I tried to look this up, but my very specific question seems to drown among the "regular" questions about the Arduino Vin pin...
So, my Arduino Uno R3 is getting power only from the USB cable. My multimeter tells me there's ~4.5V on the Vin pin (I get ~5V on the 5V pin). Where does the 4.5V come from? What path does it follow, and how much current can it source?
The Uno R3 schematic only adds to the mystery - I didn't see any path from the USB power to Vin pin, unless it goes "backwards" through the 5V on-board regulator...
You can't source any current safely, ignore it. You could study the auto-power switching circuit as that's
probably the point of commonality. Vin is for power in, not power out, the name is there for a reason.
MarkT:
You could study the auto-power switching circuit as that's
probably the point of commonality.
Do you mean the OpAmp and the transistor that select the power source?
MarkT:
Vin is for power in, not power out, the name is there for a reason.
Ok, but still, when you use the power jack, I understand it's safe to take some power from Vin (e.g. for an external "power ON" indicator lamp).
Of course, since Vin under USB power is fishy/undefined, it should not be used - somehow all the guides I've seen ignore this issue (or take it for granted without bothering to explain).
I have the same question that you, I have been checking the schematics of the arduino uno and mega and the circuit that selects the voltage source is the same...I measure 4.45V in Vin pin of the arduino mega when this is only connected by the USB pin. Maybe the stuff is in the Mosfet and/or the Op-Amp, the datasheet say that the mosfet is P channel.
I have the same question that you, I have been checking the schematics of the arduino uno and mega and the circuit that selects the voltage source is the same...I measure 4.45V in Vin pin of the arduino mega when this is only connected by the USB pin. Maybe the stuff is in the Mosfet and/or the Op-Amp, the datasheet say that the mosfet is P channel.
So you have only ever used USB power to supply power to the arduino? And have never used DC input jack to supply power to the arduino, right?
Anyway, what you could do is .... remove all power from arduino. Short Vin to ground (temporarily only). Then remove that temporary short. Then reconnect USB power..... then see what voltage you measure at the Vin pin.
Even if you do measure 4.45 V at the Vin pin (under USB power conditions only), then it's unlikely that the arduino will be able to supply anything from the Vin pin. It'll probably drop to zero volt real quick upon connection of a load to that pin.
I momentary connected Vin to GND without any power and then disconnect that two pins and finally plug to USB. Vin pin still show 4.4 V. I find this post:
Maybe the voltage regulator is broken. I havent tried yet supply something from Vin with only USB connected.