Cathode Anode Zener Diode Transistor Alternator output Alternator field Ground 9-41 Figure 9-74.  Inverter. Figure 9-73.  ACU circuitry. Figure 9-72.  Zener diode.  AC power systems are becoming more popular on modern  aircraft. Light aircraft tend to operate most electrical systems  using DC, therefore the DC battery can easily act as a backup  power source. Some modern light aircraft also employ a small  AC system. In this case, the light aircraft probably uses an  AC inverter to produce the AC needed for this system.  Inverters are commonly used when only a small amount of  AC is required for certain systems. Inverters may also be  used as a backup AC power source on aircraft that employ  an AC alternator.  Figure 9-74  shows a typical inverter that  might be found on modern aircraft.  A modern inverter is a solid-state device that converts  DC power into AC power. The electronic circuitry within  an inverter is quite complex; however, for an aircraft  technician’s purposes, the inverter is simply a device that  uses DC power, then feeds power to an AC distribution bus.  Many inverters supply both 26-volt AC, as well as 115-volt  AC. The aircraft can be designed to use either voltage or both  simultaneously. If both voltages are used, the power must be  distributed on separate 26-and 115-volt AC buses.  As an ACU monitors alternator output, the zener diode is  connected to system voltage. When the alternator output  reaches the specific zener voltage, the diode controls a  transistor in the circuit, which in turn controls the alternator  field current. This is a simplified explanation of the complete  circuitry of an ACU.  [Figure 9-73]  However, it is easy to see  how the zener diode and transistor circuit are used in place  of an electromechanical relay in a vibrating-type regulator.  The use of solid-state components creates a more accurate  regulator that requires very little maintenance. The solid-  state ACU is, therefore, the control unit of choice for modern  aircraft with DC alternators.  Since certain electrical systems operate only on AC, many  aircraft employ a completely AC electrical system, as well  as a DC system. The typical AC system would include  an AC alternator (generator), a regulating system for that  alternator, AC power distribution buses, and related fuses and  wiring. Note that when referring to AC systems, the terms  “alternator” and “generator” are often used interchangeably.  This chapter uses the term “AC alternator.”  AC alternators are found only on aircraft that use a large  amount of electrical power. Virtually all transport category  aircraft, such as the Boeing 757 or the Airbus A-380, employ  one AC alternator driven by each engine. These aircraft  also have an auxiliary AC alternator driven by the auxiliary  power unit. In most cases, transport category aircraft also  have at least one more AC backup power source, such as  an AC inverter or a small AC alternator driven by a ram-air  turbine (RAT).  AC Alternators Power Systems