Jun 122016
 

So I’ve managed to get the board up and going, sort-of. I’m developing the firmware, getting acquainted with the ATTiny24A’s hardware.

The logic is that it’ll be sensing the battery voltage and two inputs from mains and solar, and so when it goes into charge mode, it picks one of the two sources and starts pumping current. Simple enough.

Except testing it has proven “fun”. I hooked everything up, using a power supply with a diode to stand in for the battery. I noticed the ADCs were seeing a voltage on their inputs. How? Why? Of course, the answer was plain to see in the datasheet if I bothered to look!

That little body diode, was of course, passing the current from my “battery” back to the outside world, and that was messing with measurements.

Great. So I’ll be needing a series diode to cram in there somewhere, and the MOSFET is expected to switch up to 30A, so the diode needs to handle that too. The challenge, is there isn’t much room for a heatsink.

Actually, the MOSFETs can do over 70A, so I’ll aim for a diode that can do about 60A, with a view that it won’t be stressed doing 30A even without the heatsink. The Vishay VS-60EPU02PBF is looking like a good option, although expensive.

One annoyance is there doesn’t seem to be a diode that has the cathode connected to the tab of a TO-220, as then I’d just solder the MOSFETs and diodes back-to-back and clamp a heatsink to the pair of them.

I guess for now I can try a few experiments to get acquainted with how it’ll all work, perhaps de-solder the tabs of the MOSFETs (again) and perhaps put a small 3A diode in as a stand-in for testing so I can at least get the firmware written.