New answers tagged protection
As pjc50 said, you can't put resistors in the power or data of the USB without interfering with the communication. If you want to avoid any damage to the USB port, I'd recommend leaving disconnected the VCC pin of the USB breakout board, powering the MBED with a different power source. You will prevent any shortcircuit due to incorrect wiring.
I came up with the following circuit that does what you ask: 5 volts from the "SWITCH" source will turn on M1 and M2, connecting the 3 volt rail to the load, and pulling current out of the right hand transistor of the differential pair Q3 and Q4. This turns off Q2 and shuts down the 5 volt rail. When Q1 turns off by applying 0 volts to "SWITCH", Q4 is ...
Measure currents around the 3V MOSFET M2 when the switch is closed. I have a feeling that 5V on its drain is forward biassing the body diode and limiting VOUT to 3.6V (or some value dictated by M6's ON resistance. You may need a pair of MOSFETs there, in series, (usually with their sources connected together) to provide isolation in both directions. (Edit: ...
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