals when you place them on the drawing, otherwise use the wire function. Press F3 or the wire button (the pencil and blue line). Click at the first point, click at any intermediate points where you need to make 90 o^ turns, then click at the second terminal point. Crossed lines are not connected. If you want a junction of wires and not a crossing, you need to click at the junction (look for the blue square that indicates a junction.)
To assign values to components, move the cursor over the component until the pointing finger appears. Right-click and type in the value. For sources, just put in the DC value if you are doing DC analysis. For Transient analysis, click Advanced, go to the left side, click Sine (usually) and enter the amplitude (peak value) and frequency. For AC (frequency response) analysis, go to the Small Signal AC section and put AC, which assigns the default peak value of 1 V or 1 A to the source, or a different value in the amplitude block.
In assigning units , you can use p for pico, n for nano, u for micro, k for kilo, m for milli and MEG for mega. (Be careful: a common mistake is using M for mega, but it will give you milli!) You can use either conventional American 4.7k for a 4.7k-ohm resistor or the international 4k7. You do not have to put V for volts, Hz for hertz and so on, but there is no problem if you do.
LTSpice labels components as R1, R2, C1, C2 and so on. You can change them as you like by right-clicking the label and typing in the new name. To label nodes, press F4 or the “label net” button (the box with an ‘A’ in it) and type in a name. Place the dot over the wire or node and click. There are a couple of reasons to do this:
- you can give logical names like “out” and “in” to nodes so it is easier to pick out the ones you want to plot from a list
- if a certain node connects to many points in the circuit, you can eliminate a lot of messy wiring on the drawing by giving all the nodes the same name. For example, call your battery (+) terminal V+ and then put the same V+ name on all points connecting to that bus. It has the same effect as connecting them with wires.
If you want to add text comments, press ‘T’. Type in the text, ending each line with ctrl-m, and place on the drawing. Under Tools / Control Panel / Drafting Options, you can select the font size.
Dependent sources are modeled as four-terminal devices. The two input terminals are connected to the controlling signal and the two output terminals are connected to the output of the dependent source. The four types of dependent sources are called e, f, g and h for voltage-controlled voltage source, current-controlled current source, voltage-controlled current source and current- controlled voltage source, respectively. The gain value is set by right-clicking on the source and setting the number in the Value line.
Finally, some miscellaneous commands:
- to delete , press the scissors button on the Toolbar or F5. Move the scissors icon to the desired component, wire or other entity and click.
- to move a placed component: press the move key (the hand with spread fingers) or F7. Click the component and move it to its new location, it is disconnected from any wiring.
- to copy : press the copy button (two sheets of paper) or F6 (use Move to select first.)
- to rotate ctrl-r. If the component is already selected (has not been placed), just press ctrl-r. If it has been placed, select Move (F7), select the component, ctrl-r, and place again.
