Arduino power distribution?

What are the details on the power distribution for the Uno? If USB and the DC-in power is connected, which is preferred?

The reason I am asking is that i have an ethernet shield that draws a lot of power. If just USB or just DC-in power is connected, it doesn't work, but if both are connected it does work. This seems a little odd to me, I would think just having DC in would provide plenty of power (2 amp supply).

:astonished: what are you DC power ?? You don't have a jumper to select DC or USB power ?

My arduino mega + Ethernet Shield + LCD KeyPad draws 270mA, so with an DC Power supply it's full working !

You don't have a jumper to select DC or USB power ?

The Decillia was the last Arduino to have a header.

Since then, an auto selector circuit determines USB or Vin/Barrel.

What are the details on the power distribution for the Uno? If USB and the DC-in power is connected, which is preferred?

If both are connected, and DC-in is about 7.5V or higher then the DC-barrel/Vin will be selected. As for why having both connected allows for operation? Not clear why that would be the case. Maybe as current draw goes up, voltage on the rails drops and the MOSFET keeps switching back and forth?

What else do you have connected? An Arduino and just the Ethernet shield should operate fine over USB.