monitor is dim

I have a HP ZV5000 pavilion laptop. When the a/c adapter was plugged in, the screen was bright and if I worked directly off the battery the screen would be dim. Now from out of nowhere, when the a/c adapter is plugged in, the screen is very dim and if I unplug the a/c adapter from the computer the screen gets brighter. I have checked for settings in "power options" and "display" in the control panel. Do you have an idea what the problem could be? Please advise. Thank you.

Dave