A variac (also called a dimmer coil, variable autotransformer, or AC voltage controller) is an industrial device that allows smooth, step-less control of AC output voltage from 0V to the full mains voltage — or above in some designs. Unlike fixed transformers, a variac provides continuously variable output by moving a carbon brush along an exposed section of the winding.
How a Variac Works
flowchart LR A["⚡ AC Input 230V / 415V Mains"] --> B["Toroidal Autotransformer Core"] B --> C["Carbon Brush slides along exposed winding"] C --> D["Output taken from brush position"] D --> E["0V → 270V Step-less control"] F["Manual Knob or Motor Drive"] --> C
The entire coil acts as both primary and secondary. Moving the carbon brush changes the number of turns between input and output, changing the output voltage smoothly with no switching or distortion.
Types of Variacs / Dimmer Coils
| Type | Description | Capacity Range |
|---|---|---|
| Single Phase Variac | Single winding, 230V input | 2A – 100A |
| Three Phase Variac | Three independent windings, 415V 3ph | 10A – 500A+ |
| Enclosed Dimmer | Variac in a steel cabinet with knob/digital control | Custom |
| Motorised Variac | Variac driven by servo motor (used inside servo stabilizers) | All sizes |
Industrial Applications
- Heating Control — Furnaces, ovens, and heating elements
- Testing Laboratories — Equipment testing at variable voltages
- Manufacturing — Plastic moulding, printing, packaging machinery
- Lighting Control — Stage lighting, studio lighting
- Motor Speed Control — Variable speed drives for small motors
- Quality Control — Testing products at different voltage levels
Variac vs Dimmer — What’s the Difference?
| Feature | Industrial Variac / Dimmer Coil | Electronic (TRIAC) Dimmer |
|---|---|---|
| Technology | Variable autotransformer | TRIAC/thyristor switching |
| Efficiency | ~98% (magnetic transfer) | ~80–90% (resistive losses) |
| Power Range | 1A – 1000A+ | Small loads only (up to ~10A) |
| Output Quality | Pure sine wave | Chopped / distorted sine wave |
| Inductive Loads | Fully compatible | Not suitable |
| Applications | Furnaces, motors, labs, industry | Residential lighting only |
ServoKai Dimmer & Variac Products
ServoKai manufactures dimmer coils, single phase dimmers, three phase industrial dimmers, and enclosed variac dimmers with manual or motorized control. All units use precision copper winding and high-quality toroidal cores. Browse our dimmer product range →