Problem with displays on external power supply (dim)

Hello!

I've been working with arduinos for a while now and the software side isn't an issue, but sometimes I lack the skill to troubleshoot hardware stuff

I've bought several TFT screens from saintsmart and others, also diff sizes 2.8, 3.2, 7.0, etc

I'm using shields to connect them to several arduino mega, I lose in size but win in not having a huge cable mess, simplifies news projects and also I also have switched between different shields in my attempts

Currently using UTFT libraries

I've googled alot to no avail, usually people report issues on wiring and pullout pins, since I'm using a shield to connect the display to the arduino those threads didn't help me

I've programmed menus forms and all sorts of cool stuff and all works well, from speed to touch, but when I started porting things to external power sources, things just doesn't work out

When powered through USB the display is bright, works 100%

I simply remove the USB and plug in an external power font and on some cases the display gets way dim than through the USB, something looks wrong with this, shouldn't be dimmer...

I've tested several displays, with several shields and different arduinos too, as for the fonts I've tried using 5v 9v and 12v, I also tried using batteries 4x 6x 8x to see if maybe it'd make a difference

Everytime I connect power through the power jack, something wrong happens, in the 2.8 and 3.2 displays they only get dim, which is annoying but works, one of the displays seems to work good under 12v input, but I think this would be a little overkill..

The 7 inch displays are the worst, they flicker and looks like it's gonna fry, though USB is all 100% well, switch to 5v or 9v or 12v external jack and the problem returns

I might be missing something very simple in here, shouldn't arduino work off the external jack supply the same as from the USB supply?Iis this a problem within the TFT screens maybe? Is there a recommended voltage supply for when externally powering?

Do not power external devices - such as LCD screens, relays, motors, steppers, servos etc. - from the 5V line on the Arduino. Use a separate power supply - a separate regulator or even better, a switchmode well-regulated "plug pack" or "brick" power supply - which can fully power the external device and the Arduino through its 5V terminal.

The on-board regulator can only supply the Arduino itself and a few LEDs.