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LTspice

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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. ↩

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