Those packages were already old when I was young, so please forgive me for not knowing
I do not recommend to bend the leads too much though. Bending them apart so you have more room for the pads is generally OK, but if you bend them to change the pin order, its quite easy to generate a short, either directly, or later if the transistor gets bumped accidentally and bends a bit.
To prevent this, itâs also advisable to put fragile parts not too close to the edge, and preferably near to bigger parts.
I sort of agree, but:
I always try to start with an existing footprint. That way pad 1 is in the right place and I remember to include all the layers so my footprints are consistent with Kicad library layouts.
Same goes for schematic symbols. I get an idea of the size of the box and it is easy to keep to the right grid.
Okay I looked up the pinout, and the symbol I used in my schematic has gate and source reversed. So I should have used the DSG configuration rather than DGS. Thanks to everyone for the helpful advice. Now I can just erase the FET symbol and replace it with the right one. But KiCad still doesnât have footprints for FETs as far as I can see. I can make a new footprint but whatâs this about layers? Something I need to attend to? The KiCad Getting Started PDF doesnât seem to go into much detail about layers and such.
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.
Iâll try to do that for you tomorrow⌠maybe⌠maybe notâŚ
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 âSymbol Propertiesâ 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 donât want the fourth pad on your board, you will have to find a different footprint or modify the TO72. Iâd 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.
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.
Thanks. Thatâs 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 canât I find those?
EDIT: I just found it. It is the library called âPackage_TO_SOT-THTâ. 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 doesnât seem to be a noise problem.>
Donât forget: all the kicad footprints are âlooking down at top of the componentâ, 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 donât try to attach your FET to a TO3 footprint
It is a starting point, but not a particularly good one.
That post has pictures of MOS fets, not J fetâs.
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 itâs 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. Donât be afraid to use them to modify existing, or create new parts.
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 donât 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 donât do a prototype but start from ordering a small production serie (like 50 pcs).
Thatâs an important point. I would have thought the view was looking at the leads of the component, not looking down at the top. Thanks Iâll keep that in mind!
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 âFlip Board Viewâ which is at the bottom of âViewâ.
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 layersâŚ
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 didnât have the case grounded in my protoype and it worked well.
I will definitely use the âFlip Board Viewâ 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:
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.
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 Kicadâs idiosyncrasies. I can explore this on my own but where can I find these tutorials? Just google âLibrary management in KiCad version 5â or âHow to make a symbol in KiCad version5â? Or are those subjects listed in some KiCad site that I donât know about?
