The standard symbol for an independent DC power source is a pair of parallel lines (one long, one short) under the US-based IEEE 315 standard, or a simple circle with an internal DC/AC designation under the international IEC 60617 standard. If you are reading a schematic, identifying the correct source type dictates how you calculate bias currents, simulate the circuit in SPICE, and safely probe the physical board.
Power Source Symbol Reference Table (IEEE vs IEC)
Before tracing a circuit, confirm which drafting standard the engineer used. North American schematics heavily favor IEEE 315 (ANSI Y32.2), while European and global designs default to the IEC 60617 graphical symbols database. Use this table to decode the specific source type.
| Source Type | IEEE 315 (US) Symbol Description | IEC 60617 (Intl) Symbol Description | Practical Meaning on the Bench |
|---|---|---|---|
| DC Voltage (Battery / Multi-Cell) | Alternating long/thin (+) and short/thick (-) parallel lines. | Circle with a '+' and '-' inside, or a standard battery glyph. | Provides fixed DC potential. Internal resistance is assumed non-zero in real life. |
| DC Voltage (Single Cell) | One long/thin line (+) and one short/thick line (-). | Circle with '+' and '-' (identical to multi-cell in basic IEC). | A single electrochemical cell (e.g., 1.5V AA or 3.7V Li-ion). |
| AC Voltage Source | Circle with a sine wave inside. | Circle with a sine wave inside (identical). | Mains or transformer secondary. Polarity alternates; do not use DC multimeter settings. |
| Independent Current Source | Circle with an internal arrow pointing in the direction of conventional current. | Circle with an internal arrow. | Ideal source delivering fixed amps regardless of load resistance (often a SPICE abstraction or active LED driver). |
| Dependent (Controlled) Voltage Source | Diamond shape with '+' and '-' polarity marks. | Diamond shape with '+' and '-'. | Voltage output is controlled by another circuit variable (common in op-amp and transistor small-signal models). |
| Dependent (Controlled) Current Source | Diamond shape with an internal arrow. | Diamond shape with an internal arrow. | Current output scales with a controlling voltage or current elsewhere in the schematic. |
Regional Variants and the "Rows People Get Wrong"
While the table above covers the official standards, real-world schematics are often drawn by engineers using custom CAD libraries that mix conventions. Here is how to determine which standard applies to your region, followed by the most common schematic misinterpretations.
Which Standard Applies to You?
- North America (US/Canada): Expect IEEE 315. You will see the long/short parallel lines for batteries and circles for AC/DC generators. Ground symbols will typically be the three descending horizontal lines (earth) or a zigzag (chassis).
- Europe, UK, and Asia: Expect IEC 60617. Power sources are almost exclusively represented by circles with internal text or symbols. Ground symbols are often a single horizontal line with three diagonal hash marks beneath it.
- Legacy UK (Pre-1990s): You may encounter old BS 3939 standards, which heavily overlap with early IEC but sometimes use unique resistor rectangles and distinct switch glyphs. Treat these as IEC variants but verify component values with a meter.
The Rows People Get Wrong
1. Confusing the DC Battery with a Polarized Capacitor
In IEEE 315, a polarized capacitor also uses parallel lines, but one line is typically curved (representing the outer foil or negative terminal), and the gap between the lines is much wider than a battery symbol. A battery symbol has alternating long/thin and short/thick lines packed closely together. Misreading this in a power supply feedback loop will completely break your SPICE transient analysis.
2. Dependent vs. Independent Sources in Datasheets
When reading an internal block diagram for an IC (like a switching regulator or an op-amp), manufacturers use the diamond shape for dependent sources. If you see a diamond with an arrow, it is a voltage-controlled current source (transconductance), not a fixed current supply. Treating it as an independent source in your head will lead to fundamental misunderstandings of the IC's compensation network.
3. The "Lazy CAD Library" AC Source
An AC voltage source should never have fixed '+' and '-' polarity markers outside the circle, because the polarity reverses continuously. However, many engineers reuse DC source CAD footprints and simply swap the internal text to a sine wave. If you see an AC source with fixed polarity markers, it is a drafting error; trust the sine wave, not the external pins.
Safe Interpretation When Schematic Markings Fade or Fail
Schematics are only useful if they match the physical board. When working on legacy equipment—like a 1990s Tektronix oscilloscope, a repaired SMPS (Switch-Mode Power Supply), or a custom board where the silkscreen has flaked off—you cannot rely on printed polarity markers or faded schematic PDFs.
When the symbol on the schematic doesn't match the physical reality, or the physical markings are gone, follow this bench procedure:
- Establish a Known Ground: Locate a verified chassis ground or the negative terminal of the main bulk filter capacitor. This is your 0V reference.
- Measure Open-Circuit Voltage: Using a true-RMS multimeter (like a Fluke 87V), measure the DC voltage from your reference ground to the suspected positive power rail. If the meter reads a negative value (e.g., -12.0V), your assumed positive terminal is actually the return path.
- Check for AC Ripple: Switch your multimeter to AC voltage mode and measure the same DC rail. A healthy DC power source should read less than 50mV of AC ripple. If you read several volts AC, the source is either an unfiltered rectifier output, or the filter capacitors have dried out and failed.
- Trace the Copper: If the board is single-layer or you have a clear view of the inner layers via vias, trace the power symbol's physical pad back to the regulator output pin or the transformer rectifier diode. The cathode stripe on the diode points toward the positive DC rail.
Power Source Symbol FAQ
What is the schematic symbol for a power source vs a battery?
In common parlance, "power source" is a broad term that includes generators, bench supplies, and batteries. In strict IEEE 315 schematic terms, a generic DC power source (like a bench supply or an ideal voltage node) is often just labeled with a voltage value (e.g., "+5V") and a ground return, without a specific glyph. A battery specifically uses the alternating long/thin and short/thick parallel lines to denote electrochemical cells. IEC 60617 uses a circle for both, relying on internal text (like "BAT" vs "PSU") to differentiate.
How do I read a dependent power source symbol on an op-amp datasheet?
Op-amp and transistor datasheets use small-signal equivalent models filled with dependent sources. Look for the diamond shape. A diamond with '+' and '-' is a Voltage-Controlled Voltage Source (VCVS), representing the op-amp's open-loop gain (A_OL). A diamond with an arrow is a Voltage-Controlled Current Source (VCCS), representing the transconductance (g_m) of a MOSFET or BJT. These are mathematical abstractions for AC analysis; you will not find a physical component shaped like a diamond on the PCB.
Why does my European schematic use a circle for a DC power source instead of parallel lines?
European schematics follow the IEC 60617 standard, which standardizes almost all active sources and generators as circles. The IEC philosophy prioritizes a uniform envelope (the circle) with internal modifiers (text or small glyphs) to denote the exact type of source, whereas the US IEEE 315 standard uses distinct outer shapes (lines, circles, diamonds) for every source type. If you are reading a schematic from a German or UK-based engineering firm, expect circles for everything from a 3V coin cell to a 400V AC mains feed.
What does a power source symbol with a barbed arrow inside a circle mean?
A circle with an internal arrow (often barbed or straight) represents an independent ideal current source. Unlike a voltage source that maintains a fixed voltage regardless of the load, an ideal current source will adjust its output voltage to whatever level is necessary to push a specific, fixed amount of current through the load. In physical hardware, this is typically implemented using an active constant-current LED driver circuit or a precision current mirror, rather than a basic power supply.
