I have a few power related questions:

1. If both USB and external power to Vin are supplied, then what does the mbed run off?

2. With just USB and nothing connected to Vin I measure about 4.5 volts on Vin. Apparently this comes from the USB connection, somehow backward through the regulator? I can actually run a small toy motor off this (the voltage then drops to about 3 volts). Do I run the risk of damaging anything, and if so what is the maximum amount of mA I can draw this way.

Regards, Bert

http://mbed.org/projects/handbook/svn/MicrocontrollerTRM/mbed-005.1.pdf?format=raw

1. It will run on the one with highest voltage.

2. Are you sure it was Vin and not Vu? D2 should stop current from passing out on Vin. On Vu IC3(FPF2123) limits the current to 460mA from USB

Hi Bert,

1. The two supplies are merged by a pair diodes with an ultra low Vf (0.1v) - namely the BAT60A. The highest voltage supply will dominate, assuming it is more than a few Vf's higher than the other, remembering the diodes do not have perfect cut off. When the two supplies are within a few Vf's current is sourced from both, the exact split depends on the supply voltages, individual diode characteristics and output impedance of the supplies.

2. Yes, this is reverse leakage through the diodes. I have just tested my board here. When I put a 10k resistor from Vin to ground, I get 3.6v across it, so 0.36mA. being leakage, I suspect this will vary a lot from board to board, and that it isnt harmful.

Hope that helps,

Chris