Nuts and Volts - December 2008

LTspice/SwitcherCAD III Button Icon Cheat Sheet Continued

ICON

FUNCTION

Here are the basic passive building blocks: a resistor, a capacitor, and an inductor. Remember to give the new component a value once it’s placed. Then, use the right mouse button to enter the component’s description dialog. Most components also have a Select button to pick from a database of predefined standard values.

The Diode icon requires a selection of a specific diode model from the database. The Component icon invokes a dialog box to select from the component model database. The component can be of two classes: Spice symbols (a voltage generator for example) or commercial components (a LM741 op-amp, for example).

The ‘hands’ allow editing of an existing schematic. The Move icon (open hand) grabs a component and severs the connections. The Drag icon (closed hand) maintains the connections with rubber bands. Both act on single components or can lasso a group. Use these to clean up or edit a schematic.

Mistakes in editing can be corrected with the Undo icon and Redo icon, which are un-ghosted during some editing steps.

For better appearance, parts can be positioned in the schematic using the Rotate icon or Mirror icon.

Both of these icons invoke the same dialog box, but have quite different effects. The Text icon is used to add annotation to a schematic without affecting the simulation. The SPICE Directive icon allows an instruction to be added to the schematic that controls how the simulation is run.

the toolbar icons will be un-ghosted. Now hit the Tile Windows toolbar button to bring our blank schematic to full screen size. For clarity, I’ll refer to the icon buttons by name (refer to the sidebar for an LTspice button icon cheat sheet). Don’t forget to download the example circuits to be used in this tutorial from the Nuts & Volts website ( www.nutsvolts.com) — this will speed things up so that you learn the schematic drawing tools later while playing with these examples.

Our first circuit simulation demonstrates Ohm’s Law. We are going to do some work with a virtual resistor or two, using a virtual battery and a virtual meter to read voltage and current. This will all happen inside the

computer, so the next problem to overcome is how do we see the results? Actually, first we have to tell SPICE what we want it to do.

Fortunately, LTspice uses a GUI (Graphical User Interface) running under Windows. The original Berkeley SPICE was run on a UNIX box and humans had to type the circuit (called a netlist) and device models in as text that could be interpreted by the computer.We’re already familiar with schematics that use symbols to represent electrical components and connections. These don’t look like the physical parts (in most cases) and it’s also true of the elements in a SPICE schematic. Where the schematic for a real world device can call out a part

number and a symbol — a 9V battery, for example — the SPICE program has to know how a 9V battery behaves in the simulator to make sense of it in the real world. So, let’s start there.

Take a shortcut by loading the schematic called NV_SPICE_ 11.as c. Although very simple, it may have taken you a while to create exactly this with the LTspice drawing tools. Having said that, don’t let me stop you from doodling! The final schematic should look like Figure 2.