that has nothing to do with the ac adapter
i had the same problem on my laptop
its the Inverter that lights p the screen on laptops
if your warenty is still good get it repaired
if not
then contact dell and ask them to fix it
im not sure if you can do it your self never tried
but just tell dell your inverter is broken
this is a comon problem in all laptops
Today at work I fixed the same problem he described by simply removing the battery.