The standard electrolytic capacitor symbol consists of two parallel lines—one straight line representing the positive anode, and one curved line representing the negative cathode, typically accompanied by a plus (+) sign on the anode side. Because electrolytic capacitors are polarized, misinterpreting this symbol or the physical component markings can lead to catastrophic failure. Below is the definitive reference for reading, drawing, and verifying these symbols across global standards.
The Electrolytic Capacitor Symbol Reference Table
The following table outlines the standard schematic representations you will encounter on circuit diagrams, PCB silkscreens, and engineering drawings. Refer to this when verifying your schematic against your bill of materials (BOM).
| Symbol Variant | Visual Description | Polarity Indicator | Standard / Region | Practical Meaning & Application |
|---|---|---|---|---|
| Standard Polarized (Aluminum) | One straight vertical line, one curved vertical line | Plus (+) sign next to the straight line | ANSI/IEEE (US), IEC (Alt) | Standard radial or axial aluminum electrolytic. The curved line always denotes the negative (cathode) terminal. |
| IEC Standard Polarized | Two straight parallel vertical lines | Plus (+) sign next to one of the straight lines | IEC 60617 (International) | Used in European and international schematics. Relies entirely on the (+) sign rather than the curved line to denote polarity. |
| Solid Tantalum | One straight line, one filled-in rectangle (or curved line with a +) | Plus (+) sign on the straight line / rectangle is positive | ANSI/IEEE, IEC | Tantalum capacitors. In ANSI, the filled rectangle is the positive anode. In IEC, a plain rectangle with a (+) sign is used. |
| Bipolar / Non-Polarized (NP) | Two curved lines facing outward, or two straight lines with no (+) | None (Non-polarized) | Universal | Back-to-back electrolytics used in audio crossovers and AC coupling. Can be installed in either direction. |
| Variable / Trimmer | Standard polarized symbol with a diagonal arrow crossing through it | Plus (+) sign on the anode | Universal | Rare in electrolytics, more common in ceramics/films, but denotes an adjustable capacitance value. |
Regional Standards: IEC vs. ANSI/IEEE Variants
When reading schematics from different global regions, the electrolytic capacitor symbol can look subtly different depending on the drafting standard used by the engineer.
ANSI/IEEE Std 315 (North America)
In the United States, the ANSI/IEEE standard strictly enforces the straight-line/curved-line convention. The straight plate represents the positive anode (the foil connected to the positive terminal), and the curved plate represents the negative cathode (historically representing the outer aluminum can). The plus (+) sign is highly recommended but sometimes omitted by lazy CAD drafters; in ANSI, if you see a curved line, the curved side is always negative.
IEC 60617 (International / Europe)
The International Electrotechnical Commission (IEC) standardizes the base capacitor symbol as two straight, parallel lines. To denote a polarized electrolytic capacitor, IEC 60617 simply adds a plus (+) sign next to one of the straight lines. While IEC does permit the straight/curved variant as an alternative, most modern European CAD libraries (like Altium or KiCad defaults in the EU) default to the two straight lines with a (+) marker. If you are designing a board for international manufacturing, the two-straight-line-plus-sign variant is the safest, most universally understood choice.
Markings and Rows People Get Wrong
Even with a perfect schematic, translating that symbol to the physical component on your workbench is where most mistakes happen. Here are the most common misinterpretations and how to handle them.
1. The Tantalum Polarity Reversal Trap
Beginners often assume the physical markings on a capacitor match the aluminum electrolytic convention. They do not. On a standard aluminum can, the painted stripe denotes the negative lead. On a solid tantalum capacitor (often a small epoxy-dipped teardrop or SMD rectangle), the painted stripe or bar denotes the positive anode. Reversing a tantalum capacitor doesn't just vent gas; it frequently results in a short circuit and a localized fire.
2. Faded or Missing Aluminum Can Markings
If you are repairing vintage gear or salvaging parts, the negative stripe on an aluminum electrolytic capacitor may be faded, scratched off, or obscured by heat-shrink tubing. Identifying the pinout safely requires secondary physical cues:
- Lead Length: From the factory, the positive (anode) lead is intentionally cut longer than the negative lead. If the part has never been clipped, the longer leg is positive.
- The Sleeve Seam and Crimp: Look closely at the bottom rubber bung and the plastic sleeve. The negative side often features a slight flat spot on the can, or the plastic sleeve's vertical seam is aligned with the negative terminal.
- Bench Testing (Last Resort): If physical cues are gone, you can use a multimeter in resistance mode. Apply the positive meter probe to one lead and the negative to the other. Note the resistance reading. Swap the probes. The orientation that yields a higher final resistance reading (lower leakage current) is the correct forward-bias polarity.
3. Confusing Bipolar (NP) with Standard Polarized
In audio crossover networks, you will frequently see non-polarized (NP) electrolytic capacitors. These are essentially two polarized capacitors wired back-to-back in series inside a single can. Their schematic symbol features two curved lines facing away from each other (like a bowtie) or two straight lines with no polarity marker. Treating an NP capacitor as polarized and aligning it to DC bias won't damage it, but assuming a standard polarized cap is NP and subjecting it to AC swing will destroy it.
Frequently Asked Questions
Which way does the curved line go on an electrolytic capacitor symbol?
In the ANSI/IEEE standard, the curved line always represents the negative terminal (cathode). The straight line represents the positive terminal (anode). A helpful way to remember this is that the curved line represents the outer aluminum can of a traditional radial capacitor, which is internally connected to the negative electrolyte, while the straight line represents the internal foil roll connected to the positive pin. Always look for the accompanying plus (+) sign to confirm.
How do I safely identify polarity if the physical capacitor markings are faded?
If the negative stripe is unreadable, first check the lead lengths; the positive lead is longer from the factory. If the leads have been clipped, inspect the bottom of the capacitor. The negative side often has a physical asymmetry, such as a flat edge on the aluminum can or a specific alignment of the plastic sleeve seam. If all visual markers are gone, use a multimeter's resistance or diode-testing function to measure leakage current in both directions. The orientation showing the lowest leakage (highest resistance) is the correct forward-bias polarity. If in doubt, replace the capacitor with a new, clearly marked unit.
What is the schematic difference between aluminum and solid tantalum electrolytic capacitors?
While both are polarized electrolytics, their schematic symbols differ to warn designers of their unique failure modes. In ANSI standards, an aluminum electrolytic uses the straight/curved line pair. A solid tantalum capacitor is drawn with a straight line and a filled-in rectangle (or sometimes a standard polarized symbol with the word 'TANT' noted). This distinction is critical because tantalum capacitors have a much lower tolerance for reverse voltage and surge currents, often failing as a dead short rather than venting like aluminum caps.
Does the IEC electrolytic capacitor symbol look different from the ANSI standard?
Yes. While ANSI relies heavily on the straight-line/curved-line geometry to denote polarity, the IEC 60617 standard prefers using two identical straight parallel lines for the base capacitor symbol. To indicate that it is a polarized electrolytic capacitor, the IEC standard simply places a plus (+) sign next to the positive terminal's line. If you are reading a European schematic and see two straight lines with a plus sign, treat it exactly as you would the straight/curved ANSI symbol.
