LittleRain:
I'm really sorry to post to a really old topic by Lydecker is right. The mosfet is the wrong way around, not just on the schematic, but the board too, the FDN340p is literally doing nothing on the board.
When Gate is off current flows, and when gate is on current flows.
No, that's how it's supposed to be.
It's not a switch, it's an 'ideal diode' circuit.
A normal diode (the fet's body diode) when the fet is 'off', and a short (ideal diode) when the fet is 'on'.
Watch this.
Leo..
So, I see that you are not looking at the WHOLE circuit but only at
that part you think is important. The question is which power supply
do you want to use: the USB supplied one or the Vin source?
The higher voltage source is then automatically selected! Do not fret
about the body diode because if Vin is selected, it will be reversed
biased and out of the circuit.
Herb
Wawa:
No, that's how it's supposed to be.It's not a switch, it's an 'ideal diode' circuit.
A normal diode (the fet's body diode) when the fet is 'off', and a short (ideal diode) when the fet is 'on'.Watch this.
Leo..
Yeah that's what it was saying in the link Jim posted, what I don't understand is why they would add the comparator to check to see if the external voltage is active, why not pull it to ground and call it a day?
LittleRain:
why not pull it to ground and call it a day?
The fet has two functions.
Provide true 5volt from the PC/laptop to the Arduino instead of ~4.6volt through a backflow protection diode.
(like the Nano).
Keep that backflow protection diode there, so you don't power to the PC/laptop's 5volt circuits if the Uno is on external supply.
The fet needs to be switched if you want those two modes.
Leo..
Ah ok, makes much more sense now.