if you look, the MA it says on the power supply device is the MAX it can do.
And in the sanner manual it should say how many it needs, Most want atleast 300 ma. So you want to get one that does the same or more than what your scanner says.
I know my Pro 96 says atleast 300 and the wall cord I got from Radio Shack said it will do upto 900.
I manual for the 500 does show 500mA