I am working on a breadboard to study my power requirements. I have an external power supply (5V, 4A). THere is a 5V to 3V3 DC/DC also in the circuit. On my external supply I carefully monitor the V and A with a meter so I can assess the overall power required for the project.

The problem is that I also have a USB cable (from mbed to PC). I am confused as to how the power gets merged/used between these two power supplies. It seems the mbed can be powered from either source - Thus it just isnt clear that my power monitor is correct.

Is there any way to ensure that all devices are powered from the external power and not the USB power?

- Jim

The mbed has some internal diodes that allow power to come from either USB or Vin. The highest voltage will 'win', but there is probably going to be some leakage current. Note that VU (pin 39, 5V out) is always derived from the USB input voltage. Dont use that output pin in case you want the attached circuitry to work independently from USB or when you want to measure overall power using your external powersupply.