JFet models library for LTspice? - diyAudio
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Old 17th May 2014, 05:07 PM   #1
kouiky is offline kouiky  United States
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Default JFet models library for LTspice?

I've seen alot of model parameter strings posted on the web, but no solid layman instructions on how to use them to create model files. It's a shame, since I would guess that this situation might prevent some people from modeling some interesting circuits. I know that it has got in my way a few times this year, and searching was a dead end.

Libraries that can be downloaded and dropped into the proper folder seem to be the reliable way to go. I went this route with vacuum tube models and it was a godsend.

Are there any Jfet libraries for common devices, like the 2SK and 2SJ families and others, that can be shared here to benefit the DIY community?
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Old 17th May 2014, 09:41 PM   #2
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Every month it seems like another JFET gets discontinued, and when that happens the manufacturer's SPICE model tends to disappear from their web site. Sometimes there are individuals, or publishers of simulation software, who continue to make the model available. Even so there are probably more versions of SPICE models for JFET's than there are types of currently available JFET's.

For specific devices the LTSpice Yahoo user's group is probably the first place to look if you can't get a model from the manufacturer. Several group members have posted extended versions of the "standard.*" component libraries over the years. My "standard.jft" library has over 600 entries. I don't know how many of those come with the current LTSpice installation - how many are models I've accumulated over 20 years or so of activity - how many are models of discontinued parts - how many come from different manufacturers with minor electrical differences - and how many are essentially duplicates (e.g., listed under both the thru-hole and SMD variation of a part) - but that's a lot of component models to choose from.

Dale
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Old 17th May 2014, 10:00 PM   #3
jcx is online now jcx  United States
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modding the standard distribution lib files is bad practice - learn to use the .inc <file> declaration, make a "mylib" for your collections

the most portable for sharing your .asc on forums is to just include the .model or .sub definition on the schematic as a spice directive (far right tool bar button)

Last edited by jcx; 17th May 2014 at 10:08 PM.
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Old 18th May 2014, 12:48 AM   #4
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Bob Cordell's book has a chapter on making models from datasheets -- you can use Microsoft Excel's "optimization" program to tweak the variables.
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Old 18th May 2014, 04:44 AM   #5
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You're bringing up my second biggest criticism of LTSpice: Library and model management. (And don't get the idea I'm simply throwing that out as a gratuitous complaint just to be cantankerous. Even among professional CAE packages costing multi-kilobucks per copy I haven't found a satisfactory solution to the problem.)
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Originally Posted by jcx View Post
. . . the most portable for sharing your .asc on forums is to just include the .model or .sub definition on the schematic as a spice directive (far right tool bar button)
This is the standard advice given in the Yahoo user's group. It's a nearly fool-proof way to make a portable file, and is actually quite effective for small circuits. The disadvantage is that the schematic drawing becomes cluttered with device definitions as the circuit complexity increases, or the designer wants to investigate several possible conditions. (E.g., the schematic may include models for several different candidate parts in one or more locations; or for best-case, worst-case, and nominal performance levels of one or more parts.)

My suggested solution is to enhance the program's UI by adding a separate "Models" tab in the schematic, where you could park models and edit them as necessary, but generally keep them out of sight while working with the schematic. I honestly have no idea how much effort this would require at the program design, or coding, levels.

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. . . modding the standard distribution lib files is bad practice - learn to use the .inc <file> declaration, make a "mylib" for your collections . . .
I'm generally in agreement with this sentiment, except that LTSpice encourages you to behave contrary to the advice. The "Help" file entry for "Third-party Models" mentions three ways to incorporate an external models, including editing the " standard.* " files to include the new models. The Help file doesn't mention any warnings or drawbacks to the practice - and even points out that putting new models into the " standard.* " files makes the model easily accessible via the "Pick New Device" feature in the component's information menu. The "Pick New Device" menu is a convenient way to browse and choose from available models - much easier than picking through a library file in a text editor. As far as I know there isn't any way to point the "Pick New Device" feature at any file structure except " standard.* ". If it could access a parallel structure - e.g., "user.bjt", "user.jft", "user.dio", etc - I would gladly park my models in the "user.*" files and leave "standard.*" untouched.

Dale
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Old 21st May 2014, 03:00 PM   #6
kouiky is offline kouiky  United States
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I know that I would like to model with the 2SK170 and 2SJ74, etc. I've been to the Yahoo groups and the intructions skipped steps, ending up being un-usable. I spent alot of time last year trying to get them to work, to no avail. The people who write how to create your own components using LTspice's models seem to assume that we're computer programmers will extensive knowledge in code and file systems.

A few months ago, a member linked me to a library download for vacuum tubes, and it was basically drag and drop. That worked out very well, with predictable results. An equivalent download for Jfets would be very useful for "the rest of us".
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