Powering Arduino with 5V pin and using USB for serial comms

Hi Andrew,

I'm not super familiar with Arduino Nano/Mega, I took a look at the Nano schematic, and there is a diode between +5V and VUSB which should meet your need.

I think the VIN supply won't interfere with the ability to plug in via the USB. Everything on the Nano board appears powered up via the VIN supply, and similarly everything  is powered by the VUSB supply if that's all that is available. If both are available, then everything is powered by the VIN supply since there's not a diode drop of voltage difference between VIN and VUSB.

Have I misunderstood the requirement? Or the circuit?

EDIT: Having re-read, I think you're saying you wish to connect 5V, not a higher VIN (which has to be probably ~ 6V or higher for a regulated output). In that case, you could wire your 5V supply directly to the +5V connection, if you don't use the VIN pin at all. The IC2 (LM1117) might be ok with this (I've not checked the LM1117 datasheet). Worst case, it could be desoldered off, but I suspect it should be fine. Worth a try!

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I missed that on the Nano schematic - I looked at the PDF on the official site and the auto-selector circuit is tucked up at the top partially obscured by something else!  But, yes that would give the protection if I used a Nano, great find thanks. 

On the Mega, there doesn't appear to be such a circuit.  There's a diode between the DC connector and VIN; and the USB and VIN is selected through a comparator and FET (basically, if VIN is > 2x3.3V then it wins.)  The best it gives is a poly fuse (F1) and I don't think I could rely on that body diode in the FET.

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So you're right, I want to use the 5V pin with a regulated 5V supply, and I don't want to use Vin (or a DC connector); I don't even want to power from USB, just be able to communicate to the board whilst it's powered on in-circuit.  The Nano works fine because of that diode.

My choice will come down to memory - the Nano Every has 48KB + 6KB which may be enough; if it isn't I would have to shift up to the Mega.  Using Vin would bring power dissipation concerns (and expense!) into play so whilst I could do it, it would be a bit of a pain.

I missed that on the Nano schematic - I looked at the PDF on the official site and the auto-selector circuit is tucked up at the top partially obscured by something else!  But, yes that would give the protection if I used a Nano, great find thanks. 

On the Mega, there doesn't appear to be such a circuit.  There's a diode between the DC connector and VIN; and the USB and VIN is selected through a comparator and FET (basically, if VIN is > 2x3.3V then it wins.)  The best it gives is a poly fuse (F1) and I don't think I could rely on that body diode in the FET.

3343.contentimage_207302.png5428.contentimage_207303.png3857.contentimage_207304.png

So you're right, I want to use the 5V pin with a regulated 5V supply, and I don't want to use Vin (or a DC connector); I don't even want to power from USB, just be able to communicate to the board whilst it's powered on in-circuit.  The Nano works fine because of that diode.

My choice will come down to memory - the Nano Every has 48KB + 6KB which may be enough; if it isn't I would have to shift up to the Mega.  Using Vin would bring power dissipation concerns (and expense!) into play so whilst I could do it, it would be a bit of a pain.