Which statement about diodes in rectifiers is true?

The assertion that six diodes are usually present in rectifiers is correct because many common rectifier circuits, particularly three-phase bridge rectifiers, typically employ six diodes to efficiently convert alternating current (AC) to direct current (DC). In such configurations, three phases of AC input allow each diode to conduct during each half of the AC cycle, providing a continuous flow of DC output.

Using six diodes ensures that each phase is adequately serviced, allowing for smoother and more effective rectification without overloading any single diode. This design is crucial in power applications where voltage stability and efficiency are essential. In contrast, the other statements misrepresent the typical configurations and role of diodes in rectification processes.