Hello
I was wondering what happens when there is no USB power, if the comparator is not powered, then what is the state of the FDN304 FET ?
must be open to let external power go to VIN, but this seems so unstable, what makes it work ?
regards
It is a power selector. It selects USB power when that is available.
Think of a battery-powered Arduino. While you are programming it you don't want to run the battery down.
MorganS:
It is a power selector. It selects USB power when that is available.Think of a battery-powered Arduino. While you are programming it you don't want to run the battery down.
ok but can you please take time to read and answer the actual question ?
phil123456:
must be open to let external power go to VIN...
This does not make sense.
USB power goes to the 5volt rail, and V-in should not be used as output on USB power.
The output state of the opamp (and the fet) depends on the voltage on V-in.
Switchpoint it 6.6volt (the output of the 1:1 divider compared to 3.3volt on the - input).
6.6volt from an external supply switches the fet (USB) off.
Watch this.
Leo..
Wawa:
This does not make sense.
yes, but it is the official arduino uno schematic AFAIK
watched the video, got me lost around 3:37 where it stoped making sense...-12V...whut...from where ?
if the comparator is not powered, then what is the state of the FDN304 FET ?
If there is no power to the comparator then there is no power to the Arduino so the Arduino and all its components are off.
If there is power from somewhere, Power Jack, Vin pin, USB or 5V then the comparator is powered and switches according to if the USB is connected or not.
so actualy got the answer somewhere else, the lm1117 on an arduino micro/uno whatever seems to tollerate usb power at its output without the need of a diode
I been confirmed the whole FET/comparator setup was rather pointless since 2 diodes were enough to keep USB/battery power to leak towards the other