Circuit Symbols
Drawing style

Transformer symbol

A transformer is drawn as two windings facing each other: the primary, driven by the source, and the secondary, which feeds the load. In the separate-winding form no wire joins them. In the drawings here, lines between them mark a magnetic core.⁠Source 1Source 2Source 3

Reference Designator T⁠Source 4

On this sheet

Symbol and terminals

US style

IEC style

Fig. 1 — Transformer, US windings as loops and IEC windings as arcs; terminals P1, P2, S1, S2

Terminals

⁠Source 1
P1P2
The primary winding is the one driven by the source.
S1S2
The secondary winding is the one that delivers power to the load.

Parts of the drawing

  • Transformer symbol in US style, with numbered leaders to the primary winding, the secondary winding, the core lines and the dots
1 Dots
Transformer dots mark corresponding voltage polarity: each dotted terminal has the same instantaneous polarity relative to its own winding's undotted terminal. The dots do not indicate equal voltage or current direction.⁠Source 2
2 Primary winding
As in the terminal key.
3 Secondary winding
As in the terminal key.
4 Core lines
In the transformer drawings here, parallel lines between windings indicate a magnetic core, drawn as iron-core examples. The air-core example omits these lines; another drawing's legend may use a plain coil more generally.⁠Source 2Source 3
  • Transformer symbol in IEC style, with numbered leaders to the primary winding, the secondary winding, the core lines and the dots
1 Dots
Transformer dots mark corresponding voltage polarity: each dotted terminal has the same instantaneous polarity relative to its own winding's undotted terminal. The dots do not indicate equal voltage or current direction.⁠Source 2
2 Primary winding
As in the terminal key.
3 Secondary winding
As in the terminal key.
4 Core lines
In the transformer drawings here, parallel lines between windings indicate a magnetic core, drawn as iron-core examples. The air-core example omits these lines; another drawing's legend may use a plain coil more generally.⁠Source 2Source 3
Fig. 2 — Transformer symbol, US style, its marks numberedTransformer symbol, IEC style, its marks numbered
  • Transformer symbol, US style, turned 0°, with its terminal names attached

    0°

  • Transformer symbol, US style, turned 90°, with its terminal names attached

    90°

  • Transformer symbol, US style, turned 180°, with its terminal names attached

    180°

  • Transformer symbol, US style, turned 270°, with its terminal names attached

    270°

  • Transformer symbol, US style, mirrored, with its terminal names attached

    Mirrored

Rotating or mirroring a whole symbol, with its terminal identities and connections preserved, does not change the component. The terminal labels move with the drawing.⁠Source 5

  • Transformer symbol, IEC style, turned 0°, with its terminal names attached

    0°

  • Transformer symbol, IEC style, turned 90°, with its terminal names attached

    90°

  • Transformer symbol, IEC style, turned 180°, with its terminal names attached

    180°

  • Transformer symbol, IEC style, turned 270°, with its terminal names attached

    270°

  • Transformer symbol, IEC style, mirrored, with its terminal names attached

    Mirrored

Rotating or mirroring a whole symbol, with its terminal identities and connections preserved, does not change the component. The terminal labels move with the drawing.⁠Source 5

Fig. 3 — Transformer symbol, US style at 0°, 90°, 180° and 270°, and mirroredTransformer symbol, IEC style at 0°, 90°, 180° and 270°, and mirrored

Variants

  • P1P2S1S2CT
    P1P2S1S2CT

    Centre-tapped transformer

    A centre tap is an electrical connection at the midpoint of a winding, shown as an extra lead from that winding.⁠Source 2Source 3

  • P1P2S1S2
    P1P2S1S2

    Air-core transformer

    As in Fig. 2, the parts of the drawing.

