As can be seem from various post's here, I'm an Altium user. I'm lucky enough to have access to it work, and I've been using it since it was good old Autotrax for DOS. Yes, I'm that old....
For work that's fine but with my home projects I'm keen on sharing these as Open Source Hardware and releasing designs in Altium goes against the grain here - the community prefers that you share using open source tools.
So with this in mind, I decided to take the plunge and try KiCAD. My following thoughts will be comparisons between 'what I know' about Altium and what my impression are about KiCAD and I fear it will be far from fair. Altium does set the bar, and thus my expectations of what a PCB CAD tool can do, pretty high...
It Can Be Done!
And besides, I need something to do while I wait for my next revision of Alarm PCBs to land....
Back in the start of my Alarm interface project, I ran into problems with my current limiting circuit. My little 3A FET would go up in smoke with limited to 500mA and I just put this down to a need to heatsink the FET. The thinking here is that passing 500mA when switched on was too much for the little SOT23 package.
I was wrong!
But before I knew that, I decided to beef up my circuit, switching the PMV48XP for a TD2955PT4G which comes in a D-PAK package, and rated to run with a Heavy Duty 12A steady state current. 500mA should be nothing for this beast, and the bigger case might compensate for the lack of heatsink (I was trying to get a physically small solution).
However, the only thing I knew was that I was going on guesswork here. I didn't have any hard maths that proved what I was thinking was correct, and rather than commit to winging it on my next run of Seeed studio PCB's I built a test circuit to check it out.
I've made no secret that I took my inspiration from Lior's Alarmino project, and I was quite pleasantly surprised when he contacted me on one of my posts. He suggested that I take a look at the value of the filter cap I'm using on the 4V rail supplying the SIM900.
In his design, the bulk storage cap was charged by a linear regulator that delivered a maximum of 1A, but as my design uses a switch mode that's (nominally) rated to 3A I figured I could go smaller with my filter cap as my circuit can deliver a higher rate of charge - even with the substitution of the eBay sourced LM2596 circuit.
I'm getting closer to finishing but there's still one functional block that I'd not yet tested and it's my active filter. Again, a big hat's off to Lior for the inspiration behind my build, but I have done some things my own way.
For example I used a Sallen-Key low pass active filter to get a 1.5kHz Low Pass Filter.
Thanks to the internet, rather than hand crank the numbers, I used this on-line calculator from OKAWA Electric Design. From that design I whipped up this LT-SpiceFile.