I understand that if the Arduino is powered by Jack or Vin when the USB cable is connected, the internal circuitry would inderdict the USB powering. But what if I power the arduino througn 5V pin while the USB cable is connected? How does the internal circuit do the choice? Did I just connect the 5V power supply directly with USB port of the laptop (since the 5V rail directly connects 5V pin and USB port)? Is this allowable? Is this dangerous? Thank you!
If u use the VIN pin of arduino, You shouldn't have a problem , as arduino selects it required voltage automatically, it's voltage regulator senses the voltage from USB and External source, if the power of the external source is higher than usb, the the power is bypassed to the External power. But don't know if you are using the 5 volt pin to turn on the arduino.
Well everything I said is applicable for UNO boards don't know other ones.