Hi, this is something I really should be able to work out myself, but I'm really tired and its got me scratching my head.
I've built a 12+v DC step-down 'power-board' to pull out a 5V supply via a LM7805 and a 3.3v line off a LM3940. It features all the caps and diodes and an inductor as well to clean it all up. Designed to run off a motorbike supply.
I run the out of the 7805 via the 3940 and on to the VIN pin.
My understanding was that if there was no external supply connected to the DC jack, then it would switch to taking power raw off of the VIN pin.
The mega powers up fine, but its only putting out 4.09V via the 5V rail.. im assuming this is because theres a series diode or something on the VIN?
Also - whats the best way to get a split 5V + 3.3V rail sorted ? Similar to what I'm doing or is there some magical reg that outputs both ?