Voltage Source (DC) Symbol
Definition: The Voltage Source (DC) symbol represents an ideal two-terminal source of constant direct voltage, defined in IEC 60617-02 as a circle with a long line (positive terminal) and a short line (negative terminal) inside or adjacent to it, and similarly in ANSI Y32.2 / IEEE 315, indicating a source that maintains a specified DC voltage across its terminals regardless of the current drawn from it.
Also known as: DC source, direct voltage source, ideal voltage source, battery symbol, DC supply symbol, EMF source.
What the Voltage Source (DC) symbol means
The DC Voltage Source symbol denotes an ideal source that maintains a constant voltage between its two terminals, with the long line representing the positive terminal (higher potential) and the short line representing the negative terminal (lower potential, or reference). An ideal DC voltage source has zero internal resistance, meaning the terminal voltage remains perfectly constant regardless of load current. Real sources such as batteries, DC power supplies, and regulated voltage rails are modelled as an ideal DC source in series with an internal resistance (r) that causes terminal voltage to drop under load.
In schematics, the DC Voltage Source symbol communicates that the energy source is unidirectional and constant, and that standard DC circuit analysis (Ohm's law, Kirchhoff's laws with constant values) applies. The voltage value in volts is labelled next to the symbol (e.g. 5 V, 12 V, 9 V). The positive terminal is always at higher potential. The designator V or E (for electromotive force) is used adjacent to the symbol.
How to identify the Voltage Source (DC) symbol
The DC Voltage Source symbol is a circle with two parallel horizontal lines inside or directly adjacent: a longer line (representing the positive terminal) and a shorter line (representing the negative terminal). In many conventions only the two lines are drawn without the circle, especially for battery symbols. A plus (+) sign is often placed near the longer line and a minus (-) sign near the shorter line. Some schematics use only the + and - signs attached to the terminals instead of the line-length convention. The absence of a sine wave or tilde inside the circle distinguishes this symbol from the AC Voltage Source.
Function in a circuit
In a circuit, the Ideal DC Voltage Source maintains a specified constant voltage V across its terminals and supplies whatever current the connected circuit requires. The positive terminal sources current into the external circuit; the current returns through the negative terminal to complete the loop. In real circuits, a DC source is modelled with internal resistance r_int in series, and terminal voltage drops to V - I x r_int under load current I. The DC source symbol is the fundamental starting point for all DC circuit analysis, powering resistors, capacitors, inductors, transistors, and integrated circuits.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-02 defines the DC voltage source symbol as a circle with a long line (positive) and a short line (negative). The long-line/short-line convention is also used for the battery symbol (multiple cell pairs) in IEC 60617. The standard document reference is IEC 60617 database symbol S00273. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 defines the same circle with long-positive / short-negative line convention for the DC voltage source. The battery symbol in ANSI uses the same alternating long-short line pairs to represent multiple electrochemical cells. |
| Key difference | IEC and ANSI symbols for the DC voltage source are visually identical: a circle (or simply two lines for a battery) with a longer positive line and shorter negative line. Some older ANSI schematics showed a + and - sign without the circle, but modern practice in both standards uses the same long-line/short-line glyph. |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Common DC supply voltages: 1.5 V (single alkaline cell), 3.3 V (logic), 3.7 V (Li-ion cell nominal), 5 V (USB), 9 V (PP3 battery), 12 V (automotive, ATX PSU), 24 V (industrial), 48 V (PoE, telecom). Theoretical ideal source: zero internal resistance. Real battery internal resistance: 0.05 Ohm to several Ohms depending on type and state of charge.
Where the Voltage Source (DC) symbol is used
- Representing battery supplies in portable device schematics (phones, remote controls, flashlights)
- Modelling regulated DC power supply rails (5 V, 3.3 V, 12 V) in digital and analogue circuit analysis
- Thevenin equivalent source in DC circuit simplification for network analysis
- Representing an EMF source in electrochemical cell and battery modelling
- DC bias supply for transistor amplifier operating point (Q-point) analysis
- Representing the output of a rectifier-filter section before a voltage regulator in power supply schematics
- DC motor and solenoid driving circuit schematics
Example
In a basic transistor biasing schematic, a DC Voltage Source symbol labelled V_CC = 12 V has its positive terminal connected through a collector resistor to the NPN transistor collector and through a voltage divider bias network to the base, while the negative terminal connects to the circuit common (GND). The DC source symbol communicates that a steady 12 V supply is present, and the circuit analysis proceeds using DC Kirchhoff's laws to determine the quiescent base current, collector current, and operating point.
