Edison was not a scientist, or all that good of an engineer. He had no formal training, so he stumbled around in the design process a lot. When he would realize that he couldn't do what needed to be done, he went out and tried to find scientists and engineers. Sometimes he would pay them to solve the problem, and sometimes he would just walk off with the ideas he saw.
But he was an admirable inventor for the sole reason that, like a bull dog, once he latched into something he would pursue it to the end. He was a very obsessive and compulsive person. I've seen this many times in my life, and those types of people usually end up being "the winners". Didn't he say "innovation is 1% inspiration and 99% perspiration"? Edison perspired a lot. He often dabbled in things that were over his head, academically.
So he didn't really "invent" a lot of things that are attributed to him... he just tinkered around and refined things to make them manufacturable and bring them to market.