A high-pressure sodium (HPS) ballast is used in conjunction with an HPS grow light. The role of the ballast is to help provide enough power to get the arc started when you turn your light on. After this, the ballast's job is to regulate the current, providing a steady beam of light. Without a ballast, the grow light would get too hot and pop. Similar to an HPS ballast, the metal halide (MH) ballast operates an MH grow light. It has the same responsibility of regulating the current inside the gas-charged tube, ensuring it doesn't burn too hot.