LCD 16*2 with external source

Good evening, I have a small problem, it turns out that I'm developing a project where I have to feed an lcd with an external source, my problem is that when I feed it with this external source I get black pictures, on the other hand if the food with the 5v of the arduino works normally. Any ideas?. Looking at the internet, I notice that they only feed them with arduino and not with a source.

Are you keeping a common ground between the LCD and the Arduino ? (recommended)

Thank you, yes i do, the ground is common but the black square is same, now, i conet the 5v arduino and ground of arduino and the code works, but if i conet 5v of source y ground common between lcd, arduino and source dont work

If you dont have too much else connected to the arduino then keeping the Arduinos 5V to the LCD should be OK as many 16x2 displays are connected that way.

Larger displays sometimes need a seperate supply but even then not always.