So I'm making a shield for the Arduino mega that requires more power than the Arduino on its own provides. Since it connects to some 12V devices, I was thinking of just using a down step to drop it to 5V for some more amps for other 5V devices.
Bottom line: If I coneect a 5V 1.5a power source to the 5V on the Arduino to power devices, will it simple increase the amperage? Or will I break something?
Thanks,
Generally combining power supplies in this way is a bad idea, because the voltages are never quite the same, and they will "fight", and short circuit.
The proper way, is to connect the high power consumption device directly to the step-down regulator. Two devices don't need to share a Vcc to talk to each other. They need to share a common ground, because the ground is the reference point for all voltages.
I helped with a circuit that we needed to demo in a hurry, that involved 8 servos, but the power adapter was only good for 6. We had an extra adapter, so we connected the negative leads together of the wall warts, and connected 4 servos to each power adapter. If we had connected the positive rails the whole thing probably would have shorted out.
You may power up arduino via +5V pin, making shield to supply power to your arduino,