arjscott:
Hi all,Quick question - the specs state "The Arduino Uno can be powered via the USB connection or with an external power supply. The power source is selected automatically."
How is the power source selected automatically?
The board has a simple two resistor voltage divider wired to the Vin voltage, if there is a voltage of sufficient voltage, a op-amp comparator circuit switches off a MOSFET switch that then isolates the USB voltage from the board's Vcc bus. So if there is both USB voltage and Vin voltage available, then the Vin voltage takes priority and is used to power the board.
Is there an in-chip relay? (perhaps thats a stupid question...)
No, just an op-amp circuit driving a MOSFET used as a switch.
Or some tiny SMD components which handle this? (ditto)
The reason I ask is, as you have probably read, I'm working on a solar power monitor.
Instead of auto-switching between USB bus and DC power, perhaps I can make use of this feature to auto switch between (solar powered) battery and DC input.
Perhaps, but it would probably be better to design your own auto-switching between solar power and battery power externally and then wire the resulting selection voltage to the arduino external power connector. External allowable voltage input is is around 7.5 to 12 vdc.
LeftyThanks in advance.
Andrew.