Variable Transformer (Variac) Symbol
Definition: The Variable Transformer (Variac) symbol represents an adjustable autotransformer in circuit diagrams, depicted as two magnetically coupled coils — or a single tapped coil — with an arrow superimposed on the secondary winding to indicate continuous voltage adjustment from zero to above mains voltage, as defined by IEC 60617-06 (transformers and inductors) and ANSI Y32.2 / IEEE 315-1975; the designator is T or AT.
Also known as: Variac, variable autotransformer, variable transformer, adjustable transformer, powerstat, autotransformer with variable tap, slidac.
What the Variable Transformer (Variac) symbol means
The Variable Transformer symbol denotes an autotransformer whose output voltage is continuously adjustable by means of a carbon-brush wiper that slides along an exposed section of the toroidal winding, delivering any voltage from 0 V to typically 110–140% of the mains input without the galvanic isolation of a two-winding transformer. The symbol is drawn as a standard transformer pair of coils with an arrow overlaid on the secondary coil, visually communicating the adjustable nature of the output tap.
In a schematic, the Variable Transformer symbol carries the critical implication that the output is electrically continuous with the mains — there is no isolation barrier — meaning chassis and output conductors can reach line potential if connected incorrectly. Engineers reading the diagram understand this safety distinction immediately from the autotransformer topology shown, and they know the output voltage is set manually by rotating the wiper knob, making the variable transformer indispensable for controlled AC voltage test environments.
How to identify the Variable Transformer (Variac) symbol
The Variable Transformer symbol shows two adjacent sets of arc-shaped coils (or a single coil loop for autotransformer variants) representing the winding, with a vertical or angled arrow touching or overlapping the secondary coil set to indicate the adjustable wiper tap. Four pins are present: L (line/live input) and N (neutral input) on the primary side, and V+ (variable voltage output) and V− (neutral/return output) on the secondary side. The arrow distinguishes this symbol from a fixed transformer; the autotransformer layout (shared winding) may be shown explicitly or implied by the label 'VARIAC' or 'AT'.
Function in a circuit
The Variable Transformer provides a smoothly adjustable AC output voltage derived from the mains supply, allowing engineers to power equipment under test at voltages from 0 V to above nominal mains, to perform voltage-stress testing, lamp dimming, motor speed adjustment on single-phase AC motors, and heater element power control. The wiper's position sets the turns ratio, and because the output winding shares conductor with the input, efficiency is very high — typically 98–99% — and the device adds negligible waveform distortion.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-06 (transformers) defines the variable transformer symbol as a transformer pair of coils with an arrow on the adjustable secondary winding. The autotransformer variant shows a single coil with a common terminal between input and output, plus the adjustable arrow. The applicable product standard is IEC 61558-2-14 (variable autotransformers). |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 shows the variable transformer as two magnetically coupled inductors (winding arcs) with an arrow through the secondary coil, consistent with the adjustable-component arrow convention used for variable resistors and capacitors. The autotransformer connection is shown with a common winding node. |
| Key difference | IEC 60617-06 and ANSI Y32.2/IEEE 315 are functionally identical for the variable transformer: both use the transformer coil pair with a superimposed arrow on the variable winding. Minor difference: IEC may show the core bar between windings more prominently, while ANSI sometimes omits the core bar for air-core autotransformers. |
Terminals / pins
| Pin | Name |
|---|---|
| pri_l | L |
| pri_n | N |
| sec_h | V+ |
| sec_l | V- |
Typical values
Input voltage: 120 V AC (North America) or 230–240 V AC (IEC/European); output range: 0 V to 140% of input (e.g., 0–140 V from 120 V input, 0–270 V from 240 V input); frequency: 50/60 Hz; power ratings: 0.5 kVA to 20 kVA typical bench/industrial; efficiency: 98–99%; waveform: sinusoidal (no distortion added).
