There’s more than
one way to create
an electronic
circuit and
make it work.
One powerful tool
in particular is
simulation
(previously only
available to
professionals).
With simulation,
you can accurately
predict the circuit’s
performance even
before turning on
your soldering iron!
by Peter J. Stonard
The most popular computer
simulation tool is called SPICE
(Simulation Program with
Integrated Circuit Emphasis). Simply
put, it’s an analog electronic circuit
simulator used in IC and board-level
design to predict circuit behavior.
I have used SPICE for some time
in industry and quite frankly it took a
bit of getting used to because it has a
steep learning curve. Once I got
comfortable with the process, I could
judge when it was valuable (and
believable) and I quickly added it to
my engineering bag of tricks.
My first SPICE tools were costly,
commercial implementations of the
original academic SPICE software
developed at the University of
California at Berkeley (UCB). Today, I
use a freeware version that was
created and is supported by the folks
at Linear Technology Corp (LTC;
www.linear.com/designtools/
software/). There are several other
freeware sources for circuit simulators
(that trace their roots back to UCB’s
SPICE engine), including Texas
Instrument’s TINA-TI http://focus.ti.
com/docs/toolsw/folders/print/
tina-ti.html.
LTspice is very popular and has
50 December 2008
been downloaded more than half a
million times. Plus, nearly every semiconductor house has free SPICE
macro models for their own products,
to be used with generic SPICE tools.
We’re getting ahead of ourselves,
so let’s see why anyone would need
these tools, and what we can expect
them to do for us. This is in the
context of hobby electronics, and if
you have used SPICE at school or do
so at work you might want to skip
ahead. My goal in this article is a
transition from simple circuit theory
with a pocket calculator to running the
same circuits in SPICE (see the sidebar
“Getting Started with LTspice”).
Experience Counts
An effective way to do
something is to employ an expert or
learn from an expert. That’s all well
and good, but we would be limited to
only doing things that are already
known. A better approach is to
conceive of an idea, try it out, and
make corrections when and where we
got it wrong. This is called Research
and Development (R&D). It offers a
way to make incremental progress, but
takes time and resources (money,
materials, labor, and usually more
money). Another way to reach a goal
is to prepare a scale model of the end
product, test it for suitability, and then
scale it up to final size. Models can
easily look exactly like the final full
size project, but making them work
exactly that way is not always possible.
For example, a model of a road bridge
may be the right size (scale) but
may not have the right strength of
materials. Once understood, these
limitations can be ignored (if trivial) or
factored in to make the scale model
behave like the real thing.
To understand the model, we can
break it down into smaller parts and
materials. These are collectively called
elements and represent the smallest
component. Practical electronic
circuits also have elements, many of
which are very common and well
understood (such as resistors). From
time to time, we may have to invent
new elements for our library, as new
electronic components appear in the
market, for example.
A very useful tool was adapted to
this type of work and it is called simulation. It can take many forms, but the
idea is that a machine of one type is
made to behave like that of another
