You don't. There should be one attached to the unit, most likely in the control panel. If it is missing or destroyed, you need to call a Hobart service office and tell them the model and serial number, and any other technical info they need. If you are lucky, they will fax or mail you a diagram. Technical info is hard to come by for Hobart units. They tend to keep it on the down low. There techs are good, and they want you to hire them, not fix it your self. I know this cuz I am one.