The auto regulator ballast is also called a constant-wattage autotransformer (CWA). It can limit the current and transform the voltage. They can handle a +10% line voltage variation. Over the range, they can maintain the wattage within +5%, which is better than reactor and lag auto ballasts. They can also keep the light on when there is a sudden voltage drop. They are widely used in HID street lights.
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