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LTspice Frequently Asked Questions

Q: Is there a version of LTspice® for Mac?

A: It's released! A whole new and improved user experience....



Q: I have just overwritten an LTspice jig. How do I get the original back?

A: Save the modified file to a directory of your choice (if needed) then go into Tools-> Sync Release to restore the original file. Checking the date code of the updated file should show the current date.


Q: I get the message: Port(pin) count mismatch between the definition of subcircuit "xxxx" and instance "xxx".

A: This is normally encountered when a Spice model has been imported into LTspice and the Spice model definition has a different number of pins to the actual symbol used. If you have created your own symbol, Go into File -> Open, Change the 'File of Types' dropdown menu to Symbols (*.asy) and open the symbol. Then select View -> Pin Table to see the pin assignments.


Also check that your SPICE model specifies the expected number of pins. The SPICE model below specifies an N channel FET with 4 pins, labelled 20, 10, 30, 50, but its datasheet shows a circuit symbol with only 3 pins. In this case, it is better to use another FET rather than trying to edit the SPICE model


*FDD6630A at Temp. Electrical Model
.SUBCKT FDD6630A 20 10 30 50






This error message can also occur if incorrect text has been entered into the Component Attribute Editor. In the Schematic View, right click on the component to bring up the dialogue box below



If text has been added against the SpiceModel Attribute, it will throw up this error. This line should be left blank.


For further instructions on how to import external models, see the LTspice Tutorial: Part 4 on this site.


Q: I get the message: This schematic uses symbols that couldn't be found. Saving it will remove references to these symbols from the schematic.

A: This normally happens after a Spice model has been imported into LTspice and the original file containing the model has been deleted. Even if the file has been undeleted, LTspice can throw up this error. If undeleting the model file does not solve the problem, redrawing the circuit in a fresh file normally works.


Q: I get the message "Unknown SPICE device type"

A: This can happen if there is text in the SPICE file that LTspice does not recognise. If the SPICE file has been saved as a text file (eg using Notepad) the text editor can sometime place extra characters in the file, especially if the file is not saved as ANSI (see Notepad). Open the text file in LTspice and check for unwanted characters. Also ensure that the original SPICE file is a PSPICE file


Q: I get errors referring to circuit elements I do not recognise

A: If you get an error message similar to the one shown below:


you have probably downloaded the incorrect format of SPICE model. LTspice works best with PSPICE models. The above error message was generated when trying to use a SPICE3 model. The syntax is different.


Q: I get the message: Failed to create empty document

A: This may be due to one of the following:

· The TEMP or TMP environment variable may point to a folder that does not exist.

· The drive containing the TEMP directory may be full.

If the drive is full, clear some space.

If the TEMP environment variable does not exist, or points to a folder that does not exist, use the following steps to set the variable to a valid folder:

1. Click Start. Point to Settings, then click Control Panel.

2. Double-click the System icon in Control Panel.

3. Click the Environment tab in the System Properties dialog box. (in Window 7, this is in the Advanced System Settings)

4. In the Variable edit field, type TEMP.

5. In the Value edit field, type the folder name that will receive temporary files. For example, C:\TEMP (if the TEMP folder exists on drive C).

6. Click OK.


Q: My LTspice simulation runs slowly when I import a 3rd party MOSFET model

A: LTspice uses only the simplest MOSFET parameter set to describe the MOSFET. This parameter set is defined by the simple '.model' statement and defines the MOSFET using approximately 12 parameters. MOSFET manufacturers use a much more complex (and sometimes inaccurate) model defined using the more complex '.SUBCKT' model, incorporating many more parameters. This added complexity slows down simulation time with little improvement in simulation accuracy. Writing your own MOSFET model for LTspice is awkward but not impossible. Please either pick a MOSFET from the internal MOSFET libraries with similar Qg and RDSON to the FET you would like to simulate or create your own MOSFET model by referring to LTspice Tutorial 6


Q: Does LTspice simulate IGBTs?

A: It depends on what model you use. A member of an electronics forum asked this question and the model they were using was impossible to simulate. I believe this is because the model called up the component 'NIGBT' which is an unknown part in LTspice. However, looking on the Fairchild website, they specify their IGBTs using the .SUBCKT model and thus can be treated like a standard component using the methodology above. Here is the post, for reference:



Q: When opening up my LTspice files, some of the files have a padlock symbol () over them?

A: The Windows Virtual Store framework is preventing the user from modifying the directory in Program Files. Either turn off User Account Control or take ownership of the directory in which LTspice is installed


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