Again: Symbol Library not consistent even with default datasheet data and pin numbering?

It seems redundant but it isn’t.

I selected the KiCAD’s Default transistor MMBT3906 for a project
the pin numbering on the schematics (on the SYMBOL) is:

1 = B
2 = E
3 = C

And it’s footprint is a SMD DOT-23

BUT when I open the datasheet linked into the Symbol Properties, I find:

  1. It’s a 2N3906 and not an MMBT
  2. The datasheet reports a TO92 instead of a SOT-23
  3. The numbering is totally different:

1 = E
2 = B
3 = C

Since I was almost aggressed in this topic for similar issue, i want to understand what to follow now.

Because to me I start losing any trust on the symbols/footprint assignments in the KiCAD’s library

Here the screenshot that illustrates the content of this topic:

The correct datasheet to be assigned, is this one (not the one in default), that for the MMBT reports the correct SOT-23 with consistent numbering and it will NOT lead in error who is busy with the design.
Hopefully will it be corrected for 7.0.3?

Screenshots form the correct Datasheet and the correct Component

I hereby certify that I am not simply asking someone else to design a footprint for me.

This is an auto-generated message that is in place on the “footprints” section of the forum. If I remove it and ask for a footprint to be designed anyway, I understand that I will be subject to forum members telling me to go design my own footprint or referring me to a 3rd party footprint site.

I think it’s a fair request that the MMBT3906 should have a different datasheet from 2N3906. Maybe even the same datasheet for three packages like the one you found. Do submit a patch to the library entry.

But then this raises the question, why would you assume that the pin assignments in the SOT-23 model are the same as the TO-92 model, since there are no pictures of the SOT-23 model in the Onsemi datasheet.

Always verify it yourself. The library is created by volunteers and can have mistakes. In that case, it looks like the data sheet link is incorrect.

Feel free to submit a patch or create an issue.

I didn’t assume it.I just made notice the pletora of inconsistencies.
It’s required an update

I’m a user. So I made it notice.
I hope who it’s supposed to patch it, it will do it soon.

I created an issue report: Check datasheet of MMBT3906 (#3222) · Issues · KiCad / KiCad Libraries / KiCad Symbols · GitLab

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I agree with retiredfeline: the linked datasheet should describe the actual component. If not, it’s a bug (although a small one).

If you want to use MMBT3906 and find it in the KiCad libaries, you should be able to trust it works properly. Despite this you should always check the symbol, the pins, the footprint and the pads. Remember that in the end you are always responsible for what you use, whether it’s buggy or not.

You have to compare the symbol and the footprint against the actual datasheet of the part you actually have or will order/buy/use. If the symbol points to a wrong datasheet, you don’t have to be confused at all about what is in that wrong datasheet. In this case the KiCad symbol and footprint are correct and the only thing wrong is the datasheet link.

Forget that linked datasheet totally, and there’s no problem nor confusion. Yes, it’s a small bug and it would be good to get it fixed. But do not trust the datasheet links in any case. Trust only the latest datasheet from the part manufacturer, and validate the symbol and the footprint against it. The workflow is approximately this:

1. You have to know or find out what part you will use.
2. You have to have the correct and up to date datasheet for that part. Usually it’s found by googling or from a seller database.
3. You have to find the closest matches for the symbol and the footprint or to create them from scratch.
4. If you find an exact match – a fully defined symbol/footprint combination with that part number – you have to check them against the correct datasheet. You can also copy the library component to your personal libraries.
5. If you find a close match, you can copy them to your personal libraries and edit them to meet your needs, according to the datasheet.

In my opinion there’s no place for opening the linked datasheet, ever. If the symbol was created 5 years ago, the link may be dead or point to an outdated datasheet. Don’t trust or use any of the linked datasheets unless you have added the link yourself and know where it points to. There just isn’t a way to keep the links up to date and correct automatically, so the person who made the symbol pointed to whatever was available at that time and what was relevant for them when creating the symbol. Even if it’s not a mistake (like copying an old symbol and forgetting to check all fields), it’s not necessarily correct now.

