N-type P-type Electron Hole Conduction band Valence band Band gap Light − + Cathode Anode Expoxy lens/case Reflective cavity Semiconductor die Flat spot Leadframe Anvil Post Wire bond Load resistor Diode Transformer AC output AC input + − + 0 − Output waveform Positive half wave Diode 1 Diode 2 + − V$_{RL}$ Diode 1 Diode 2 + − V$_{RL}$ O$_{V}$ O$_{V}$ O$_{V}$ A B 11-20 Figure 11-44.  A full wave rectifier can be built by center tapping the  secondary coil of the transformer and using two diodes in separate  circuits. This rectifies the entire AC input into a pulsating DC with  twice the frequency of a half wave rectifier.  Figure 11-43.  A half wave rectifier uses one diode to produce  pulsating DC current from AC. Half of the AC cycle is wasted  when the diode blocks the current flow as the AC cycles below zero.  Figure 11-42.  A close up of a single LED (left) and the process of a semi-conductor producing light by electrons dropping into holes  and giving off energy (right). The symbol for a light emitting diode is the diode symbol with two arrows pointing away from the junction.  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.  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.