power trouble

hi everyone,
I am having trouble with power on my home built Prusa i2
Whenever I have it connected to the computer threw a USB cable but don't have my power supply on it works as normal. I have a smart controller that also works as normal while being powered by the computer. But when I turn on my power supply the printer stops communicating with the computer and my smart control or goes dim to where I can barely read it. And the only way to control the printer is through to smart controller.
Also if I only have it connected to the power supply the electronics won't work (no lights come on on the arduino or the ramps or the smart controlor and only when I plug it into to computer does it start working)
I can move the motors threw to smart controlor so I know the boards are getting power from the power supply.
Other wise I am completely clueless as to what to do.
I am using ramps and arduino with repetier host and firmwear
Hope someone can help!
Thanks in advance
Sage.

What sort of PSU do you have, and how are you connecting it up?

This could be one of multiple issues, so knowing the PSU type and how it's connected up with help determine what the problem might be.

Notes:
You mention you have a smart controller. Which brand/model is it, because there are a few designs available.
Where did you buy your RAMPS/Arduino Mega/etc from?

PS: RAMPS just refers to the board on top of the Arduino Mega, not the whole setup.

I use a 30A switching power supply i bought on Ebay.
I have a reprap discount smart controller.
I am using Reepetier 0.91 firmware with 0.91 Linix Repetier host
I have Ramps1.4 on a Arduino mega 2560 clone.
The power comes in at the normal place on the Ramps board.

Thanks for the reply!
Sage.

I too am experiencing this exact issue (minus the LCD dimming).

I have Sainsmart RAMPS 1.4 and Mega 2560 clones with the knock off RepRap Discount Smart controller - Didn't realize this when I bought it....

My power supply is an old 12v laptop charger at 6.6A

Rich

Have you tried taking the display off to see if the issue persists?

Hi everyone,
I have same power problem. When I connect with pc smart controller it works well. But when I plug in with power supply , I only see empty screen.But power is coming. ?
Please advise me.