Many aircraft contain a separate power distribution bus  specifically for electronics equipment. This bus is often  referred to as an avionics bus. Since modern avionics  equipment employs sensitive electronic circuits, it is often  advantageous to disconnect all avionics from electrical power  to protect their circuits. For example, the avionics bus is often  depowered when the starter motor is activated. This helps to  prevent any transient voltage spikes produced by the starter  from entering the sensitive avionics.  [Figure 9-93] The circuit employs a normally closed (NC) solenoid that  connects the avionics bus to the main power bus. The  electromagnet of the solenoid is activated whenever the  starter is engaged. Current is sent from the starter switch  through diode D1, causing the solenoid to open and depower  the avionics bus. At that time, all electronics connected to  the avionics bus will lose power. The avionics contactor is  also activated whenever external power is connected to the  aircraft. In this case, current travels through diodes D2 and  D3 to the avionics bus contactor.  A separate avionics power switch may also be used to  disconnect the entire avionics bus. A typical avionics power  switch is shown wired in series with the avionics power bus.  In some cases, this switch is combined with a circuit breaker  is weak or in need of charging, the external power circuit is  used to power the starter. During most typical operations, the  starter is powered by the aircraft battery. The battery master  must be on and the master solenoid closed in order to start  the engine with the battery.  Figure 9-93.  Avionics power circuit. Figure 9-92.  Multifunction starter switch. Figure 9-91.  Starter circuit.  Avionics Power Circuit + + - External power plug External power solenoid Starter solenoid Battery (master) solenoid Starter motor Master switch To split bus - + S B R L OFF +V To main bus + + - External power plug External power solenoid (NO) Avionics contactor (NC) (a.k.a. split bus relay) To starter contactor S B R L OFF To main bus A V I O N I C S B U S M A I N B U S Ignition switch D3 D2 D1 Avionics master switch 9-51