When is the right time to mention my traumatic past?

About a year and a half ago I was d, it was violent and traumatizing and has no doubt had a pretty profound effect on me. I always feel as if I need to mention it before I become physical with someone, so that they can understand my own experience a little bit. However, I don't know exactly when the right time is. I don't want to scare the guy off or make them nervous but I want to be able to connect with him. Should happen if I'm ready and without telling him about my past until further into the relationship?