What does a Transformer-rectifier unit (TRU) do?

A Transformer-Rectifier Unit (TRU) is used primarily in electrical systems to convert alternating current (AC) to direct current (DC), while also changing the voltage level in the process. In typical applications, such as in aviation or industrial systems, the AC input might come from a generator or an external power source, and the output DC voltage is needed for operating various electronic components and systems.

The TRU consists of two main components: a transformer and a rectifier. The transformer steps down (or sometimes steps up) the AC voltage to a desired level, and the rectifier converts this AC into DC. This dual function of transforming voltage and rectifying current is integral to providing the power required by many so that electronic devices and systems can operate reliably.

Choosing an option that states simply maintaining the voltage would not encapsulate the primary function of a TRU, as it inherently involves the conversion and voltage alteration processes. Therefore, the most accurate description of a TRU's functionality is that it converts AC to DC while also changing the voltage level.