MAIN BUS 15A 50A Alternator Ammeter Battery External power jack Starter B Master switch Starter switch BAT Starter solenoid Ammeter shunt Master solenoid External power relay 9-48 Alternator circuits, like generator circuits, must control  power both to and from the alternator. The alternator is  The ammeter shown in the battery circuit is used to monitor  the current flow from the battery to the distribution bus. When  all systems are operating properly, battery current should flow  from the main bus to the battery giving a positive indication on  the ammeter. In this case, the battery is being charged. If the  aircraft alternator (or generator) experiences a malfunction,  the ammeter indicates a negative value. A negative indication  off or on virtually all electrical power by controlling the  battery connection. Note how the electrical connections of  the battery solenoid are protected from electrical shorts by  rubber covers at the end of each wire. As can be seen in  Figure 9-86,  the generator switch controls  the power to the generator field (F terminal). The generator  output current is supplied to the aircraft bus through the  armature circuit (A terminal) of the generator.  Generator circuits are used to control electrical power  between the aircraft generator and the distribution bus.  Typically, these circuits are found on older aircraft that have  not upgraded to an alternator. Generator circuits control  power to the field winding and electrical power from the  generator to the electrical bus. A generator master switch is  used to turn on the generator typically by controlling field  current. If the generator is spinning and current is sent to  the field circuit, the generator produces electrical power.  The power output of the generator is controlled through the  generator control unit (or voltage regulator). A simplified  generator control circuit is shown in  Figure 9-86 .  means current is leaving the battery to power any electrical  load connected to the bus. The battery is being discharged and  the aircraft is in danger of losing all electrical power.  Figure 9-84.  Schematic of typical battery circuit.  Figure 9-85.  Battery solenoid. Generator Circuit Alternator Circuit