A capacitor symbol diagram translates a physical energy-storage component into a standardized schematic shorthand. The core symbol consists of two parallel lines (or a line and a curve) representing the conductive plates, separated by a gap representing the dielectric. Below is the definitive reference for identifying these symbols across global standards.
Complete Capacitor Symbol Diagram Reference Table
The following table maps schematic symbols to their physical counterparts. Use this as your primary bench reference when reading datasheets or reverse-engineering legacy PCBs.
| Symbol Description | Component Type | Polarity | Standard Origin |
|---|---|---|---|
| Two parallel straight lines | Fixed Capacitor (Ceramic, Film) | Non-Polarized | IEC 60617 / ANSI |
| One straight line, one curved line | Fixed Capacitor (Electrolytic) | Polarized (Curved = Negative) | ANSI/IEEE 315 (US) |
| Two straight lines with a '+' sign | Fixed Capacitor (Electrolytic, Tantalum) | Polarized ('+' = Positive/Anode) | IEC 60617 (Global) |
| Two parallel lines with a diagonal arrow through them | Variable Capacitor | Non-Polarized | IEC / ANSI |
| Two parallel lines with a T-shaped arrow ending at the plates | Trimmer Capacitor | Non-Polarized | IEC / ANSI |
| Two parallel lines with multiple diagonal arrows | Ganged / Differential Capacitor | Non-Polarized | IEC / ANSI |
| Two parallel lines with a perpendicular line crossing the gap | Feedthrough Capacitor | Non-Polarized | IEEE 315 |
Regional Standards and the 'Rows People Get Wrong'
Schematic interpretation depends heavily on the regional standard used by the drafting engineer. The two dominant frameworks are the International Electrotechnical Commission (IEC 60617) and the North American ANSI/IEEE 315 standard. While modern CAD tools like Altium and KiCad often default to IEC, millions of legacy US schematics rely on IEEE conventions.
The Most Misinterpreted Symbol: The Curved Line
The most common error hobbyists and junior technicians make is misinterpreting the one straight, one curved line symbol. Many assume the curved line denotes a specific dielectric material (like ceramic) or a non-polarized component. This is incorrect.
According to standard electronic symbol conventions derived from IEEE 315, the curved line strictly designates the negative (or outer foil) plate of a polarized capacitor. The straight line represents the positive (anode) plate. If you see a curved line in a schematic, you must use a polarized component (like an aluminum electrolytic) and connect the curved side to ground or the lower-potential node.
The IEC Shift to the '+' Sign
Because the curved line caused widespread confusion globally, the IEC standardized on using two straight parallel lines for all fixed capacitors, adding a distinct '+' sign next to the positive plate to denote polarity. If you are designing a new board in 2026, stick to the IEC two-straight-lines-plus-sign method to ensure your schematics are universally readable.
Decoding Faded Markings and Physical Polarity
Schematics are only half the battle. On the bench, you frequently encounter PCBs where the silkscreen is faded, scorched, or entirely missing. Here is how to safely interpret physical polarity when the visual markings fail.
Electrolytic Capacitors (Through-Hole and SMD)
- The Negative Stripe: Aluminum electrolytics feature a prominent stripe running down the side. This stripe contains minus ('-') signs and indicates the cathode (negative) lead.
- PCB Silkscreen: If the cap is missing, look at the PCB pads. A filled-in or shaded half-circle on the silkscreen indicates the negative side. For through-hole pads, the square pad almost always denotes the positive (anode) lead, while the circular pad is negative.
- SMD Cans: Surface-mount aluminum electrolytics have a black plastic base with a painted semi-circle. The painted side is the negative terminal.
Tantalum and Niobium Capacitors
Tantalums reverse the visual logic of aluminum electrolytics, which traps many beginners. On a tantalum bead, the painted stripe or bar indicates the positive (anode) lead. On SMD tantalums, the thick white or black line across one end of the molded case is the positive terminal.
Bench Verification with a Multimeter
If a component is completely unmarked and you suspect it is a polarized tantalum or a degraded electrolytic, use your multimeter. Set a Fluke 87V (or equivalent) to Diode Test mode. Place the red probe on one leg and the black probe on the other. A polarized capacitor will show a brief charging spike followed by an 'OL' (open loop) reading in the forward bias direction. If it reads a dead short or conducts continuously in both directions, the dielectric has failed and the component must be replaced. Always consult a comprehensive capacitor application guide when selecting replacements to match Equivalent Series Resistance (ESR) and ripple current ratings.
Frequently Asked Questions
What does the capacitor symbol diagram with a plus sign mean?
The '+' sign explicitly marks the positive (anode) terminal of a polarized capacitor. This is the IEC standard method for denoting polarity. You must connect this terminal to the higher voltage potential in the circuit. Connecting it to a lower potential or ground will reverse-bias the dielectric oxide layer, leading to rapid thermal runaway and venting.
Why are there two different capacitor symbol diagrams for non-polarized caps?
Historically, ANSI/IEEE used two straight lines for non-polarized caps and a straight/curved combo for polarized. IEC used two straight lines for everything, relying on the '+' for polarity. Today, most modern EDA software defaults to the IEC standard (two straight lines for all fixed caps) to reduce symbol library bloat, but you will still see the curved line in older American military, aerospace, and consumer electronics schematics.
How do I read a capacitor symbol diagram for a variable or trimmer cap?
Look at the arrow. If the diagonal arrow passes completely through the parallel plates, it represents a user-adjustable variable capacitor (like a tuning gang in an RF circuit). If the arrow ends at the plates with a perpendicular 'T' shape at the end of the arrow, it represents a trimmer capacitor, which is meant to be set once at the factory with a non-conductive alignment tool and left alone.
What is the capacitor symbol diagram for a supercapacitor?
There is no universally unique IEC symbol exclusively for supercapacitors (EDLCs). In modern schematics, they are typically drawn using the standard IEC polarized capacitor symbol (two straight lines with a '+'). However, because supercapacitors are often used in banks and have massive ESR/leakage implications, engineers will frequently annotate the symbol with 'SC', 'EDLC', or draw a double-line variant to signal to the PCB layout engineer that high-current, wide-trace routing is required for that specific node.
