I have 12 times a 8 piece 7 segment led connected using the max 7219 library
for that i am shoudl use a external power supply of course
but I also need the USB connected to the computer to change the values on the displays ( for a flight sim project)
the external psu will power the leds and preferably the board as well
when I use a standard 5 V , once i plugin the USB the leds turn off more or less as ( I assume ) the power is drawn from USB and that is by far not enough.
You can always use a data-only USB cable that doesn't try to power the Arduino.
Or you can use the external power supply for the LEDs only and power the Arduino via the USB...just remember to connect the power supply ground to the Arduino ground.
Steve
slipstick:
You can always use a data-only USB cable that doesn't try to power the Arduino.Or you can use the external power supply for the LEDs only and power the Arduino via the USB...just remember to connect the power supply ground to the Arduino ground.
Steve
tnx for your reply
a power delivery USB to arduino cable does not work nicely with the external power ( using 5.5 V )
on only usb power the displays hardly work at all.
still a bit frustrated 
im now having a common 6.6V and GND rail where the Arduino (mega) plugs into towards the Vin and GND
pins
once I plugin the usb cable things go beserk.
pmaartense:
once I plugin the usb cable things go beserk.
Is it a data-only USB cable?
Steve
Where did 6.6V come from?
How about posting a block diagram sketch of your pieces and how you have them connected.
If I understand your setup correctly, a very rough estimate of the maximum current required for 12x8x7 LED segments, at 20 mA per segment, is 13 Amperes total if all the segments are on continuously.
If only 1 digit per display has all segments on at any one time, 1.2 Amperes total.
Always oversize power supplies by at least 50%.