Acer 4520 light going dim

Normally the way it works is if you plug in your AC Adapter to a laptop the computer screen will Brighten up since its no longer running off of battery and is actually charging . The reason why it should go dim is when you unplug the Ac adapter it goes dim to save battery life and last longer. If you are getting a really dim you can check and make sure that the Ac adapter jack on the adapter or motherboard isnt loose and it could be having contact to the unit and be loose inside and every small movement causes it not to charge / charge in and out and would cause this Dim effect. Again this depends on how old the computer is and I dont really think its the LCD or backlight going bad but you can try to mess with the screen brightness and If im not mistaken on that model it should be the FN key and the Right > and Left < Key for lower and higher brightness. Give that a try and check the adapter connection and you can pin point the issue . Hopefully it doesnt come worst to worst which could be a bad LCD going bad or damaged lcd internal cable. Hope my answer helped guide you closer to a solution.

Fn and f8 which is the arrow up button when it is plugged in will brighten the screen, press fn and f7 which is the arrow down button when unplugged to save power
or fn and which ever keys have the sun face on it,it kind of looks like a star figure