Motorized Autotransformer Voltage Regulator with Preset Control

Mains voltage in many areas swings well outside the nominal 230 V, stressing appliances, lab equipment and anything with a motor or transformer inside. The classic industrial answer is the servo stabilizer: a motor-driven autotransformer whose brush is moved until the output is correct — smooth, stepless correction, unlike relay tap-changers that jump in coarse steps. This project builds that servo loop at bench scale and makes it a control-systems teaching instrument: the sensing, the comparison against a setpoint, the motor actuation and the deadband that prevents oscillation are all visible and explainable. Three preset buttons (220/230/240 V) turn it into a practical bench AC source as well as a feedback-theory demonstrator.

1. The variac output is stepped down by a sensing transformer, rectified, and scaled to the Arduino's ADC range for measurement.
2. The firmware compares the measured voltage with the active preset (220, 230 or 240 V selected by push-button).
3. If the output is below the setpoint minus the deadband, the Arduino drives the L298N motor driver forward; above the setpoint plus the deadband, it reverses.
4. Inside the ±2 V deadband the motor stops, holding the brush position without hunting.
5. Limit switches at both ends of travel open the drive path if the brush ever reaches the mechanism's limits.
6. The digital meter shows the corrected output live while the lamp load demonstrates stable brightness as the input voltage is varied with an external source.

Tech stack:

• Toroidal autotransformer (variac), 0–250 V, 500 VA
• 12 V DC gear motor with reduction gearing
• Arduino Uno (ATmega328P)
• L298N dual H-bridge motor driver
• Sensing transformer + rectifier + divider
• Digital AC voltmeter (panel instrument)
• Preset push-buttons, AUTO/MANUAL toggle, limit switches

• [Closed-loop servo regulation] The Arduino reads the output voltage, compares it with the selected preset, and drives the variac brush toward the setpoint — a complete feedback loop on real mains voltage.
• [Three voltage presets] Push-buttons select 220, 230 or 240 V targets, covering the common nominal standards and making the setpoint change a one-press demo.
• [Deadband anti-hunt logic] The motor stops inside a ±2 V band around the setpoint, so the mechanism does not oscillate or chatter around the target.
• [Manual jog mode] An AUTO/MANUAL toggle hands brush control to jog buttons in both directions, useful for demonstrations and for parking the brush safely.
• [End-of-travel limit switches] Micro-switches at both ends of brush travel cut the motor drive, protecting the winding and the gear train from overrun.
• [Live output metering] A digital AC voltmeter displays the regulated output continuously, so correction is visible as the input is varied.
• [Demonstration load] A lamp load on the output shows regulation under load and gives the classic "dim/bright" visual as voltage is corrected.

• Fully wired regulator (variac, motor and gearing, control box, meter, lamp load)
• Arduino firmware implementing the servo loop (presets, deadband, jog, limits)
• Calibration procedure for the voltage-sensing stage
• Complete wiring diagram and schematic
• Servo-control theory notes for the report
• Project report PDF (background, feedback theory, methodology, calibration)
• PPT presentation for final review
• Viva Q&A preparation document (closed-loop control, deadband, variac vs tap-changer, limit protection)

• Correction speed is limited by the gear-motor travel time (a few seconds end to end); it handles slow mains drift, not sub-cycle sags — fast sag correction needs electronic tap-switching, listed as future scope.
• The 500 VA variac rating bounds the demonstration load; the documentation states the safe load limit clearly.
• Mains wiring is involved: assembly, testing and any servicing must follow the documented safety procedure with faculty supervision.
• Regulation accuracy is a design target verified by the buyer-run calibration procedure, not a factory-measured claim.
• The prototype is a bench/lab instrument, not a certified voltage stabilizer for household use.