Nuts and Volts - December 2008

type (the unknown target, in many cases). Unknown design questions can be answered and data collected before the final project is attempted. What does this have to do with hobby electronics and SPICE? Hopefully, we’ve added a few technical terms and some background to make it easier to understand the SPICE world (hint: element, model, simulation). These terms will appear in SPICE work, and each has a specific meaning and job to do.

■ FIGURE 1. SwitcherCAD III/LTspice start-up screenshot.

Virtual Simulator

We’ve talked about physical machine simulators, but simulation can also be done in a totally artificial environment inside a computer, where the output is the result of mathematic operations performed upon initial data

using rules that mimic the behavior of the real world materials. Simulations speed up the design cycle and reduce costs, and those advantages apply directly to all of us. Now we’re getting closer to electronics! Take the principles of electronic circuits, the behavior of the components, add a schematic and power supplies, input controls and signals, add a little SPICE, and see what comes out!

Jumping In With Both Feet

That’s enough background to set

the stage. Let’s try to do something simple with LTspice. Take another look at the sidebar, download the software, and set it up on your own PC. Next, start the program and compare your screen to Figure 1. Hit the File > New Schematic button on the toolbar to start a new project, and notice that

Getting Started With LTspice

I use a typical PC running a Microsoft OS. I’m not sure if the LTspice software can run on other platforms. A fast machine (Pentium4, 2 GHz or higher) is very desirable as SPICE work hogs both memory and CPU time. The software is a free download from the Linear Technology (LTC) website at www.linear.com/design tools/software/switchercad.jsp.

The tool is one of several that LTC offers, and seems to have two names: LTspice and SwitcherCADIII. As far as I can tell, they are one and the same; to avoid confusion, I’ll call it LTspice (except where another name is critical, such as files and directory names). Users have the option of registering with LTC for email notification of updates, and also to download other software and device libraries (recommended).

The file is about eight megabytes (it varies with updates: at the time of writing the version is 2.24u and dated 7/8/08). I recommend downloading and saving the file to a

new temporary subdirectory, then running it so if anything goes wrong (or you move it to another computer later), you already have the file saved.

There is a legal agreement screen that requires acceptance, and the default installation is to a new subdirectory called LTC in your computer’s Program Files directory. (See Figure A.) If you have already installed a previous version, it prompts before overwriting it (recommended). There’s quite a bit to the automatic installation, so be patient! (Also, it’s better to not tax your computer by running other programs during installation.)

When the installation has finished (Figure B), you may run the program and, for future ease, add the LTC spice icon to your computer’s start menu (or desktop). The download also includes subdirectories for examples and lib (library); the LTC directory should look something like Figure C.

Next, navigate to the Nuts &Volts website with your browser and download the companion file for this article. Unzip the file to a local directory. (You can use the same subdirectory as the LTspice install (Figure C again) and put the article files and your new work in a fresh subdirectory. (I call mine NandV.)