Well, last weekend I had finally acquired the bits after some delays getting everything together. This included some ATTiny24As, some P-channel MOSFETs and some 5V DC-DC PSUs.
Last weekend I got around to building the PCB, and yes, I’m not as handy with a dalo pen as I used to be. That, and the ferric chloride I was using had seen better days (the bottle has “Dick Smith Electronics” printed on it — enough said).
So I spent much time last weekend finding shorts between tracks and attacking those with a sharp knife. Last Sunday I managed to get something built, but not tested.
Today, I got around to testing it. At first I plugged it into a bank of 6 AA cells, and got 0.8V across the power input. WTF? Okay, maybe the DC-DC converter needs a little more current. So I wire up a cable loom with a 5A blade fuse and 30A Andersen connector. Plug everything together: *BOOM*, a tantalum capacitor blows up!
The tantalum was a 330µF that was scavenged from an old computer motherboard, probably only rated for 10V, and when you over-voltage a tantalum, they do throw a tantrum!
Lesson learned: don’t use those scavenged parts for 12V! I swap that out for an electrolytic (rated at 16V).
The reason for my high voltage drop though? A faulty MOSFET. I found that by de-soldering the tab on both input MOSFETs until the problem disappeared. Then pressing down the faulty one caused the short to re-appear. Strange, as the MOSFET has never seen use.
I proceeded minus a MOSFET for a bit, see if I could get some code into the MCU. I was able to program that okay, but then fun came when I tried to use the timer interrupt: including the timer ISR would cause the MCU to not boot. It’d just sit there.
The problem disappeared if I compiled my code without optimisation, or at -O1, but would return at -O2 or -Os (I was using -Os). So something in my toolchain was broken for the ATTiny24A.
Whilst waiting for a toolchain re-build, I decided to tackle the faulty MOSFET. I had one spare, so I carefully soldered that into place, only for the original issue to re-appear, so now I’m completely lost.
I guess next weekend, I’ll take a closer look and the cause will become obvious, but right now I’m more confused than a moth in a light shop!