Um auto transformer symbol is the graphic representation used in circuit diagrams and electrical schematics to identify an autotransformer. Unlike the conventional two-winding transformer symbol, the autotransformer symbol uses a single continuous winding with one or more tap connections, visually communicating its shared-coil design without separate primary and secondary windings.
Correctly reading this symbol is essential for anyone working with electrical drawings — whether you are designing a motor starter panel, reviewing a power distribution schematic, or sourcing a transformer for a project. This guide explains what the Johsun Tec auto transformer symbol looks like, how it differs from other transformer symbols, the key variations for single-phase, three-phase, and variable types, and how to interpret it in a real circuit diagram.
In electrical schematics, the auto transformer symbol consists of a single coiled winding symbol — typically drawn as a series of semicircular loops or a continuous spiral — with one or more tap points marked along its length. The symbol may also include a core representation (two parallel lines) to indicate an iron-core design, or it may omit the core lines for air-core or simplified drawings.
The defining characteristic that separates the auto transformer symbol from the conventional transformer symbol is the absence of separate primary and secondary coil symbols. Instead, a single winding symbol serves both input and output, with taps that indicate where voltage is taken or applied. This directly mirrors the physical construction of an autotransformer, where one continuous winding acts as both the primary and secondary circuit.
For a deeper understanding of how autotransformers work, see our guide on O que é um autotransformador?.
The quickest way to distinguish an autotransformer from a conventional transformer in any schematic is to look at the number of separate windings drawn. The table below summarizes the visual differences.
| Visual Feature | Auto Transformer Symbol | Conventional Transformer Symbol |
|---|---|---|
| Winding count | One continuous winding | Two separate windings (primary and secondary) |
| Tap points | One or more taps along the winding | No shared taps between windings |
| Core symbol | Two parallel lines between or beside the winding | Two parallel lines between the windings |
| Electrical connection | Winding is continuous — input and output share the conductor | No direct electrical connection between primary and secondary |
| Polarity dots | Can appear at winding ends or tap points | Typically appear at the start of each separate winding |
| Common drawing style | A spiral or looped line with an arrow or tap mark at the connection point | Two independent sets of loops, often drawn on opposite sides of the core |
This distinction matters in practice: if you see two separate coil symbols with a core between them, the drawing shows a conventional isolation transformer. If you see one continuous coil with a tap, it is an autotransformer — and there is no galvanic isolation between input and output.
The single-phase autotransformer symbol is the most basic form. It consists of a continuous winding drawn as a series of loops alongside a core, with at least three electrical connection points: two end terminals and one intermediate tap.
In a step-up configuration, the input is connected across a portion of the winding (fewer turns), and the output is taken across the entire winding (more turns). The symbol reflects this with the input leads attached to a tap point and one end, while the output leads connect to both ends of the full winding.
Key symbol features for step-up:
In a step-down configuration, the input is connected across the full winding (more turns), and the output is taken from a tap point (fewer turns). The symbol shows the input across the entire coil and the output connected between one end and an intermediate tap.
Key symbol features for step-down:
Many schematics do not explicitly label the direction — it is inferred from the voltage values written next to the terminals. A good rule of thumb: the side with fewer turns carries the lower voltage.
A three-phase autotransformer symbol represents three single-phase autotransformer units or a single three-limb core with shared windings. In a schematic, the symbol typically shows three identical autotransformer winding symbols arranged in either a star (wye) or delta configuration.
The most common three-phase autotransformer symbol uses a star connection — three windings connected at a common neutral point, with each winding drawn as a continuous coil with a tap. The delta-connected variant shows the windings arranged in a closed triangle.
| Connection Type | Symbol Appearance | Uso típico |
|---|---|---|
| Star (Y) autotransformer | Three windings originating from a common neutral point, each with a series winding and tap | Transmission interconnection, distribution voltage regulation |
| Delta autotransformer | Three windings connected end-to-end in a closed loop, each with a tap | Industrial motor starting, phase balancing |
| Zigzag autotransformer | Interconnected winding segments drawn in a zigzag pattern | Grounding transformers, harmonic mitigation |
On a three-phase autotransformer symbol, look for the common neutral point in star connections — it is a key indicator that distinguishes it from a conventional three-phase transformer symbol, where star windings on the primary and secondary sides are electrically separate.
