The standard schematic symbol for a non-polarized capacitor is two parallel lines separated by a gap, representing the conductive plates and the dielectric. For polarized capacitors, one plate is marked with a plus (+) or a straight line, while the negative side uses a curved line or a minus (-). Below is the complete reference for what the symbol for a capacitor looks like across global standards.
Complete Capacitor Symbol Reference Table
Before wiring a board or debugging a schematic, verify which standard your ECAD software or legacy blueprint uses. The table below maps the component type to its graphical representation in both North American and international standards, alongside the physical PCB silkscreen you will encounter on the bench.
| Component Type | ANSI/IEEE 315 Symbol (North America) | IEC 60617 Symbol (International) | Common PCB Silkscreen / Footprint |
|---|---|---|---|
| Non-Polarized (Ceramic, Film) | Two parallel straight lines | Two parallel straight lines | 'C' designator, no polarity marks |
| Polarized (Electrolytic, Tantalum) | Straight line (+) and curved line (-) | Two parallel lines with '+' on one side | '+' mark, or shaded stripe for negative |
| Variable Capacitor | Parallel lines with diagonal arrow crossing | Parallel lines with diagonal arrow crossing | Rare on modern PCBs; usually off-board |
| Trimmer Capacitor | Parallel lines with T-shaped arrow | Parallel lines with T-shaped arrow | Small SMD footprint with adjustment screw |
| Feedthrough Capacitor | Capacitor symbol with line passing through plates | Capacitor symbol with line passing through plates | 3-terminal footprint (In, Out, Ground) |
| Capacitor with Integral Resistor | Capacitor symbol with a line across the plates | Capacitor symbol with a line across the plates | Standard 2-pin footprint, no special marks |
Decoding the Standards: ANSI/IEEE vs. IEC Symbols
When asking what the symbol for a capacitor is, the answer depends heavily on your region and the era of the schematic. In North America, the governing standard is ANSI/IEEE 315, which historically relies on the "curved line" to denote the negative terminal of a polarized capacitor. This originated from early manufacturing techniques where the outer foil of a rolled capacitor was connected to the negative terminal to act as a shield.
Conversely, the international standard IEC 60617 rejects the curved line entirely. In IEC schematics, both plates are drawn as straight, parallel lines, and polarity is indicated strictly by a plus (+) or minus (-) sign next to the respective terminal. Modern ECAD tools like Altium Designer or KiCad often allow you to toggle between these symbol libraries, but mixing them on a single schematic is a frequent source of assembly errors. Always check the title block of a schematic to confirm which symbol set the drafter used before ordering PCBs.
The "Rows People Get Wrong" Notes
Even experienced bench technicians misinterpret specific capacitor symbols. Here are the most common pitfalls from the reference table above:
- The Curved Line Polarity Trap: In ANSI schematics, beginners often assume the curved line represents the "positive" side because it looks like it is "hugging" or receiving the current. It is strictly the negative terminal. Wiring an aluminum electrolytic capacitor backward will cause the dielectric oxide layer to break down, leading to rapid heating, venting, and potentially a violent explosion.
- Variable vs. Trimmer Arrows: The diagonal arrow crossing the plates indicates a user-adjustable variable capacitor (like a tuning gang in a radio). The arrow with a "T" bar at the end indicates a trimmer capacitor, which requires a tool (like a ceramic screwdriver) to adjust and is meant for one-time factory calibration.
- The "Line Across" Confusion: A straight line drawn horizontally across the two plates (bridging the gap) does not mean a short circuit. It represents a capacitor with a built-in discharge resistor, commonly found in high-voltage power supplies and motor-run circuits to safely bleed off charge when power is removed.
Safe Interpretation When PCB Markings Are Faded or Missing
When repairing legacy gear or reverse-engineering a board, the silkscreen indicating capacitor polarity is often burned off, scratched away, or obscured by conformal coating. If you cannot rely on the printed symbol for a capacitor, use these physical footprint rules to determine polarity safely:
- The Square Pad Rule: For through-hole and SMD polarized capacitors (especially tantalums and aluminum electrolytics), the positive terminal is almost universally assigned to Pad 1. On a PCB footprint, Pad 1 is indicated by a square copper pad, while the negative pad is round or oval.
- The Ground Plane Trace: In DC power supply circuits, the negative leg of a bulk filter capacitor ties directly to the system ground. Set your multimeter to continuity mode. Place one probe on a known ground point (like the metal chassis, a USB port shield, or a large ground via). Touch the other probe to the capacitor pads. The pad that beeps (reads near 0 ohms) is the negative terminal.
- SMD Can Markings: If the board silkscreen is gone but the physical SMD aluminum electrolytic capacitor is still installed, look at the top of the metal can. A black painted semicircle on the top indicates the negative side. If you are replacing it, align the new component's black mark with the footprint's shaded area or the round pad.
For deeper troubleshooting on identifying unknown SMD components, referencing manufacturer datasheets for footprint dimensions is critical. Resources like the SparkFun Capacitor Tutorial provide excellent visual guides for matching physical component markings to schematic values.
Capacitor Symbol FAQs
What is the symbol for a polarized capacitor on a PCB silkscreen?
On a physical PCB silkscreen, a polarized capacitor is typically outlined with a box or circle where one half is shaded or filled in solid black. The shaded area indicates the negative terminal. Alternatively, you may see a simple '+' sign printed next to the positive pad. Never rely solely on the '+' sign if it conflicts with the shaded area; always cross-reference the component datasheet, as some older Japanese equipment used the shaded area to denote the positive terminal.
How do you read a capacitor symbol with three digits underneath?
When a schematic or PCB silkscreen shows a capacitor symbol with a three-digit code (e.g., 104) instead of a direct value, it is using the EIA (Electronic Industries Alliance) standard. The first two digits are the significant figures, and the third digit is the multiplier (number of zeros) in picofarads (pF). For example, '104' means 10 followed by four zeros: 100,000 pF. This converts to 100 nF or 0.1 µF. A trailing letter indicates tolerance: J = ±5%, K = ±10%, M = ±20%.
What does a capacitor symbol with a line through it mean?
If the line is drawn diagonally through the plates with an arrowhead, it is a variable capacitor. If the line is drawn horizontally across the gap between the plates (connecting them), it represents a capacitor with an internal parallel discharge resistor. This is common in high-voltage microwave oven circuits and HVAC motor-run capacitors, ensuring the capacitor bleeds down to a safe voltage within seconds of power removal.
