Hello!
I know this topic has been treated a million time but I still have one question and I dond found the answer
let's say I have a stabilized 5V external supply to power a lot of LEDs and an arduino UNO.
meanwhile I'd like to use the USB serial or be able to update my code.
so I connect my 5V stabilized power to the 5V pin of my arduino and I unsold the polyfuse to protect my computer. 
then I would like to upload my code, with the external power source off. I just install a wire were my polyfuse was. 
Now Is that possible that by placing a diode where the polyfuse was I could be abble to use upload my code with no danger for my computer regadless my external power is on or off? 
I'm a hobbyist I have now a little bit of experience with arduino but I'm still learning, just tell me if what I wrote sounds crazy 
thanks a lot!
Leave the fuse alone.
Connect external power to the leds directly (if WS-led-string) or through MOSFETs if bare leds making sure that you connect that ground to Arduino ground.
You don't want more than 200mA flowing through the board. Limit what your pins supply by using transistors and external power to get the work done. No need to mess with your dev board.
Thanks for your reply
I forgot to say that I don't want my computer to power the board,
My LEDs strips are indeed powered directly by the external source but I'd like my board be powered by this external source too. 
my computer will be plug only for debug and upload
Hi,
What model Arduino are you using?
Tom.. 
Hi, again.
T1 in the UNO circuit provides the isolation so you can plug your powered USB plug in while you have a separate external 5V supply connected.

I have done it many times and no problems.
Just don't turn your external supply OFF while USB is connected.
You could use a modified USB cable that you have disconnected the 5V wire, so you have gnd and the two signal wires only.
Tom.. 
Thanks for this well documented answer!
woah! I though it was just not possible to do that! I'll try as soon as I receive my board!
thanks again!
R.