Which device is used to change alternating current into direct current?

A bridge rectifier is the correct device for changing alternating current (AC) into direct current (DC). It consists of four diodes arranged in a bridge configuration that allows both halves of the AC waveform to be converted into a unidirectional output. This results in a smoother DC output, as the rectification process takes the positive and negative halves of the AC signal and effectively combines them into a single-direction flow of electrical current. This is essential in many applications where DC is required, such as powering electronic devices or charging batteries.

The other options relate to different functions or configurations in electrical engineering but do not serve the primary role of converting AC to DC. A step-down transformer reduces the voltage of AC but does not convert it to DC. A dual inductor is used in circuits to manage current and voltage independently but does not perform rectification. Lastly, while three silicon-controlled rectifiers (SCRs) in series could conduct current in a specific manner, they are typically used for control rather than the fundamental conversion of AC to DC like a bridge rectifier does.