Understanding Arduino Uno External Power

Hi all!

I am using an Arduino Uno hooked up to my Macbook Pro to power 10 Macetech Megabrite LEDS.

Presently I have the Uno hooked to my Macbook via a USB cable, which powers the Arduino.

Then, I have a 7.5V DC 4 AMP power supply hooked to the LEDs, and it shares ground with the Arduino via the Uno's ground pin.

I have noticed a frustrating problem that ocassionally when I touch the casing of my Macbook with a new USB cable, the lights will flicker. I think it's because the Macbook chassis is connected to ground and it makes ground have a small charge for a moment.

I have purchased a USB cable with two ferrite chokes for now thinking that might mitigate this occasional problem, but I had the idea that maybe not using USB power or ground linked to my computer at all might be best, since then all power will be isolated inside the acrylic enclosure I've made.

Two questions:

1) If I connect my power supply to the GND and VIN pin headers on the POWER connector, will the Arduino detect that it should use that supply even if it's not on? I would not like the Arduino to be connected to the Macbook ahead of time, then switch to that power when I switch the supply on.

Would it be better to solder a 2.1mm center-positive plug coming off my power supply and plug that into the board? Would that cause the Arduino to always use my power supply's power / gnd and not the USB?

2) Regardless of if I go the approach above or continue to share GND with USB power, is there anything I can do to "clean up" my shared ground? Is it a good idea to put a filtering cap or RC circuit of some kind on the power coming off USB? Is there anything I could read up on regarding this?

Thanks all, I'm excited to hear more about this!

If Vin/2 is > 3.3V, the 5V regulator output is used vs USB power. Vin is after the reverse polarity protection so could be up to 0.7V below the barrel jack (if power is brought in the to the barrel jack vs the Vin header). USBVCC goes thru a resettable fuse and T1 if enabled and connects to a 47uC electrolytic and the 0.1uF caps scattered around the board.

If you look at the Uno schematic on the Products tab, you will see there is already components separating the USB shield from the Arduino Gnd (lower left corner) and also a solder jumper that allows for the shield to be connected to the rest of the ground plane.

We have seen posts where one of those components got smoked. Not really sure how. Not sure if USB/Serial adapters like this FTDI Basic connect the USB-Mini or USB-Micro shell to Gnd, or if only the USB Gnd is carried thru to the board. https://www.sparkfun.com/products/9716 This Mikroe483 certainly seems to have the USB shell connected to Gnd with the rest of the board. I have not had problems using them in my designs. http://www.mouser.com/ds/2/272/usb_uart_manual_v100-32094.pdf

So the USB shell separation may be overkill, or it may be a way to prevent ground loops, with the shield/shell only connected (as far as AC signals go) to Gnd at the PC end.