Autotransformer / Variac
Transformer
A transformer with a single tapped winding shared by primary and secondary. The lab 'variac' version has a sliding brush that gives a smoothly variable AC output from zero to above line voltage.
Schematic symbol
Transformer
How autotransformer / variac appears in a circuit diagram.
Types & variants
Key specs
Input voltage
V (AC)Rated line voltage, typically 230 V or 415 V three-phase.
Output range
V (AC)Variacs typically give 0–270 V from a 230 V input — the winding extends past the input tap.
Current rating
ASet by the brush and winding wire; exceeding it burns the track at one spot.
kVA rating
kVABecause part of the power passes conductively, a small winding handles a large throughput — the key autotransformer advantage.
Markings
Panel marks input terminals, output terminals and a dial calibrated 0–100% or in volts. Fixed autotransformers have a nameplate with ratio and kVA like any transformer.
Standard values
Bench variacs: 230 V input, 0–270 V output at 2 A, 4 A, 8 A, 10 A, 15 A, 28 A. Three-phase dimmerstats: 415 V input, 0–470 V output.
How to choose
1) Decide the maximum current the load draws and add margin. 2) Prefer a closed metal case with a fuse for bench safety. 3) Remember there is NO isolation — output shares a conductor with the mains, so pair with an isolation transformer for service work on mains equipment. 4) For fixed voltage correction, an autotransformer is smaller and cheaper than a two-winding unit when the ratio is near 1.
Pinout & package
Toroidal winding on a laminated core with a carbon brush riding on a bared track, dial on top, terminals for input, output and common. Fixed types are conventional transformer packages with three terminals: common, tap, and full winding.
Example circuits
- Slowly ramping mains voltage to reform capacitors in vintage equipment
- Auto-transformer starting of a large induction motor (reduced-voltage start)
- Servo voltage stabiliser: motorised variac driven by a comparator
- Testing a product's behaviour at brownout voltages (e.g. 170 V)
Common failures
Worn or chipped carbon brush (crackling, dead spots), burnt track from overloading at one position, open winding, and shocks from users forgetting the output is not isolated from mains.
How to test
With power off, check winding continuity end to end and from brush to each end (should vary as you rotate). Power through a bulb limiter first; verify output tracks the dial smoothly with no dead zones or sparking.
Substitutes
For isolated variable AC, use a variac feeding a 1:1 isolation transformer. For DC experiments a bench supply replaces the variac + rectifier combo. Fixed-ratio units can be replaced by a two-winding transformer of the same ratio (bulkier but isolated).