LTspice

LTspice is a simulator for analogue circuits with broad DC and AC capabilities, based on SPICE.

Simulation

• To simulate the voltage across an element, we can select a node and drag across the element to the next node.
• To simulate a waveform, see this page.
• For transient circuits and AC circuits, we can simulate with the transient option.
• We can add SPICE directives with s or alternatively Edit > SPICE Directive.

Keybinds

• F2 opens a toggleable element chooser. Supported components are mostly from Analog Devices.
• F3 opens a wire connector tool.
• F4 opens a tool to assign net names.
• F5 opens the cut tool, where elements/components can be deleted.
• F7 opens the move tool, to move components.
• g will add a ground terminal. It is necessary to connect it to the circuit for simulation.
• v will add a voltage source, c a capacitor, r a resistor, l an inductor.
• Right-clicking on an element will open an edit menu.
  • Holding Ctrl and right-clicking will open an advanced edit menu.
• Ctrl+R will rotate an element.
• Ctrl+E will flip an element.

Third-party elements

Unfortunately LTspice doesn’t contain a full breadth of components that alternatives (like Multisim) support. We can work around this by following these steps:¹

• Find the .model directive for the component on the Internet. The LTwiki may be a good place to start.
  • For example, this page on the LTwiki has many models that are not otherwise included.
• Copy the .model directive into a .txt file in the same subdirectory as your simulation files.
  • Technically speaking, it’s not necessary to put it in the same subdirectory nor in a .txt file. This is easiest, though.
• Then, as a SPICE directive, add .include filename.txt.
• Whenever a component is being used, rename it to the model specified in the directive (i.e., if using an NPN BJT, add a generic NPN to the schematic, then rename the component).
• Run the simulation and check if it works. If it doesn’t, something may have went wrong in the above intermediate steps.

Resources

• The LTwiki is an excellent resource for reference and documentation

Footnotes

1. Derived from this slideshow given by an engineer at Linear Technology. ↩