The device used to control alternator field current is the ________.

• A. Diode trio
• B. Regulator
• C. Brush assembly
• D. Rotor

Answer: (A)

Regulation of the alternator’s field current is done by the regulator. It watches the system voltage and adjusts how much current flows through the rotor’s field winding. More field current strengthens the magnetic field, increases the generator’s output, and raises the charging voltage; less field current weakens the field and lowers the output. This feedback keeps the battery voltage at the desired level (around 13.8–14.4 volts in most cars). The regulator accomplishes this by modulating the current sent to the rotor via the brushes and slip rings, using electronic or mechanical means. The diode trio is part of the rectifier network that converts AC to DC and can contribute to supplying excitation in some designs, but it does not regulate the field current. The brush assembly and rotor are involved in delivering current to the field, not in controlling it.