Which battery charger component rectifies AC output of the power transformer to DC?

The correct answer is the power diode, as it serves the specific role of converting alternating current (AC) produced by the power transformer into direct current (DC). This process is essential in battery charging applications because batteries require DC to store energy effectively.

When AC flows through the power diode, the diode allows current to pass in one direction while blocking it in the opposite direction. This characteristic of diodes enables them to rectify the AC waveform, resulting in a pulsating DC output. The rectified output is typically then smoothed and filtered using additional components, but the fundamental conversion from AC to DC is accomplished by the power diode.

In contrast, a power transistor is primarily used for switching and amplification rather than rectification. A power resistor is involved in dissipating excess energy and controlling current flow but does not facilitate the conversion of AC to DC. Regarding a power capacitor, it is mainly used for filtering and energy storage, helping to smooth the rectified output rather than performing the rectification itself.