LTspice® Compatibility

SIMetrix can load schematics and symbols created by LTspice® XVII. Visually the result will be close to the original display in LTspice® except for colour differences. If the schematic does not use encrypted models, it will usually be possible to run simulations although it is usually necessary to make some minor modifications especially to the simulator commands.

Individual symbol files can be opened directly and it is also possible to convert the entire symbol library to a single SIMetrix symbol library file for easier and faster access.

Note that in order to load schematic files, the symbol files that they use must be available. See Symbols for LTspice® below for details.

In this topic:

Symbols for LTspice® Schematics

LTspice® schematic files do not embed their symbol definitions and cannot be opened standalone. Most regular symbols such as resistors, transistors and diodes are shipped with LTspice® itself and if LTspice® is installed on your system most or all the symbols it uses will be available. SIMetrix will locate the symbol files using the same search rules that LTspice® uses. If you don't have LTspice® installed on your system you will need to provide copies of the symbol files.

SIMetrix uses the same search path for symbol files as detailed in LTspice® documentation and established by experiment. The following locations will be searched for schematic symbols:

  1. Local folder (i.e. same folder as the schematic)
  2. MyDocuments\LTspiceXVII\lib\sym and all sub-folders
  3. Search paths specified in LTspice .INI file. This is located at AppData\LTspiceXVII.ini. Section [Options], key SymbolSearchPath. Value is a list of paths separated by semi-colons.

A search will be made for a file with the name symbolname.asy. Sometimes the symbol name is prefixed with a folder name in which case that folder will also be searched for the file.

Note that if a symbol file is found in multiple locations and the files in the different locations are not identical, an error will result and the symbol will not load. The schematic will open but the symbol will be missing and an error message will list the conflicting files. In this situation, you can remove or rename one or more of the conflicting files then reload the schematic.

The various locations for LTspice® files are believed to be fixed in the design of LTspice® and it is not expected that they will be found in different locations. If this is not the case, or the locations change in a future version, it is possible to configure them to different locations. See LTspice® Configuration

Building Symbol Library

You can build a SIMetrix symbol library that contains all LTspice® symbols found in its search path. This is useful if you want to use LTspice® models in your own SIMetrix schematics.

To build the library select menu Tools > LTspice > Build/Rebuild LTspice Symbol Library. The process takes a few seconds to complete. For more information about using LTspice® models, see Using LTspice® Models

Opening LTspice® Schematics

The procedure for opening an LTspice® is the same as opening a SIMetrix schematic. Use File > Open... then select Schematic Files from the file type drop down box. Alternatively you can simply drag and drop an LTspice® .asc file into SIMetrix. Note that double clicking an LTspice® schematic file will open the file in LTspice® only.

As mentioned in LTspice® Compatibility above, the symbols for the schematic must be available for the schematic to open correctly. If you have LTspice® installed on your system and the schematic opens correctly in LTspice® it should also open correctly in SIMetrix.

Simulating LTspice® Schematics

Some schematics will simulate correctly without modification, but many will need changes of some kind. The most common difference is that LTspice® stores its simulation commands within the schematic. SIMetrix can do this too and schematics converted from LTspice® will use that method. But the Choose Analysis GUI only reads and writes simulation commands in the F11 window. To allow GUI control of the simulation you will need to copy the commands across.

Note that SIMetrix and LTspice® simulation commands are only compatible at a basic level. Single step transient, device-based DC sweeps and AC frequency sweeps are usually compatible but that is about all. Multi-step commands are different and will need to be converted; the schematic loader does not perform that conversion.

Many models designed for LTspice® can be used with SIMetrix. For more information, see Using LTspice® Models

Saving LTspice® Schematics

SIMetrix is not able to save schematics in LTspice® format. When you save an LTspice® schematic (.asc file) you will be prompted to save to a new file name using .sxsch or .wxsch. You should not overwrite the original .asc file as the resulting schematic will not be readable in LTspice®.

LTspice® Text and Property Styles

SIMetrix will attempt to use the same typefaces and font sizes as the original LTspice® schematic. To do this it needs to know what fonts LTspice® is using. Initially SIMetrix will use the factory default fonts that LTspice® uses. At the time of writing this was the 'Tahoma' font family set to bold. To change this setting or import the current LTspice® settings, proceed as follows:

  1. Select menu Tools > LTspice > Font Settings
  2. To import the current settings used by LTspice®, click on the Load Current LTspice Settings button. This should make the LTspice® schematic rendered in SIMetrix have the same text sizes and styles as LTspice® although note that the colours will not usually be the same.
  3. You can alter the font characteristics to deviate from LTspice® if you prefer.

When you make the above changes, a series of schematic styles designed for LTspice® schematics will be altered. You can manually alter the styles using the style manager GUI but note that running the above sequence will overwrite any such changes.

LTspice® Configuration

A number of option settings are available to allow for non-standard LTspice® installations. The following can be changed:

Configuring LTspice® Documents Folder

All LTspice documents including symbols, models and example schematics are stored in a location in the My Documents folder. The standard location is "My Documents\LTspiceXVII". This location appears to be hardwired in the LTspice design and can't be changed. However, if for some reason it becomes necessary to relocate it, the new location can be specified using the LTspiceDocsPath option variable. Use this command:

Set LTspiceDocsPath=New-location

The symbol and model library locations are specified relative to this location. See Folder where symbol files are stored and Folder where model files are stored.

Configuring Symbol File Location

The default location for LTspice symbol files is: "My Documents\LTspiceXVII\lib\sym". You cannot change the absolute location of this folder but you can change its location relative to the LTspice Documents Folder.

To change the relative location, execute this command from the command line:
Set LTspiceSymbolsSubDir=New-location

The default value for LTspiceSymbolsSubDir is /lib/sym. Note the leading '/' is necessary.

Configuring Model File Location

The default location for LTspice® model files is: "My Documents\LTspiceXVII\lib". You cannot change the absolute location of this folder but you can change its location relative to the LTspice® Documents Folder.

To change the location, execute this command from the command line:

Set LTspiceModelsSubDir=New-location

The default value for LTspiceModelsSubDir is /lib. Note the leading '/' is necessary.

The actual models are in sub directories /lib/cmp and /lib/sub. By specifying /lib, both sub directories will be searched.

The model library location is not needed for reading schematics for display only, but is required if LTspice® models will be used in simulation. The model library will be searched when an LTspice schematic is loaded and if a model for a part is found, the schematic instance will be decorated with properties to locate that model using the .LIB simulation statement.

Configuring .INI File

The default location of the LTspice® .INI file is [AppDataPath]\LTspiceXVII.ini where [AppDataPath] is your Application Data folder. This is usually C:\Users\[username]\AppData\Roaming where [username] is your login username.

To change this location, execute this command from the command line:
Set LTspiceIniPath=New-location

New-location should be a full path to the .INI file

The .INI file is read by SIMetrix to locate additional symbol and model search locations and for font settings

Configuring Font Families

LTspice® uses a fixed range of font families that appears to be hardwired in the application, as opposed to being enumerated from fonts installed on the system. At the time of writing those font families were 'Arial', 'Arial Narrow', 'Palatino Linotype', 'Tahoma', 'Times New Roman', 'Verdana', 'MS Shell Dlg'. If the range of fonts changes or is different on your system, you can change the selection with this command line command:

Set LTspiceFontFamilies=List-of-fonts

List-of-fonts should be the font family names separated by a pipe ('|') symbol. They must appear in the same order as they show in the LTspice® control panel font selection box.