On the other hand, saturation is exploited in some electronic devices. Saturation is employed to limit current in "saturable-core transformers", used in arc welding, and ferroresonant transformers which serve as voltage regulators. When the primary current exceeds a certain value, the core is pushed into its saturation region, limiting further increases in secondary current. In a more sophisticated application, saturable core inductors and magnetic amplifiers use a DC current through a separate winding to control an inductor's impedance. Varying the current in the control winding moves the operating point up and down on the saturation curve, controlling the alternating current through the inductor. These are used in variable fluorescent light ballasts, and power control systems.