when to use a semi colon vs comma with a run together sentence

Either a semi colon (alone) or a comma (plus a conjunction) can serve to join two independent clauses. Think of the differences this way.

A period (two separate sentences) represent a full stop in the flow of speech or thoughts. A comma plus conjunction represents a slight pause in the flow. A semi colon represent more of a pause than the comma, but less than the period. The choice belongs to the writer to determine how much emphasis is given to the connection/separation. The shorter pause (comma) represents greater connection. The longer pause (semi colon or period) represents greater separation.  Do not use a semi colon to link unrelated thoughts.

Hope this helps,