Also known as lead style ballasts, Constant-Wattage Autotransformer (CWA) ballasts are the most common ballasts used on 175 W or higher MH lamps. It offers the best compromise between cost and performance, means of providing stabilized light output (lamp regulation) as opposed to the Magnetically Regulated circuit. It is a high power factor devise, utilizing a capacitor in series with the lamp. A ±10% line voltage variation will result in a ±5% change in lamp wattage for Mercury, or ±10%
change in wattage for Metal Halide. Below picture shows a typical Constant-Wattage Autotransformer (CWA) HID Ballast Circuit Diagram.

Constant-Wattage Autotransformer HID Ballast Circuit Diagram The circuit consists of a high reactance autotransformer with a capacitor in series with the lamp resulting in a high power factor ballast. A CWA ballast also has a significantly lower dropout voltage (the voltage at which the lamp extinguish) than the lag types. The CWA ballast delivers better light output control also. These regulation characteristics are greatly improved over the reactor and the high reactance circuits. CWA ballasts tend to be higher in CCF, typically in the range of 1.6 to 1.8 and high power factor (0.9) due to a capacitor. Additionally, the ballast input current during lamp warm-up does not exceed the current when the lamp is stabilized. They are rated to handle supply voltage variations of 10% or higher.

In some CWA ballast designs a 10% variation in supply voltage will vary the lamp wattage and light output by only 5%. This circuit requires an ignitor for QMH (Quartz Metal Halide) pulse start, CHM (Ceramic Metal Halide) and HPS (High Pressure Sodium).

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