Often confused with

    • Transformer symbol in US style; ringed: two or more windings facing each other

      Transformer

      Two or more windings facing each other

    • Inductor symbol in US style; ringed: single winding

      Inductor

      Single winding

    A transformer has two or more windings drawn facing each other, with core lines between them when it has a magnetic core. An inductor is a single winding with two terminals.⁠Source 1Source 2Source 3

    • Transformer symbol in IEC style; ringed: two or more windings facing each other

      Transformer

      Two or more windings facing each other

    • Inductor symbol in IEC style; ringed: single winding

      Inductor

      Single winding

    A transformer has two or more windings drawn facing each other, with core lines between them when it has a magnetic core. An inductor is a single winding with two terminals.⁠Source 1Source 2Source 3

    Fig. 4 — Transformer and inductor, the mark that tells them apart ringedTransformer and inductor, the mark that tells them apart ringed

Where you meet it

  • A transformer between an AC source and a lamp, drawn with no values
  • A transformer between an AC source and a lamp, drawn in IEC style with no values
Fig. 5 — A transformer between an AC source and a lamp; values left offA transformer between an AC source and a lamp; values left off ⁠Source 4

Notes

An autotransformer shares one winding, so its sides are not isolated

An autotransformer is drawn as a single winding with taps rather than separate windings. Its primary and secondary connections share that winding, so there is no electrical isolation between them.⁠Source 2 Every transformer on this sheet has separate windings, with no wire between them; the autotransformer is the exception to look for.

Loops and humps draw the same winding

Looped and semicircular coil drawings are alternative inductor forms. This sheet uses loops in its US view and humps in its IEC view; the winding and terminal roles are unchanged.⁠Source 3 A single winding on its own is drawn the same way as an inductor; two windings facing each other, with or without core lines, make a transformer.

Check your reading

More questions on the practice page

Pick an answer to see the reason.

  1. Question 1

    Is there a wire connection from the primary winding to the secondary in this drawing?

    P1P2S1S2

    Two rows of loops facing each other, with two straight parallel lines between them and a dot near the top of each row. The left row's leads, going left, are labelled P1 at the top and P2 at the bottom; the right row's leads, going right, are labelled S1 and S2.

    P1P2S1S2

    Two rows of humps facing each other, with two straight parallel lines between them and a dot near the top of each row. The left row's leads, going left, are labelled P1 at the top and P2 at the bottom; the right row's leads, going right, are labelled S1 and S2.

    Not this one Right Answer

    The answer is “No: the windings are linked magnetically, not by a wire”. In the transformer drawings here, parallel lines between windings indicate a magnetic core, drawn as iron-core examples. The air-core example omits these lines; another drawing's legend may use a plain coil more generally.An autotransformer is drawn as a single winding with taps rather than separate windings. Its primary and secondary connections share that winding, so there is no electrical isolation between them. The separate-winding transformer shown here transfers energy magnetically between windings without an internal conducting connection between them. This description excludes an autotransformer, whose windings share a conducting connection.

  2. Question 2

    Its labels are hidden. How do you know which winding is the primary?

    Two rows of loops facing each other, with two straight parallel lines between them and a dot near the top of each row. The left row's two leads go out to the left and the right row's two leads go out to the right.

    Two rows of humps facing each other, with two straight parallel lines between them and a dot near the top of each row. The left row's two leads go out to the left and the right row's two leads go out to the right.

    Not this one Right Answer

    The answer is “It is the winding the source drives”. Transformer dots mark corresponding voltage polarity: each dotted terminal has the same instantaneous polarity relative to its own winding's undotted terminal. The dots do not indicate equal voltage or current direction.Rotating or mirroring a whole symbol, with its terminal identities and connections preserved, does not change the component. The terminal labels move with the drawing. The primary winding is the one driven by the source.

  3. Question 3

    What do the dots beside the two windings mark?

    P1P2S1S2

    Two rows of loops facing each other, with two straight parallel lines between them. A dot sits near the top of each row: beside P1 on the left row and beside S1 on the right row.

    P1P2S1S2

    Two rows of humps facing each other, with two straight parallel lines between them. A dot sits near the top of each row: beside P1 on the left row and beside S1 on the right row.

    Not this one Right Answer

    The answer is “Ends with the same polarity at the same instant”. In the drawing convention used here, a dot marks a wire connection and a plain four-way crossing without a dot is not connected. Older drawings may use another convention, so check their legend. Transformer dots mark corresponding voltage polarity: each dotted terminal has the same instantaneous polarity relative to its own winding's undotted terminal. The dots do not indicate equal voltage or current direction.

  4. Question 4

    What is the extra lead from the middle of the secondary winding?

