A Constant-Wattage Autotransformer (CWA) is a type of ballast, specifically for high-pressure mercury lamps and high-pressure sodium (HPS) lamps. It provides the necessary starting and operating voltage for these types of lamps. A CWA ballast typically consists of input terminals for connection to alternating current lines, output terminals for connection across the lamp, and an autotransformer. The autotransformer has a primary winding connected to the input terminals and one output terminal, as well as a secondary winding connected to the other output terminal.