I am powering an arduino and shield through the shield's power jack. Is it okay to plug in the arduino's usb at the same time?
Yes, the Uno will use the power port for power if its voltage is greater than the USB's 5v (and the power port really needs voltages at least 6v, preferably 7-12v to function, so it should always favor the power port over the USB). You don't want to plug in voltage directly to the VIN pin while having the USB connected.
MichaelMeissner:
Yes, the Uno will use the power port for power if its voltage is greater than the USB's 5v (and the power port really needs voltages at least 6v, preferably 7-12v to function, so it should always favor the power port over the USB). You don't want to plug in voltage directly to the VIN pin while having the USB connected.
Its getting the power from the shield's power port not its own.
jarwulf:
MichaelMeissner:
Yes, the Uno will use the power port for power if its voltage is greater than the USB's 5v (and the power port really needs voltages at least 6v, preferably 7-12v to function, so it should always favor the power port over the USB). You don't want to plug in voltage directly to the VIN pin while having the USB connected.Its getting the power from the shield's power port not its own.
So would power jack shield and usb arduino be kosher?
MichaelMeissner:
Yes, the Uno will use the power port for power if its voltage is greater than the USB's 5v (and the power port really needs voltages at least 6v, preferably 7-12v to function, so it should always favor the power port over the USB). You don't want to plug in voltage directly to the VIN pin while having the USB connected.
Why do you say that? I see no problem with powering the arduino via USB and at any time later plugging in a DC voltage in the range of 7-12vdc into the DC connector or even the Vin pin directly even with USB still connected. Upon insertion of the external voltage the on-board auto voltage selector will isolate the USB voltage and allow just the on-board 5 volt regulator to supply current for the board.
The auto voltage selector function does not compare the USB +5 volts with the on-board 5 volt regulator's output. Rather the auto-voltage selector decision is simply based on if there is Vin voltage (above 6 volts or so) applied to the board or not, if so that is the voltage source used (via the on-board 5 volt regulator), if not then USB voltage is used.
Lefty
In general its OK to plug two circuits together both using 5v regulation. The current will be drawn from whichever supply is set at a slightly higher voltage. They are never exactly the same.
Problems can arise from ground loops if the ground connection is poor.
Barrel jack power is used if Vin/2 is > 3.3V.
Vin is the voltage after the reverse polarity diode.
So Vbarrel > 6.6+ (0.3 to 0.7), whatever the specs are on the dode.
One might expect 0.38 to 0.445V for Schottky 1A rated parts:
Ok, I wasn't quite correct in my explanation. As others have said, if you plug both the barrel jack with 7v+ and USB with 5v, it will use the power from the barrel jack over the USB.