Where the Variable Transformer (Variac) symbol is used
- Laboratory bench testing of transformers, motors, and power supplies at controlled input voltages
- Burn-in and voltage-stress testing of electronic equipment from below nominal to above nominal mains
- Incandescent lamp and resistive heater dimming where a pure sine wave is required without waveform chopping
- Single-phase AC motor speed adjustment in educational and research settings
- Pre-charging and commissioning large capacitor banks by ramping voltage gradually from zero
- Electroplating and electrolytic bath current control via variable voltage input
- Medical equipment testing requiring precise mains-voltage simulation at ±10% and ±20% of nominal
Example
In a motor-testing lab schematic, the Variable Transformer symbol (1 kVA, 0–240 V) appears between the 230 V mains supply and a 0.37 kW single-phase induction motor; the engineer sets the Variac to 115 V (50% of nominal) to measure starting torque at reduced voltage, then increases it to 253 V (+10%) for voltage-tolerance verification, with the schematic clearly showing the autotransformer topology and the absence of galvanic isolation.
Key facts
- The Variable Transformer (Variac) symbol is a transformer coil pair with an arrow on the secondary winding, defined in IEC 60617-06 and ANSI Y32.2/IEEE 315-1975.
- The designator for a variable transformer is T or AT (autotransformer) in schematics, per IEEE 315-1975.
- A Variac is an autotransformer — input and output share the same winding — so there is NO galvanic isolation; the output can reach mains potential if the wiper is at 100% or above.
- Output voltage range is typically 0 V to 110–140% of the input voltage, achieved by the brush tapping above the mains input connection on the toroidal winding.
- Efficiency of a variable autotransformer is typically 98–99% because most power is transferred conductively through the shared winding, not magnetically.
- IEC 61558-2-14 is the specific product standard governing the design, testing, and ratings of variable autotransformers.
- The four standard schematic pins are L (live) and N (neutral) on the primary, and V+ (variable output) and V− (neutral/return) on the secondary.
- Variac is a registered trade name historically associated with General Radio; genericised terms include Powerstat (Superior Electric) and Slidac; the IEC term is 'variable autotransformer'.
Frequently asked questions
What does the Variable Transformer (Variac) symbol mean in a circuit diagram?
The Variable Transformer symbol represents an adjustable autotransformer that provides a continuously variable AC output voltage from 0 V to above mains voltage. The symbol shows transformer coils with an arrow on the secondary winding, indicating that the output tap is adjustable by a rotating wiper.
What does the variable transformer symbol look like?
The variable transformer symbol shows two sets of arc-shaped coil loops representing primary and secondary windings, with an arrow superimposed on the secondary coil to indicate the movable tap. Four terminals are shown: L and N inputs on the primary side, V+ and V− outputs on the secondary side.
Is the variable transformer the same as a Variac?
Yes. 'Variac' is a trade name (historically General Radio) for a variable autotransformer; it is synonymous with variable transformer, Powerstat, Slidac, and the IEC term 'variable autotransformer'. All refer to the same device: a toroidal autotransformer with a sliding brush output tap.
What is the difference between a variable transformer and a fixed transformer symbol?
A fixed transformer symbol shows two sets of coils with no arrow, implying a constant turns ratio. A variable transformer adds an arrow through the secondary coil, indicating an adjustable wiper tap. An autotransformer variant shows a single tapped coil rather than two separate windings.
Is a Variac galvanically isolated?
No. A Variac is an autotransformer — input and output share the same continuous winding — so there is no galvanic isolation between the mains supply and the output. The output terminals can reach mains potential, which requires appropriate PPE and circuit protection when working at the output.
What standard governs variable transformer symbols?
IEC 60617-06 defines the variable transformer symbol for schematic use; IEC 61558-2-14 is the product standard for variable autotransformers. ANSI Y32.2/IEEE 315-1975 uses the same adjustable-arrow transformer symbol in North American schematics.
What is the designator letter for a variable transformer?
The variable transformer is designated T (transformer) or AT (autotransformer) in a schematic, followed by a number (e.g., T1, AT2), per IEEE 315-1975 and ANSI Y32.2.
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