pile regulator and the three-unit regulator. Each of these  units controls field current using a type of variable resistor.  Controlling field current then controls generator output. A  simplified generator control circuit is shown in  Figure 9-57 . The carbon pile regulator controls DC generator output by  sending the field current through a stack of carbon disks  (the carbon pile). The carbon disks are in series with the  generator field. If the resistance of the disks increases,  the field current decreases and the generator output goes  down. If the resistance of the disks decreases, the field  current increases and generator output goes up. As seen  in  Figure 9-58 , a voltage coil is installed in parallel with  the generator output leads. The voltage coil acts like an  electromagnet that increases or decrease strength as generator  output voltage changes. The magnetism of the voltage coil  controls the pressure on the carbon stack. The pressure on  the carbon stack controls the resistance of the carbon; the  resistance of the carbon controls field current and the field  current controls generator output.  Carbon pile regulators require regular maintenance to ensure  accurate voltage regulation; therefore, most have been  replaced on aircraft with more modern systems.  In general, modern GCUs for high-output generators employ  solid-state electronic circuits to sense the operations of the  generator or starter-generator. The circuitry then controls a  series of relays and/or solenoids to connect and disconnect the  unit to various distribution buses. One unit found in almost  all voltage regulation circuitry is the zener diode. The zener  diode is a voltage sensitive device that is used to monitor  system voltage. The zener diode, connected in conjunction  to the GCU circuitry, then controls the field current, which  in turn controls the generator output.  A typical generator control circuit for low-output generators  modifies current flow to the generator field to control  generator output power. As flight variables and electrical  loads change, the GCU must monitor the electrical system and  make the appropriate adjustments to ensure proper system  voltage and current. The typical generator control is referred  to as a voltage regulator or a GCU.  Since most low-output generators are found on older aircraft,  the control systems for these systems are electromechanical  devices. (Solid-state units are found on more modern aircraft  that employ DC alternators and not DC generators.) The  two most common types of voltage regulator are the carbon  Armatu re Generator control Field circuit Generator output to electrical loads Field winding Armatu re Carbon stack Field Voltage coil Figure 9-58.  Carbon pile regulator. Figure 9-57.  Voltage regulator for low-output generator. Figure 9-56.  Generator control unit (GCU).  Carbon Pile Regulators Generator Controls for Low-Output Generators 9-36