controlled by the pilot through the alternator master switch.  The alternator master switch in turn operates a circuit within  the alternator control unit (or voltage regulator) and sends  current to the alternator field. If the alternator is powered by the  aircraft engine, the alternator produces electrical power for the  aircraft electrical loads. The alternator control circuit contains  the three major components of the alternator circuit: alternator,  voltage regulator, and alternator master switch.  [Figure 9-87] The voltage regulator controls the generator field current  according to aircraft electrical load. If the aircraft engine is  running and the alternator master switch is on, the voltage  regulator adjusts current to the alternator field as needed.  If more current flows to the alternator field, the alternator output increases and feeds the aircraft loads through the  distribution bus.  All alternators must be monitored for correct output. Most  light aircraft employ an ammeter to monitor alternator output.  Figure 9-88  shows a typical ammeter circuit used to monitor  alternator output. An ammeter placed in the alternator circuit  is a single polarity meter that shows current flow in only one  direction. This flow is from the alternator to the bus. Since  Figure 9-86.  Simplified generator control circuit.  Figure 9-88.  Typical ammeter circuit used to monitor alternator  output. G F A Bus Generator control unit Current produced by generator Current to generator field F A B U S Alternator output Alternator To ACU 0-60A Ammeter + - To aircraft loads 60A 0A 35A MAIN BUS 5A 15A 50A AVIONICS BUS Alternator Ammeter Battery External power jack Starter B F A Starter switch Master switch BAT ALT Starter solenoid Ammeter shunt Master solenoid External power relay F A S B S Voltage regulator Avionics master switch Figure 9-87.  Alternator control circuit.  9-49