Question:
I the Vin pin on the Arduino "direct" access to the external power supply? Or is the Vin pin getting the power supply voltage through an onboard chip?
Explanation:
I am using the Arduino to drive an external 5 V motor. Now I have the motor powered by external batteries and controlled by the Arduino PWM output and a transistor. But I don't want to drop the external batteries from my design. I am powering the Arduino with a 5V power supply. Can I somehow access the "Real" Vin (not current limited by the Arduino) from somewhere on the Arduino board? I know there is a Vin pin on the Arduino, but I think that the "real" input voltage is going through some chips before going to that Vin pin, and thus current limited by the chipset. Is that the case or not? I don't want to burn out my board by drawing too much current.
You will find that the "barrel jack" - the 2.1 mm coaxial connector - feeds through a diode to the "Vin" terminal on the board which in turn leads to the on-board regulator.
Neither of these in any way constitutes a power output as you do seem to realise that you cannot power a motor from any point on the Arduino.
It may possibly be a convenience if you are powering the system from a "wall wart" supply into the "barrel jack", to take some power from it through the "Vin" terminal but you do need to remember in this case, that this is through the reverse-polarity protection diode which is rated at no more than 1 Amp.
The concept of the reverse-polarity protection diode is that you might find a rogue "wall wart" with non-standard polarity (centre positive being essentially standard), but should never be so foolish as to connect "Vin" to a negative voltage.
I did just that a few months ago without problems.
If your motor draws less current than the protection diode can manage, then probably you're safe, especially if the motor runs at a lower speed as a result of the PWM control. The Arduino board itself draws very little current.