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The problem
Mains voltage in many areas drifts well above or below the nominal 230 V, and sensitive loads — motors, pumps, lab instruments — suffer from both sags and swells. The common home stabilizer fixes this with a relay that switches between transformer taps, which corrects only in steps and produces a small jump every time it changes taps. A servo-variac stabilizer works differently: it moves a carbon brush wiper along a continuously wound transformer coil, so the correction is smooth. This project builds that continuous stabilizer from real parts — a toroidal variac, a servo motor coupled to its shaft, a microcontroller reading the mains through an isolated sensing transformer, and a digital voltmeter display. The student gets hands-on experience with mains sensing, closed-loop control, mechanical coupling and the dead-band design that keeps the servo from oscillating. It is a strong electrical-engineering project because every part of the control loop is visible and measurable on the bench.
How it works
- The step-down sensing transformer reduces the mains voltage to a few volts AC; a rectifier and filter convert it to a DC signal proportional to the mains voltage.
- The microcontroller samples this signal on its ADC, averages several readings to reject noise, and converts it back to the equivalent mains voltage.
- The firmware compares the measured voltage with the 230 V set point. If the error is outside the ±3 V dead-band (design choice), the controller steps the servo toward correction.
- The servo rotates the variac wiper along the winding; because the wiper moves continuously, the output voltage rises or falls smoothly rather than jumping between taps.
- Once the measured voltage settles inside the dead-band, the firmware starts a 1-second settle delay and then stops driving the servo, preventing hunting oscillation.
- The digital voltmeter module continuously displays the output voltage, giving a live read-out of the loop's performance.
- The control loop repeats every few hundred milliseconds (design target), tracking slow mains drifts without reacting to brief noise spikes.
Tech stack:
- Arduino-compatible microcontroller board
- Toroidal variac (variable auto-transformer)
- RC servo motor with mechanical coupling linkage
- Step-down sensing transformer
- Rectifier and filter stage
- Digital voltmeter module (LED display)
- Relay module (optional protection stage)
- Arduino IDE firmware (C++)
| Parameter | Value |
|---|---|
| Output voltage set point | 230 V AC (design target) |
| Correction type | Continuous servo-variac (smooth, not stepped) |
| Dead-band | ±3 V (design choice — see build notes) |
| Settle delay | Approximately 1 second (design choice) |
| Control loop interval | Approximately 300–500 ms (design target) |
| Variac rating | Approximately 0–270 V, 2 A class (datasheet value) |
| Servo travel | Approximately 180° rotation (datasheet value) |
| Voltmeter range | 0–500 V AC (datasheet value) |
| Mains input | 230 V AC nominal, 50 Hz |
Project features
- [Continuous servo-variac correction] The servo rotates the variac wiper along the winding, so output voltage adjusts smoothly instead of jumping between fixed relay taps — the core engineering contrast this project demonstrates.
- [Microcontroller closed-loop control] The controller reads the stepped-down, rectified mains voltage on its ADC, compares it against the 230 V set point, and drives the servo in the correcting direction until the error falls inside the dead-band.
- [Dead-band and settle delay] Build note from the prototype: the servo hunts if the dead-band is narrower than ±2 V, so the build uses a ±3 V dead-band plus a 1-second settle delay — the output holds rock-steady and the servo moves only when it should.
- [Live digital voltmeter] A digital voltmeter module displays the output voltage in real time, so the correction action is visible the moment the input voltage changes.
- [Isolated voltage sensing] A step-down sensing transformer keeps the low-voltage electronics galvanically isolated from the mains — no direct mains connection to the controller board.
- [Manual/automatic mode switch] The wiper can be parked at a safe position and the servo disengaged, so the variac can also be operated as a plain manual voltage controller for bench experiments.
- [Optional: overload protection relay] An optional relay stage trips the output contactor if the output stays outside a configurable safe window for several seconds, demonstrated as a bench-test feature only.
What is included
- Variac-servo mechanical assembly with coupling linkage
- Microcontroller control board with sensing and rectifier stage
- Digital voltmeter display module, wired and calibrated on the bench
- Complete firmware source code with the dead-band and settle-delay logic documented
- Project report PDF (background, servo-variac theory, circuit design, control logic, test results)
- PPT presentation for final review
- Viva Q&A preparation document (auto-transformers, closed-loop control, dead-band design, mains safety)
Limitations & prerequisites
- This is a teaching prototype, not a certified product: it is not BIS/ISI tested and must never be sold or installed as a commercial stabilizer.
- Mains wiring and first power-up must be done under faculty or lab supervision; the build ships with an enclosure requirement — never operated with the variac exposed.
- Correction speed is a design target of a few hundred milliseconds; it handles slow mains drift, not fast transients like lightning surges or switching spikes.
- The servo and wiper linkage have mechanical backlash, so the output can sit a volt or two off the set point — inherent to mechanical stabilizers and inside the dead-band by design.
- The voltmeter module has an accuracy of approximately ±1% (datasheet value); the report documents the bench calibration procedure rather than claiming a measured accuracy.
- The optional protection relay is a bench demonstration, not a certified safety cutoff; proper installation protection must always come from standard MCBs and isolators.
Frequently Asked Questions
How is this different from a relay-based stabilizer?
Relay stabilizers switch between fixed transformer taps, so the output jumps in steps every time the input crosses a threshold. Here the servo turns the variac wiper along the winding, so the output voltage changes continuously — no steps, no switching transients.
What does the dead-band do?
Without a dead-band the servo would chase every tiny mains ripple and jitter continuously. The ±3 V dead-band tells the controller to move only when the error is meaningful; a 1-second settle delay then confirms the correction before stopping. This is the detail most viva examiners ask about.
Why a sensing transformer instead of measuring mains directly?
Galvanic isolation. The controller board never touches the 230 V line — it only sees the few-volt replica from the step-down transformer, which is the safe way to sense mains in a student build.
Can this protect equipment from surges?
No. It corrects slow over/under-voltage drift, which is its design purpose. Fast transients need surge protection devices; the report states this distinction explicitly so the claim stays honest.
Can I run heavy loads through it?
The prototype is rated for bench-level loads consistent with the variac's datasheet current rating. Sustained high-current testing must be done in a lab under supervision, with the enclosure closed.
Is this project suitable for a final-year project?
Yes — for Electrical Engineering programs. It covers power electronics hardware, closed-loop control, electromechanical actuation and mains safety, all of which give strong viva material.
Components & software requirements
- Arduino-compatible microcontroller board
- Toroidal variac (variable auto-transformer)
- RC servo motor with mechanical coupling linkage
- Step-down sensing transformer
- Rectifier and filter stage
- Digital voltmeter module (LED display)
- Relay module (optional protection stage)
- Arduino IDE firmware (C++)
Delivery information
Built-to-order project. Delivery timeline is shared after order confirmation based on current queue.
Support terms
Complete documentation, setup guide, and viva preparation included. Support for setup and explanation provided.
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