Introduction
"With Electronics Workbench, you can
create circuit schematics that look just the same as those
you're already familiar with on paper -- plus you can flip
the power switch so the schematic behaves like a real
circuit. With other electronics simulators, you may have
to type in SPICE node lists as text files -- an abstract
representation of a circuit beyond the capabilities of all
but advanced electronics engineers."
(Electronics Workbench User's guide --
version 4, page 7)
This introduction comes from the operating
manual for a circuit simulation program called
Electronics Workbench. Using a graphic interface, it
allows the user to draw a circuit schematic and then have
the computer analyze that circuit, displaying the results in
graphic form. It is a very valuable analysis tool, but it
has its shortcomings. For one, it and other graphic programs
like it tend to be unreliable when analyzing complex
circuits, as the translation from picture to computer code
is not quite the exact science we would want it to be (yet).
Secondly, due to its graphics requirements, it tends to need
a significant amount of computational "horsepower" to run,
and a computer operating system that supports graphics.
Thirdly, these graphic programs can be costly.
However, underneath the graphics skin of
Electronics Workbench lies a robust (and free!) program
called SPICE, which analyzes a circuit based on a text-file
description of the circuit's components and connections.
What the user pays for with Electronics Workbench and
other graphic circuit analysis programs is the convenient
"point and click" interface, while SPICE does the actual
mathematical analysis.
By itself, SPICE does not require a graphic
interface and demands little in system resources. It is also
very reliable. The makers of Electronic Workbench would like
you to think that using SPICE in its native text mode is a
task suited for rocket scientists, but I'm writing this to
prove them wrong. SPICE is fairly easy to use for simple
circuits, and its non-graphic interface actually lends
itself toward the analysis of circuits that can be difficult
to draw. I think it was the programming expert Donald Knuth
who quipped, "What you see is all you get" when it comes to
computer applications. Graphics may look more attractive,
but abstracted interfaces (text) are actually more
efficient.
This document is not intended to be an
exhaustive tutorial on how to use SPICE. I'm merely trying
to show the interested user how to apply it to the analysis
of simple circuits, as an alternative to proprietary ($$$)
and buggy programs. Once you learn the basics, there are
other tutorials better suited to take you further. Using
SPICE -- a program originally intended to develop integrated
circuits -- to analyze some of the really simple circuits
showcased here may seem a bit like cutting butter with a
chain saw, but it works!
All options and examples have been tested on
SPICE version 2g6 on both MS-DOS and Linux operating
systems. As far as I know, I'm not using features specific
to version 2g6, so these simple functions should work on
most versions of SPICE. |