Anyone know off-hand what capacity the +5v reg on the Arduino, say the Duemilanove, has left for shields etc.
I'm designing a new shield and whilst it has an option for an external supply, I want to know where I should specify the cut-off from using the internal +5v to using an external supply.
Depending on how folks use the outputs of my shield it will vastly change the current consumption of the board.
Could probably work it out easy enough, but it's nice to know real world stats.
It will depend somewhat on your input voltages, since the limit of the regulator tends to be power dissipation rather than output current. But you can probably count on about 200mA
(It's a 500mA regulator, and the arduino itself is under 100mA)
You might look at the NKC Freeduino; it was intentionally left with a full-sized 7805 regulator, and it's supposed to be laid out so that you could add at least a small heatsink if needed.
(OTOH, in general 3-terminal regulators are too cheap to worry about. If your external circuitry draws enough current that the on-board arduino regulator is threatened, you might as well spend the extra $0.30 and add an extra regulator as well.)