Key facts
- The DC Voltage Source symbol is a circle with a long line (positive terminal) and a short line (negative terminal); the + terminal is always at higher potential than the - terminal.
- An ideal DC voltage source maintains a perfectly constant terminal voltage regardless of load current; real sources include an internal resistance that causes terminal voltage to fall under load.
- The DC voltage source symbol is distinguished from the AC voltage source by the long-positive/short-negative line convention inside the circle, versus the sine wave or tilde used for AC sources.
- The reference designator for a voltage source in IEEE 315 is V or E (electromotive force); the voltage value in volts is labelled adjacent to the symbol in the schematic.
- IEC 60617-02 and ANSI Y32.2 / IEEE 315 define visually identical DC voltage source symbols; both use the long-line/short-line convention that originated from the electrochemical battery cell representation.
- A battery symbol is a special case of the DC voltage source symbol, drawn as one or more pairs of alternating long-positive and short-negative lines (representing individual electrochemical cells), with the total open-circuit voltage being the sum of all cell voltages.
- In SPICE circuit simulation netlists, an ideal DC voltage source is designated with the letter V followed by a node pair and the voltage value (e.g. V1 pos neg DC 5), which corresponds directly to the DC voltage source symbol in the schematic.
Diagrams that use this symbol
- xnx xnx transmitter wiring diagram
- subwoofer wiring diagrams
- diagram of thermocouple
- sensor diagram
- encoder circuit diagram
- speaker wiring diagram
- speaker connection diagram
- speaker diagram
Frequently asked questions
What does the DC voltage source symbol look like?
The DC voltage source symbol is a circle with two horizontal lines inside: a longer line representing the positive (+) terminal and a shorter line representing the negative (-) terminal. A plus sign is often placed near the longer line. The symbol may also appear as just the two lines without a circle, especially for battery representations.
What does the DC voltage source symbol mean in a circuit diagram?
The DC voltage source symbol means a constant, unidirectional voltage supply is present. It indicates the source maintains a fixed voltage (labelled in volts next to the symbol) between its positive and negative terminals, driving current through the connected circuit under DC analysis conditions.
What is the difference between the DC and AC voltage source symbols?
The DC voltage source symbol uses a long line (positive) and short line (negative) inside a circle, indicating constant unidirectional voltage. The AC voltage source symbol uses a sine wave or tilde inside the circle, indicating sinusoidal alternating voltage. The two symbols are visually unambiguous side by side.
What do the long and short lines mean in the DC voltage source symbol?
The long line represents the positive terminal (higher potential); the short line represents the negative terminal (lower potential or reference). This convention originates from the electrochemical battery cell symbol, where longer plates represent the positive electrode and shorter plates the negative electrode.
What is the designator letter for a DC voltage source?
The standard IEEE 315 reference designator for a voltage source is V (for voltage source) or E (for electromotive force source). In SPICE netlists, ideal voltage sources are designated V. The designator appears as V1, V2, or E1 next to the symbol in the schematic.
What standard defines the DC voltage source symbol?
The DC voltage source symbol is defined in IEC 60617-02 (sources and signal generators) and ANSI Y32.2-1975 / IEEE 315-1975. Both standards specify the long-line/short-line convention inside a circle. The symbols from the two standards are visually identical.
Is a battery symbol the same as a DC voltage source symbol?
A battery symbol is a specialised form of the DC voltage source symbol. A single-cell battery uses one pair of long (positive) and short (negative) lines. A multi-cell battery repeats the pattern (two or more pairs) to indicate multiple series-connected cells. Both symbols represent a DC voltage source; the battery symbol additionally implies an electrochemical energy storage device.
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