The field winding, shown in  Figure 9-70 , is mounted to a  rotor shaft so it can spin inside of the armature assembly. The field winding must receive current from an aircraft  battery in order to produce an electromagnet. Since the field  rotates, a set of brushes must be used to send power to the  rotating field. Two slip rings are mounted to the rotor and  connect the field winding to electrical contacts called brushes.  Since the brushes carry relatively low current, the brushes  of an alternator are typically smaller than those found inside  a DC generator.  [Figure 9-71]  DC alternator brushes last  longer and require less maintenance than those found in a  DC generator. The alternator case holds the alternator components inside  a compact housing that mounts to the engine. Aircraft  alternators either produce a nominal 14-volt output or  a 26-volt output. The physical size of the alternator is  typically a function of the alternator’s amperage output.  Common alternators for light aircraft range in output form  60–120 amps.  Voltage regulators for DC alternators are similar to those  found on DC generators. The general concepts are the same  in that adjusting alternator field current controls alternator  output. Regulators for most DC alternators are either the  vibrating-relay type or solid-state regulators, which are found  on most modern aircraft. Vibrating-relay regulators are similar  to those discussed in the section on generator regulators. As  the points of the relay open, the field current is lowered and  alternator output is lowered and vice versa.  Solid-state regulators for modern light aircraft are often  referred to as alternator control units (ACUs). These units  contain no moving parts and are generally considered  to be more reliable and provide better system regulation  than vibrating-type regulators. Solid-state regulators rely  on transistor circuitry to control alternator field current  and alternator output. The regulator monitors alternator  output voltage/current and controls alternator field current  accordingly. Solid-state regulators typically provide  additional protection circuitry not found in vibrating-type  regulators. Protection may include over- or under-voltage  protection, overcurrent protection, as well as monitoring the  alternator for internal defects, such as a defective diode. In  many cases, the ACU also provides a warning indication to  the pilot if a system malfunction occurs.  A key component of any solid-state voltage regulator is known  as the zener diode.  Figure 9-72  shows the schematic diagram  symbol of a zener diode, as well as one installed in an ACU.  The operation of a zener diode is similar to a common diode  in that the zener only permits current flow in one direction.  This is true until the voltage applied to the zener reaches a  certain level. At that predetermined voltage level, the zener  then permits current flow with either polarity. This is known  as the breakdown or zener voltage.  Figure 9-70.  Alternator field winding.  Figure 9-71.  Alternator brushes. Figure 9-69.  Diode assembly.  Alternator Voltage Regulators  Solid-State Regulators Armature Diode 9-40 Field winding Slip rings