My take on this is: a MMBT3906 is needed. A MMBT3906 has a SOT-23 package. The link in properties is to a different transistor in a different package so there is a real problem.

I’d report this to the Library people and explore the internet for the right data sheet. Who cares about pin numbers? They are different packages. They are not comparable. End of story!!!

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Yes, and the problem is only that the link is wrong. Otherwise there’s no problem when using the KiCad library part, and the library items are correct according to the correct datasheet. I want to make this clear because tormyvancool seems to be more confused about this than what is warranted.


Even though the ready made library parts of KiCad are easy and quick to use, I might suggest a small exercise.

_device.kicad_sym (2.1 KB)

Take this minimal generic transistor symbol library and add it to a test project as Project library. Choose some real transistor component and modify this symbol so that it describes that component. Create a Project footprint library and create there a minimal footprint for the component. Attach the footprint to the symbol.

After this exercise you may understand better what is relevant and what is not.

The link is wrong so my alarm bells ring. If the Kicad library data sheet reference was incorrect I would not even dream of using a footprint without personal confirmation via correct data sheets, whether the symbol and footprints in the Kicad libraries were correct or not.

Personally, I only use my own libraries (often copied from Kicad), but always verified by me from data sheets found in reference books or internet searches. When matching footprints to symbols, the only person I trust is me.

Finally, as I mentioned above, MMBT3906 & 2N3906 are different packaged products so why in the world bother comparing one pin-out to the other. A pointless waste of time and effort.

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I just compared everything since it’s wrong: it’s wrong in all.
And when you are finding a component symbol in KiCAD and you have not to rely on it: just everything in this component and in all components: is “di per se” unreliable.

Yes you’re right when you stated:“you’re the only one you should trust, is yourself”.

So what’s the meaning to install full KiCAD (I mean included libraries) if these are not to be taken into consideration just because they are not reliable enough?

From the pin-out (see my previous topic I linked into my post), to the attached PDFs and so on?

In my opinion you misunderstand this sentence, as it is common to all EDA programs, not only Kicad.

As for the datasheets, I have never understood the need of including the links. I prefer to download the last datasheet from the manufacturer’s site.

just include: a symbol and footprint … completely separate to each other, and let ppl to bound them together based on their needs and component type (and related datasheets) they find.

Without creating a specific component.
Let the user build up their own libraries with their own components and eventually datasheets they need.

so, just create 6+6 symbols for PNP and NPN (due the different numbering of the electrodes)
Then a Footprint

The users will bind the one they needs.

Not necessary to insert the whole worldwide component catalogue.
The user, in this case, will be aware that has to check all things also the smallest one.

This is exactly the way I work.

But other users have other needs.

There are 6 simbols for pnp and 6 for npn. I would keep these 6 symbols and only one footprint for sot-23, one fot to-92 and so on.

As said before, there are different opinions and needs.

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6 years ago my first task before designing first PCB with KiCad was to create my libraries containing symbols and footprints I need. No one of my symbols had link to datasheet.
Two months ago I decided to add under my KiCad folder the datasheet sub-folder and add links to located there pdfs in my symbols (2 or 3 symbols already have links).
I decided to do that because I found that sometimes I have a question about parameters of element at schematic. It is typically when I consider if I can reuse the same element in the new task.
And having its datasheet a click away will allow me to find the answer faster.
I think that from time to time I will update datasheet by replacing them with new pdfs having the same name (so no changes in libraries).

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What grade did you get in school in probability and combinatorics?

This saves a huge amount of time creating symbols and footprints to place in my personal libraries.

Absolutely agree 110%!!!

“Permutations and Combinations” :slightly_smiling_face:
3! = 3 X 2 X 1 = 6


Yes indeed … not so many. Considering 12 however because PNP and NPN
however we forgot 1 case: unijunction

When they are correct, yes.

Well it’s what I did in the other topic. As result: B and C of the transistor were inverted.

I corrected because you took it literally when my meaning was just;“few more than 1”.