Voltage Source (AC) Symbol

Definition: The Voltage Source (AC) symbol represents an ideal two-terminal source of sinusoidal alternating voltage, defined in IEC 60617-02 as a circle enclosing a sine wave symbol, and in ANSI Y32.2 / IEEE 315 as a circle with a sine wave or tilde inside, indicating a source that maintains a specified AC voltage across its terminals regardless of the current drawn from it.

Also known as: AC source, alternating voltage source, sinusoidal voltage source, AC supply symbol, mains supply symbol, AC generator symbol.

What the Voltage Source (AC) symbol means

The AC Voltage Source symbol denotes an ideal source that supplies a voltage varying sinusoidally with time, expressed as v(t) = V_peak x sin(2*pi*f*t + phi), where V_peak is the peak amplitude, f is the frequency in hertz, and phi is the initial phase angle. In practical circuit analysis, the AC source is characterised by its RMS voltage (V_RMS = V_peak / sqrt(2)) and frequency; for a standard mains supply this is 120 V RMS at 60 Hz (North America) or 230 V RMS at 50 Hz (Europe, Africa, Asia, Australia).

In schematics, the AC Voltage Source symbol communicates that the source is alternating rather than direct current, and that load analysis requires AC circuit methods (impedance, phasors, and frequency-domain analysis) rather than simple DC Ohm's law. The symbol appears in power electronics, audio circuit analysis, AC motor drive schematics, signal generator representations, and any circuit where a sinusoidal excitation is applied. The two terminals are labelled + and - for instantaneous polarity reference, though AC polarity reverses every half-cycle.

How to identify the Voltage Source (AC) symbol

The AC Voltage Source symbol is a circle with a sine wave (S-shaped curve) drawn inside it. Two terminals extend from the circle: one from the top (or left) and one from the bottom (or right). In some conventions, a plus (+) sign appears at the positive reference terminal to define the reference polarity for phasor analysis, though this does not imply a fixed polarity as it would in a DC source. The sine wave inside the circle may be a simple smooth S-curve or a more detailed sinusoidal arc. This distinguishes the AC source from the DC voltage source symbol, which uses a circle with a long (positive) and short (negative) horizontal line instead of a sine wave.

Function and usage boundary

In a circuit, the Ideal AC Voltage Source maintains a sinusoidal voltage across its terminals with zero internal impedance, regardless of the load connected. Real AC sources (generators, mains supply transformers, function generators) are modelled as an ideal AC source in series with a source impedance, which limits the current and causes terminal voltage to drop under heavy load. The AC source drives current through reactive components (inductors and capacitors) that create frequency-dependent phase shifts, forming the basis of filters, resonant circuits, power factor correction, and AC power transmission analysis.

Standards: IEC vs ANSI

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Logical terminals

Where the Voltage Source (AC) symbol is used

• Representing the AC mains supply input in household appliance and power supply schematics
• Signal generator output in audio amplifier and filter test circuit diagrams
• AC motor stator supply in variable-frequency drive (VFD) circuit schematics
• Thevenin equivalent source in AC circuit analysis and power system modelling
• AC excitation source in transformer schematic models
• Sinusoidal excitation source in impedance measurement and frequency response circuit diagrams
• AC power transmission and distribution system one-line diagrams

Key facts

• The AC Voltage Source symbol is a circle with a sine wave (or tilde) drawn inside it, per IEC 60617-02 and ANSI Y32.2 / IEEE 315; the two terminals indicate the positive reference and negative reference nodes.
• An ideal AC voltage source maintains a specified sinusoidal voltage regardless of load current; its internal impedance is zero, unlike real sources which have source impedance causing terminal voltage droop under load.
• The AC source is characterised by V_RMS (root mean square voltage), frequency f in hertz, and phase angle phi; V_peak = V_RMS x sqrt(2), so a 230 V RMS mains supply has a peak of approximately 325 V.
• The AC Voltage Source symbol is distinguished from the DC Voltage Source symbol by the sine wave (or tilde) inside the circle, versus the long-positive / short-negative bar notation inside or beside the DC circle.
• Standard mains AC frequencies are 50 Hz (IEC regions: Europe, Africa, Asia, Australia) and 60 Hz (North America, Japan at 100 V/60 Hz western regions); this frequency must be noted alongside the AC source symbol in power schematics.
• IEC 60617-02 and ANSI Y32.2 / IEEE 315 define the same AC source glyph (circle with sine wave); the symbols are visually identical across both international standards.
• In AC circuit analysis, the plus (+) terminal of the AC source symbol defines the reference polarity for phasor voltage calculations; the polarity physically reverses every half-cycle but the reference direction is fixed for mathematical consistency.

Diagrams that use this symbol

• circuit diagram
• and circuit diagram
• wiring diagram
• 3 way switch wiring
• wiring a three way switch
• 3 way switch wiring diagram
• 3 way switch wiring schematic
• three way switch connection diagram

Frequently asked questions

What does the AC voltage source symbol look like?

The AC voltage source symbol is a circle with a sine wave or tilde (~) drawn inside it. Two terminals extend from the circle. The sine wave inside distinguishes it from the DC voltage source, which has a long line (positive) and short line (negative) instead.

What does the AC voltage source symbol mean in a circuit diagram?

The AC voltage source symbol means a source of sinusoidally alternating voltage is present. It indicates the circuit operates under AC conditions, requiring impedance and phasor analysis methods, and that the voltage reverses polarity every half-cycle at the specified frequency.

What is the difference between the AC and DC voltage source symbols?

The AC voltage source symbol is a circle with a sine wave or tilde inside. The DC voltage source symbol is a circle with a long line (positive terminal) and a short line (negative terminal) inside or beside it. In some conventions the DC symbol shows a + and - sign instead of lines. The AC symbol has no fixed +/- polarity implied by the sine wave.

What standard defines the AC voltage source symbol?

The AC voltage source symbol is defined in IEC 60617-02 (binary logic and analogue elements — sources) as a circle with a sine wave inside. ANSI Y32.2-1975 / IEEE 315-1975 defines the same symbol. The two standards produce visually identical glyphs.

What is the RMS voltage of an AC source and how is it related to peak voltage?

For a sinusoidal AC source, V_RMS = V_peak / sqrt(2), which is approximately V_peak x 0.707. A 230 V RMS mains supply has a peak voltage of about 325 V. V_RMS is the value that delivers the same power to a resistive load as an equivalent DC voltage of the same magnitude.

Is the polarity marking on the AC source symbol meaningful?

Yes, but only as a reference direction for phasor analysis. The + terminal on the AC voltage source defines the assumed positive polarity reference for writing Kirchhoff's voltage law equations. It does not imply the terminal is always positive; AC polarity physically alternates every half-cycle.

What frequency is assumed for an AC voltage source symbol?

The frequency is not implied by the symbol itself and must be stated in the schematic label or title block. Standard power-frequency AC sources operate at 50 Hz (IEC regions) or 60 Hz (North America). Signal generator or oscillator sources may operate at any frequency from sub-hertz to gigahertz.

Related symbols

• 3-Phase Supply symbol
• Battery symbol
• Breadboard Power Rail symbol
• Power Rail symbol
• Voltage Source (DC) symbol
• Zero-Crossing Detector symbol

Physical breadboard, PCB, and panel mapping

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