What device changes alternating current to direct current?

A rectifier is the device specifically designed to convert alternating current (AC) to direct current (DC). This process involves allowing current to flow in only one direction, effectively smoothing out the alternating waveform typically associated with AC. Rectifiers are essential in many applications such as power supplies for electronic devices, where DC is required for proper operation.

In contrast, a converter generally refers to a broader category of devices that may change voltage levels or the form of current but is not specific to the conversion of AC to DC. An inverter, on the other hand, performs the opposite function by converting DC back into AC. A regulator is responsible for maintaining a constant output voltage or current, rather than converting between AC and DC.

Understanding these roles clarifies why a rectifier is the correct choice for this question, as it accurately represents the function of changing alternating current to direct current.