Frank's Random Wanderings

Those in the know, know, that if you want to play with analog circuits, using a simulator first is a really good idea. There are many analog circuit simulators out there, some free, some mega-bucks, but the most common of all is SPICE. There are many different versions of SPICE available, and lately I’ve been using one that I’ve really been enjoying, called LTspice. You can download it here:
http://www.linear.com/designtools/software/

LTspice is a free program provided by Linear Technology. Some of you may know it by its older name, SwitcherCAD, but it’s actually a full SPICE simulator. LinearTech provides models for almost all of their parts with LTspice, in addition to a large number of models for other popular parts, like various transistors, diodes, etc. What’s nice is that if LTspice doesn’t contain a model for your MOSFET of interest for example, but you can find a spice model on the MOSFET manufacturer’s website, just copy&paste that model into LTspice and you’re good to go.

It also comes with lots of example circuits, and there’s a users manual on the website. For a free program it’s pretty well developed.

Here’s a simple example. Today I had an R-C circuit controlling a reset controller, and I wanted to know how long the R-C would take to reach a certain voltage to trip the controller. Well, it would be easy enough to work that out with a little math, but it’s way more fun to simulate it.

Click (twice) on the picture above to enlarge it. You can see the little schematic I drew, and the resulting classical R-C charging curve in red in the graph above. Quick, simple & easy. What’s nice is that I can now easily extend this circuit, adding for example a current sink on the Vout line to simulate the load of my controller, and see the result on the R-C curve. This kind of tool makes experimentation and “what-if” thought processes very easy & informative.

If you’ve never used a SPICE simulator before, or you’re just interested in learning more about how some of these simple circuits (or much more complex circuits) actually work, I’d encourage you to download LTspice & give it a try. It’s a really great tool.