■ FIGURE 9. Results plot
for NV_SPICE_ 13.as c.
constant” and a “long time
constant.” Experiment with
the Pick Visible Traces to
see the effect of these
components on the
pulses.
to just that view as shown in Figure
11. To restore the full graph, hit the
toolbar button Zoom Full Extends.
Current Probe
Oscilloscope
Analogy
■ FIGURE 10. Adding the trace
V(pulsegen) to the plot screen.
go back to the Pick Visible Traces toolbar button and select the V(pulsegen);
while holding down the Cntrl key, also
select EqualR. Many traces can be
added, but too many and the graph
becomes hard to read. In the other
branches of the circuit, I added
capacitors, and together with the
resistors these form a “short time
■ FIGURE 11. View of selected portion
of Figure 9.
Using the graph is
much like running an
analog scope. We have
control over the Node
(scope probe placement),
horizontal scale (
time-base), and vertical scale (Y
amp gain). Plus, this scope
is always in calibration and
we can print the results to
paper, if desired. LTspice
does have a graphical
probe tool, too. Activate
the schematic window so
the border is dark blue
instead of ghosted out,
and mouse over the
schematic’s wires. The
mouse pointer changes to a probe
icon, and if you then select that trace
by clicking, the waveform will be
added to the results window graph.
Very neat!
An advanced scope with dual
delay timebases can let us see a
selected piece of the displayed waveform. So too, can LTspice expand the
graph’s details. Using the mouse and
holding the right mouse button, grab a
piece of the graph waveforms. When
the area is selected, the graph scales
With a conventional scope, we
use the probe to view the voltage
(usually with respect to ground or
common). To see the current in a live
circuit, we’d need a fancy current
probe which works with our usual voltage scope. In LTspice, we can readily
view the current flowing through a
component by placing the mouse over
it. Previously, we selected wires, saw
the probe icon, and got voltage on
that node; while over a component,
the current source icon appears and
we get the current flowing through
the component.
The results are plotted on the
same graph as the generator voltage
waveform in Figure 12. Notice the Y
axis is labeled in volts on the left for
the V(pulsegen) trace and in milliamps
on the right for the I(C1) current trace.
Wrap Up For
This Session
I encourage everyone to poke
around in the LTspice environment,
and experiment with the mouse
buttons. Right-clicking while hovering
over components or graphs will open
dialog boxes not discussed here yet.
(Just remember to save your own work
under different names, to preserve
the files used here in the tutorials.)
Have a question? Please contact
the author via email ( pstonard@ix.net-com.com). Or, join us online at the
Nuts & Volts forum
( http://forum.servo
magazine.com/). In
the next installment,
we’ll use LTspice to
simulate two very
popular analog ICs,
and introduce the
concepts of subcircuits and macro
models. NV
■ FIGURE 12.
Combination voltage
and current traces.
56
December 2008