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tap the secondary coil at its midpoint and construct two circuits with the load resistor and a diode in each circuit. [Figure 11-44] The diodes are arranged so that when current is flowing through one, the other blocks current. 

When the AC cycles so the top of the secondary coil of the transformer is positive, current flows from ground, through the load resistor (V$_{RL}$), Diode 1, and the upper half of the coil. Current cannot flow through Diode 2 because it is blocked. [Figure 11-44A]  As the AC cycles through zero, the polarity of the secondary coil changes. [Figure 11-44B]  Current then flows from ground, through the load resistor, Diode 2, and the bottom half of the secondary coil. Current flow through Diode 1 is blocked. This arrangement yields positive DC from cycling AC with no wasted current. 

Another way to construct a full wave rectifier uses four semiconductor diodes in a bridge circuit. Because the secondary coil of the transformer is not tapped at the center, the resultant DC voltage output is twice that of the two-diode full wave rectifier. [Figure 11-45]  During the first half of the AC cycle, the bottom of the secondary coil is negative. 

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