Hi, I’m now to KiCad, until yesterday I did all my "PCB"s with veroboard and squared paper, loving the step up.
I’m designing a board – in the end it’s going to be a sort of a mosfet based relay board, as such, as long as the tracks are thick enough, I can decide how big a mosfet (and even the polarity of the entire circuit) at deployment time. I would like to add a generic footprint such that many different mosfets will fit, all the way from to-92 up to to-220, as well as similar surface mount parts.
I’ve seen breakout boards for sale that can take similar parts in diferent form factors like this in the past.
Unfortunately I’m not sure I’m ready to take on footprint design, and I don’t know the available parts well enough to even work out which case designs would even be appropriate at this stage.
I’m confident someone will have done this before, and I suspect that someone will know what to type into the footprint finder tool to give me exactly what I need.
I don’t think that there is such a footprint in the standard libraries. Of course you can make it yourself, but what i would suggest is that you just add as many FET’s in your schematic in parallel as you want to have different footprints on your PCB. Then assign each symbol a diffrent footprint.
In PCBnew you then can arrange the Footprint as you like and connect them with copper where needed.
Actually, I suspect that’s a rather unusual requirement. Normally designs are done with a specific part in mind. I’m not sure there is such a thing as a “generic mosfet”.
Of course, I guess you could put in jumpers to select pin out, polarities etc but I’m not sure that gives you much over a piece of stripboard.
Certainly, for a new user I would advise designing boards for known parts first, get used to the methods and then branch out into designing generic boards.
My general advice is similar to @bobc select at least a part range limited to one package and one pinout. However if you think you must have the flexibility then place one symbol plus footprint per option in parallel. Ensure the footprint pads do not overlap in this case. (You will trade performance and required board space for part selection flexibility. )
I think the advice of running several in parallel should likely do what I need for now, though I’m always (regrettably) looking to do things the fancy way.
Related question: if I setup the footprints so they overlap in a way that I can see won’t cause any electrical issues, is it going to throw any unpleasent errors at me; and is there any chance that the PCB fab will throw it back (I don’t even know if gerb files are high enough level to understand what footprints are)?
And to be clear, what I’m designing here is little more than a breakout board with a convenient pinout and a few resistors etc on it – to use it as a high side switch, great, want to use it as a low side switch, solder the opposite polarity mosfet onto another one, and plug it in backwards. The electrical side of this is totally fine for me, it’s just the KiCad usage that’s I’m unsure on.
Not exactly the same thing, but these are a similar idea:
I can see where you going. I’ve found it’s very difficult to mark such boards in a way that makes them easy to use. I tried to design some multipurpose boards for D9 connectors and it ended in quite a mess.
Given how cheap it is nowadays to get boards made, I would design single purpose boards with only minor variants. But there is no reason in principle why creating generic boards won’t work. The fab house might flag issues with clearance for fab purposes, but they won’t care what the electrical connections are.
There also cheapo chinese modules, e.g. 5Pcs MOSFET Button IRF520 but buying is not as satisfying as designing your own.
The Sparkfun module is about 1x2.5cm, so you could create a mini-panel of say 5x5 cm, and with a cheap fab like JLC (other fabs are available) get 5 “panels” for ~ $10, maybe $0.20 per board. You could have a mix of designs on the panel (e.g P-FET and N-FET etc).
There are some footprints which can easily be combined. There is another post where I replied with a picture example for a Pch SOT-6 MOSFET.
If you take a Dpak MOSFET footprint, and adjust the pad dimensions and placement to adequately reduce the spacing, you can then also place a SOT23 or even an SC70 on the same 3 pads. D2Pak (SMT version of TO-220) could also be included but in my opinion those are not so useful. You can also place a 6 pin SOT on the same pads if the package is rotated 90 degrees. You solder 4 drain pins all onto the one drain pad.
Beware of using too narrow pad spacing for high voltage. Most often this will not be an issue but it is possible.
If you want to gain more flexibility at the expense of complication, you can add another gate pin to accommodate an SO-8 or enhanced SO-8 footprint for example. I have successfully done such things but I have never heard of these footprints being widely available in cyberspace. It is not difficult to make custom footprints in KiCad and I think it is good experience. But beware that if you make a weird footprint then you need to make the pin numbers on your schematic symbols match those on the footprint. In some cases this may not match the MOSFET manufacturer’s datasheet.
So, I tried the overlapping-footprint-connected-in-paralel option discussed above – complete mess. I had naively assumed that there would have been some standardization of what order the pins came in, but no.
In the end the solution was to use the to-220 footprint, if you want to use a to-92 footprint part, splay out the legs a bit, and if you want something else either jury rig something or you’re stuffed.
Board ordered and on a ship from China as we speak.
I have to say that as a new user of the software, I’m generally very impressed. Some of the UI design stuff felt a bit odd like the way zooming works with mouse position and the mouseover with keypressed to directly do actions, and while I will get used to these, I struggle switching between different ways of doing things to the point I’m confident that a week in Kicad will mess me up the next time I try to use photoshop etc. I would suggest that adding a mode switch to turn these interface items back to something more industry standard would help new and infrequent (even frequent) users a great deal, and make the software much more approachable.