    P1P2S1S2?

    Two rows of loops facing each other, with two straight parallel lines between them. The left row's leads are labelled P1 and P2 and the right row's S1 and S2. A fifth lead, labelled ?, leaves the middle of the right row.

    P1P2S1S2?

    Two rows of humps facing each other, with two straight parallel lines between them. The left row's leads are labelled P1 and P2 and the right row's S1 and S2. A fifth lead, labelled ?, leaves the middle of the right row.

    Not this one Right Answer

    The answer is “A centre tap”. This sheet distinguishes earth, chassis and signal reference. They can be connected in a circuit, but the three labels have different meanings; ground does not always mean a connection to earth.An extra lead joined partway along a coil marks a tapped inductor: the tap is a connection to part of the winding. A centre tap is an electrical connection at the midpoint of a winding, shown as an extra lead from that winding.

  5. Question 5

    No lines are drawn between these two windings. What does that mean in the drawings here?

    P1P2S1S2

    Two rows of loops facing each other with nothing drawn between them. P1 and P2 label the left row's leads, and S1 and S2 the right row's.

    P1P2S1S2

    Two rows of humps facing each other with nothing drawn between them. P1 and P2 label the left row's leads, and S1 and S2 the right row's.

    Not this one Right Answer

    The answer is “An air core: there is no magnetic core”. A transformer has two or more windings drawn facing each other, with core lines between them when it has a magnetic core. An inductor is a single winding with two terminals.The separate-winding transformer shown here transfers energy magnetically between windings without an internal conducting connection between them. This description excludes an autotransformer, whose windings share a conducting connection. In the transformer drawings here, parallel lines between windings indicate a magnetic core, drawn as iron-core examples. The air-core example omits these lines; another drawing's legend may use a plain coil more generally.

Answer key

  1. 1

    No: the windings are linked magnetically, not by a wire

  2. 2

    It is the winding the source drives

  3. 3

    Ends with the same polarity at the same instant

  4. 4

    A centre tap

  5. 5

    An air core: there is no magnetic core

Questions

Which side of a transformer symbol is the primary?
The winding the source drives, wherever it is drawn. Turning or mirroring the symbol does not change which winding is the primary and which the secondary.⁠Source 1Source 5
What is a centre tap on a transformer?
A connection to the middle of a winding, drawn as an extra lead from that winding. Here it is labelled CT.⁠Source 2Source 3
What do the dots on a transformer symbol mean?
They mark matching polarity. At any instant, each dotted end has the same polarity, measured against the other end of its own winding. They do not show equal voltages or which way current flows.⁠Source 2

Print and files

Printable symbol sheetPrintable symbol sheet More worksheets

Variants as SVG line art: Centre-tapped transformerCentre-tapped transformer, Air-core transformerAir-core transformer

Free to reuse under CC BY 4.0. Credit: Circuit Symbols (circuitsymbols.com). Licence and credit

Sources

7 sources, numbered as cited
  1. ModEL — Components and Symbols, §§3.13–3.14 (inductors and transformers, pp. 24–26)

    Tony R. Kuphaldt, retrieved 2026-09-22.

  2. ModEL — Transformers, §§4.1, 4.3–4.4 (windings, centre taps, autotransformers and dots)

    Tony R. Kuphaldt, retrieved 2026-09-22.

  3. Lessons In Electric Circuits, Vol. V — §9, Circuit Schematic Symbols (pp. 130–139)

    Tony R. Kuphaldt, retrieved 2026-09-22.

  4. Design to Manufacturing Standards — Reference Designators (slide 33)

    Dieter Bergman, IPC, retrieved 2026-09-22. BT battery … CB circuit breaker … DS display, lamp, light emitting diode … RT thermistor

  5. Electrical Schematics, §§1.3.1–1.3.5 — nodes, crossings, references and congruent drawings

    Tufts University, retrieved 2026-09-22.

  6. Chassis, Earth and Signal Grounding: Terminology and Symbols

    NI, retrieved 2026-09-22.

  7. DC Electrical Circuit Analysis — resistive devices and schematic ground symbols (pp. 53, 59–64)

    James M. Fiore, Mohawk Valley Community College, retrieved 2026-09-22.