Where is JFET footprint?

Well, we have a starting point!

A good starting point.

KiCad does not have Footprints for devices of different types that come in the same package.

In most cases, I could use a 1206 Footprint for a resistor, capacitor, or inductor, with little complaint by a manufacture. But, there would be differences on some of the layer items.

I’ll try to do that for you tomorrow… maybe… maybe not…

Kicad has different schematic drawings for different transistors eg: Darlingtons, UJTs, IGBTs, FETS etc, but it has different footprints for different packages. eg: TO18, TO5, TO71, TO72 etc

What you need to do is to assign the footprint you require to your “Symbol Properties” of the symbol on your schematic.

In your case you need to put a TO72 package to your 2N4221 symbol.
As you have worked out you need a DSG layout, you should use Q_NFET_DSG for your symbol.
Because you have removed the case leg, you should just ignore the 4th pad.

If you don’t want the fourth pad on your board, you will have to find a different footprint or modify the TO72. I’d then recommend using a TO18 because it is the same size with pads in the same location with the same numbering.

As a different example; I have a bag of ancient TO72 cased BF200s out in my shed.
If I wanted to use those I would use a Q_NPN_BCEC schematic symbol with the name BF200 and the footprint TO72.

Kicad links the pin number on your schematic symbol to the pin number on the footprint.

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Another really great example of versatile footprints is the CMOS 4XXX range of ICs.
There are well over a hundred different chips including discreet gates, counters, shift registers, monostables, PPLs, etc, etc, and 95% of those THT chips are either DIP14 or DIP16.

So, two footprints covers nearly the whole range.

Thanks. That’s really helpful and quite specific. As it happens, when I changed the FET symbol after the first round of these posts, I chose the exact symbol you recommended: Q_NFET_DSG. But when I click on symbol Properties/ Edit Footprint and see the Footprint Library Browser, there is no category of listed footprint libraries that FETs or transistors of any kind would be in. Why can’t I find those?

EDIT: I just found it. It is the library called “Package_TO_SOT-THT”. Now I can explore the different packages and see what will work. Being a noob there is a leaning curve.

<Now that I think about it, I may use the 4th lead. This circuit is a guitar effect pedal and the FET is the final output stage before the guitar amp. It might benefit from having the FET case grounded, for noise reduction. Or not- I left the 4th lead unconnected in the prototype I breadboarded and there doesn’t seem to be a noise problem.>

Don’t forget: all the kicad footprints are “looking down at top of the component”, so, if you decided to use the TO92 footprint as @paulvdh mentioned, you would have to align your TO72 FET with the tag pointing between pins 1 & 2 (plus bend pin 2) so the footprint pad numbers match the schematic pin numbers.

There are heaps of combinations and fiddles before you decide to make your own footprints… enjoy; just don’t try to attach your FET to a TO3 footprint :smiley:

It is a starting point, but not a particularly good one.
That post has pictures of MOS fets, not J fet’s.
KiCad also has generic symbols for Jfets and they look like:

And KiCad also has them in all possible pin configurations. 6 for the N-channel and 6 for the P-channel variants:

I know of course that not all new to make footprints are best made by copying an existing one, but it is true for the majority, and certainly for a simple footprint such as this.
For beginners it’s also easier to start with a good example, then to start with an empty canvas.

But more important is to realize that KiCad has quite good editors for both schematic symbols and footprints. Don’t be afraid to use them to modify existing, or create new parts.

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I do it always only once. If I need to use any part I just make a symbol for it and link it with footprint (copied to my library to be resistant to KiCad changes in libraries). I have 0 symbols in my libraries without linked footprints. So when I place any element at schematic I don’t have to worry about its footprint - it is done automatically and guaranteed 0 errors (as was carefully tested during first usage).

I knew the pin configuration when I read elements datasheet. In many cases I don’t do a prototype but start from ordering a small production serie (like 50 pcs).

That’s an important point. I would have thought the view was looking at the leads of the component, not looking down at the top. Thanks I’ll keep that in mind!

Hi @montelee

The whole PCB programme is written for looking down at the top of the board. If you wish to turn your board over to look at the bottom copper you can “Flip Board View” which is at the bottom of “View”.

If you wish to turn layers (on multi-layer boards) on or off there is a layer manager Icon on the LHS of your screen which brings up the Layer Manager on the RHS which hides or shows different layers…

Explore! :slightly_smiling_face:

Thanks again jmk. I ended up using the TO_72_4 footprint since all the pins and layers are correct. I may use the 4th lead to ground the case of the FET. But how would I do the schematic symbol so it shows the case lead? I used the symbol Q_NJFET_DSG and it does not have pin 4, which is the case of the FET. I need to have that in the schematic so the PC board has that trace, yes? None of the Transistor_FET symbols seem to have that pin.

Or I could just ignore the 4th pad, since I didn’t have the case grounded in my protoype and it worked well.

I will definitely use the “Flip Board View” to see the copper side. It will be fun exploring the Layer Manager and other aspects of the PCB program!

Modifying schematic symbols ( or creating new ones) is done with the schematic symbol editor. It is part of KiCad, but a separate program. You start it with the icon next to the EEchema icon in the KiCad project manager:

The user interface is very similar to that of Eeschema itself, but to be able to work with it, you also have to learn some library management.

In an earlier post here I already posted a link to a tutorial for the schematic symbol editor.

Here is another one for the library management

This is a long thread so this may have been posted before if not…

Datasheet from Mouser link

PLEASE NOTICE, the pinout in the above reference is a BOTTOM VIEW. Many boards have been designed with parts reversed.

Now that is an interesting question!

Any takers???

That Mouser link from @JohnRob is a bit of a cop out. There are a couple of PNP & NPN transistors in the Kicad library that are listed as ECBC, but looking at the symbol, it seems to be two collector pins.

I dunno, and I dunno how Kicad would treat this problem.

Who else is willing to stick their neck out??? :thinking:

I will take a look at the tutorial for the schematic symbol editor. I got all distracted with this footprint business and forgot that you had posted that. And I will have to brush up on library management and familiarize myself with Kicad’s idiosyncrasies. I can explore this on my own but where can I find these tutorials? Just google “Library management in KiCad version 5” or “How to make a symbol in KiCad version5”? Or are those subjects listed in some KiCad site that I don’t know about?

“Help” at the top of Kicad and “FAQ” at the top of this forum.


Not sure your meaning here, the link I saw was for a 4 lead TO-72 JFET.

I may use the 4th lead to ground the case of the FET. But how would I do the schematic symbol so it shows the case lead?

I may be missing something but I would create a symbol with 4 leads:

  • drain
  • gate
  • source
  • case (make the case passive in case you don’t want to connect it)

another link for a Siliconix (likely the original mfg)

Its been a while since I created a library in V5. However I know the first step is to decide in what folder to put the new library.
I use Windows. I created my Kicad Libraries in a folder…

My Documents / KicadLibraries/j_active
My Documents / KicadLibraries/j_passive
My Documents / KicadLibraries/j_hardware

where the j_ is for JohnRob. This way they are all in one area in the library list.

Understood. thanks for the advice. I use Mac but the idea is the same on both systems

Oh, and when you get started, make a simple footprint 1 pad or so, save it to the new library. This way when you spend effort on the real one you don’t find there is some sort of issue that won’t allow to save your work.

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