A variable autotransformer — commonly known by the trade name Variac — uses a slightly different symbol to indicate its adjustable output. The standard symbol for a variable autotransformer shows a single winding with an arrow drawn diagonally across the coil, representing the sliding contact or wiper arm that moves along the winding to select the output voltage.
The arrow typically points toward the center of the winding and may have a separate terminal label. This is the standard IEC and ANSI convention for any variable or tapped inductive component.
In addition to the arrow, the symbol may include:
Variable autotransformers are commonly drawn in laboratory power supply schematics, test bench wiring diagrams, and voltage adjustment circuits.
Electrical symbols are not universal — two major standards bodies define how autotransformer symbols should be drawn. Understanding the differences helps you read schematics from different regions and manufacturers correctly.
| Aspecto | IEC 60617 (International) | ANSI/IEEE Std 315 (North America) |
|---|---|---|
| Winding style | Semicircular loops (scallops) | Continuous spiral or sine-wave curves |
| Step-up/down distinction | Indicated by tap position and voltage labels | Often drawn with longer winding on the high-voltage side |
| Variable arrow | Arrow at 45 degrees crossing the winding | Arrow crossing the winding symbol similarly |
| Core representation | Two solid parallel lines | Two solid parallel lines (same convention) |
| Phase marking | U, V, W or L1, L2, L3 | A, B, C or H1-H2, X1-X2 |
In practice, most modern schematics follow a hybrid approach. The key takeaway is that regardless of the standard, an autotransformer is always recognizable by its single coil with a tap — the stylistic details (loop shape, phase labels) vary by region but the underlying meaning is the same.
When you encounter an autotransformer symbol in a practical circuit diagram, follow these steps to interpret it correctly:
If you are selecting an autotransformer for an actual project, recognizing the symbol is the first step. The next is matching the right model to your load. Browse our DT/ST Series autotransformers for fixed-ratio voltage conversion or contact our team for custom winding specifications.
The auto transformer symbol is a simple but informative graphic that communicates the core design principle of the autotransformer at a glance: one winding, shared by input and output, with taps that define the voltage relationship. Whether you are reading a power system schematic, a motor control drawing, or a product datasheet, recognizing this symbol — and the variations for single-phase, three-phase, step-up, step-down, and variable types — helps you make faster, more accurate decisions. For more on interpreting transformer symbols across different types, see our transformer symbols guide.
An autotransformer symbol shows a single continuous winding — typically drawn as a series of semicircular loops — with one or more tap points along the coil and two parallel lines representing the iron core. It lacks the separate primary and secondary windings found in a conventional transformer symbol.
Count the windings. If there is one continuous coil with a tap, it is an autotransformer. If there are two separate sets of coils on opposite sides of the core, it is a conventional transformer. The autotransformer symbol never shows electrically separate primary and secondary windings.
Not inherently — the same symbol is used for both. The direction is determined by how the terminals are connected: input across a tap and one end (step-up) vs input across the full winding (step-down). Voltage labels next to the terminals clarify the ratio in a complete schematic.
A variable autotransformer symbol adds a diagonal arrow crossing the winding, representing the sliding contact or wiper. This arrow distinguishes it from a fixed-ratio autotransformer and indicates that the output voltage is continuously adjustable.
The underlying concept is the same — a single winding with a tap — but the drawing style differs. IEC uses semicircular loops (scallops), while ANSI uses a continuous spiral or sine-wave curve. Terminal phase labels also differ: IEC uses U/V/W or L1/L2/L3, ANSI uses A/B/C or H1-H2/X1-X2.
A three-phase autotransformer is drawn as three single autotransformer winding symbols connected in star (wye) or delta configuration. In a star connection, all three windings share a common neutral point. The symbol may also show a tertiary delta winding for harmonic suppression in large power autotransformers.
