LTspice/SwitcherCAD III Button Icon Cheat Sheet Continued
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
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.
Real World 9V Battery
Next, we’ll explore a real world
9V battery. You’ll need a DMM
(Digital Multimeter), a fresh 9V
December 2008 53