There are seven coordinating conjunctions in English: and, but, or, nor, for, so, and yet. When a coordinating conjunction joins independent clauses, it is usually preceded by a comma.
As soon as is a transitional expression, not a coordinating conjunction, so it cannot be used with only a comma to join independent clauses.
Exercises:
Run-on sentences 1
Run-on sentences 2
Run-on sentences 3
Run-on sentences 4
Run-on sentences 5
Run-on sentences 6
Related topic:
Commas with coordinating conjunctions