Does a comma goes before a but?

If its an complex sentence. So, if you're using a conjunction to tie two independant clauses together, a comma would be placed. However, if its an independant clause and a dependant clause, don't use the commas. Here's examples:

1: I'm going to the petting zoo, but I hear there will be bears.

2: I'm going to the petting zoo but forgot my pants.

So, it depends, but both are correct. (: