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Carbon pile regulator resistance change

Aviation Mechanic Airframe (AMA) practice question 4 of 24 on Aircraft Electrical Systems. ACS element AM.II.K.K4 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic Airframe (AMA) practice mode.

Question bank updated 2026-09-22

QUESTIONIn a carbon pile voltage regulator, what happens when the resistance of the carbon discs increases?

  1. AField current increases and output goes up
  2. BField current decreases and output goes down
  3. CField current is unchanged, but output voltage rises
Show the answer and explanation

Correct answer: B. Field current decreases and output goes down

  • A — incorrect That is the result of decreasing disc resistance, not increasing it.
  • B — correct The carbon discs are in series with the generator field, so more disc resistance means less field current and lower generator output.
  • C — incorrect The discs are in series with the field, so any change in disc resistance changes the field current.

BACKGROUND

Aircraft are designed to operate within a specific voltage range, and generator output is adjusted by changing the current in the generator field. On older aircraft with low-output DC generators the control is electromechanical, and the two common types are the carbon pile regulator and the three-unit regulator. The carbon pile regulator sends the field current through a stack of carbon discs that are in series with the generator field. If the resistance of the discs increases, the field current decreases and generator output goes down; if the resistance decreases, field current and output rise. Carbon pile regulators need regular maintenance to work correctly.

STUDY TIP

Trace the series path: disc resistance controls field current, and field current controls output, so the two move in the same direction as each other and opposite to resistance.

KEY POINTS

  • Carbon discs sit in series with the generator field winding.
  • More disc resistance means less field current and lower output.
  • Less disc resistance means more field current and higher output.

SOURCES

  • sources/ama-airframe/ch09.txt "Carbon Pile Regulators"
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MORE ON AIRCRAFT ELECTRICAL SYSTEMS

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-31B Aviation Maintenance Technician Handbook – Airframe, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.