Diodes
 A diode is a two-terminal electronic component that conducts current primarily in one direction.  It has a low resistance in the direction of current flow and a high resistance in the other direction. Diodes are made of Silicon or Germanium.

Because they only allow current to flow in one direction, Diodes are used to make rectifiers that convert the AC power produced in alternators into DC 

 Rectifiers

  Rectifiers are made up of one or more diodes and are used to convert AC to DC.

 AC electricity is produced in the stator windings when the alternator is running in vehicles.  Direct current (DC) is best for storing in batteries and powering other devices.  This is what rectifiers are used for.

 Different rectifiers are used for different purposes and characteristics like half wave and full wave.

 A rectifier circuit using 6 diodes is used to convert AC to DC from an alternator with 3 windings.

HALF WAVE RECTIFIER
 It is a star that uses only a single diode for AC output.  In this circuit, the anode (+Ve) side of the diode conducts current only during the positive half cycle of AC and is blocked during the negative half cycle.  These rectifiers are called half wave rectifiers because only half of the AC (positive half cycle) is available at the output of the diode.

FULL WAVE RECTIFIERS
Unlike half-wave, these are the type of rectifiers that help to provide power at the output equally in the positive and negative half-cycles of the output of the alternator.  Such rectifiers can be constructed using two diodes or four or six diodes.

 Alternators use 3 stator windings to generate electricity.  Full wave rectifiers are made by providing 2 diodes at the output of each of these.

 Apart from these, rectifiers (bridge rectifiers) using 4 diodes are also used in electronic circuits.
 As they operate both during the positive half cycle and during the negative half cycle and in all these cases current flows in the same direction at the rectifier output, a DC voltage is always available at the output in such full wave rectifiers as compared to half wave rectifiers.