Nuts and Volts - December 2008

■ 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