OLED with UNO, Interesting Grounding Issue ( SOLVED ) but im not sure why..

im working with a 128x32 OLED.

i downloaded the basic libraries for it and ran an example with the screen wired directly into the UNO for power supply.
this worked great, fired right up and the display cycled through the example.

i wanted to power the screen from an external power supply.

i wired it like the picture shows bellow, A4 and A5 connected to SCL and SDA ( same as before ).
the difference was, im now supplying power and ground from my bench power supply.

for a reason that escapes me this did not work, the screen would freeze up or not start up atall.


i connected the ground rail from the UNO to the bread board. im not sure why worked, but i would like to very much understand, so i can move forward.

advise of this is greatly appreciated, im brand new to the game.

excuse my pictures, i know a proper wiring diagram would be best, i hope its clear enough.
in one picture you will see it is grounded on the arduino and the other it is not

Unless you have two systems that are electrically isolated from each other (e.g., using an optocoupler, a transformer, or a relay), you always need to connect the grounds to form a circuit.

thank you

You're welcome. I'm glad if I was able to be of assistance. Enjoy!