The Electrolytic Cap Symbol and Marking Reference Chart
When reading schematics or laying out a PCB, the electrolytic cap symbol tells you two critical things: capacitance value and absolute polarity. Unlike ceramic or film capacitors, aluminum and tantalum electrolytics rely on a chemically formed oxide dielectric layer that will break down if reverse-biased. Below is the complete reference for both schematic standards and physical board markings.
How to read this table: Schematic symbols vary by region. North American designs typically follow IEEE 315 / ANSI Y32.2, while European and international designs follow IEC 60617-4. Physical markings depend on the capacitor chemistry.
| Standard | Symbol Description | Polarity Indicator | Region / Usage |
|---|---|---|---|
| IEEE 315 / ANSI | One straight line, one curved line | "+" sign next to the straight line (anode) | North America, legacy schematics |
| IEC 60617-4 | Two parallel straight lines (one thick, one thin) | "+" sign next to the thick line, or a filled rectangle on the positive side | Europe, International, modern CAD tools |
| JIS C 0617 | Similar to IEC, but often uses a specific arc for the cathode | "+" explicitly marked on the anode side | Japan, Asian manufacturer datasheets |
| Non-Polarized (NP) | Two curved lines facing away from each other (ANSI) or two thick lines (IEC) | None. Can be inserted in either direction. | Audio crossovers, AC coupling |
| Component Type | Physical Marking | Indicates Which Terminal? | Common Part Families |
|---|---|---|---|
| Aluminum Radial (Through-hole) | Dark stripe with minus signs (-) or arrows | Negative (Cathode) | Panasonic FR, Nichicon UHE, Rubycon ZL |
| Aluminum SMD (Can) | Black or painted semi-circle on the plastic base | Negative (Cathode) | United Chemi-Con, EEE-FK series |
| Tantalum SMD | White or colored stripe / bar on one end | Positive (Anode) | Kemet T491, AVX TPS series |
| PCB Silkscreen | Circle with a filled semi-circle or "+" sign | Filled area = Negative; "+" = Positive | Altium / KiCad default footprints |
Rows and Markings People Get Wrong
The most dangerous trap in electronics assembly is assuming all electrolytic capacitors use the same polarity marking convention. The "rows people get wrong" almost always involve confusing aluminum and tantalum chemistry.
On a standard aluminum electrolytic capacitor (like a Nichicon UHE 1000µF 25V), the painted stripe with minus signs indicates the NEGATIVE lead. However, on a surface-mount tantalum capacitor (like a Kemet T491 100µF 16V), the colored stripe indicates the POSITIVE lead. Installing a tantalum capacitor backward will cause a direct short, thermal runaway, and a violent fire within seconds. Always verify the chemistry before trusting the stripe.
Bipolar (Non-Polarized) Electrolytics: Used heavily in speaker crossovers and AC motor-run circuits, these contain two anodes connected back-to-back internally. They use the NP schematic symbol and lack any stripe on the can. Do not substitute a standard polarized electrolytic in these circuits, even if you place two in series, without calculating the AC ripple current derating.
SMD Silkscreen Ambiguity: Many hobbyist PCB designs use a generic silkscreen outline with a shaded half. According to standard IPC-7351 footprint guidelines, the shaded or hatched half of the silkscreen circle denotes the negative terminal for aluminum cans. However, if the board designer mistakenly used a diode footprint shape, the shading might mean the cathode (which aligns with negative), but it is always safer to cross-reference the physical component stripe against the schematic before reflow soldering.
Safe Interpretation When Markings Are Faded or Missing
When salvaging components or repairing vintage gear, the heat-shrink sleeve may be torn, the silkscreen scraped off, or the can markings faded by decades of heat. Never guess polarity. Use this bench protocol to safely identify the anode and cathode.
- Inspect Physical Clues First: For unused through-hole capacitors, the factory standard is a longer lead for the positive (anode) and a shorter lead for the negative. Inspect the bottom vent score (the K, cross, or Y shape stamped into the aluminum base); while this doesn't indicate polarity, it confirms the part is an electrolytic and not a similarly sized inductor or film cap.
- The DMM Leakage Resistance Test: Set your digital multimeter to the highest resistance range (usually 2MΩ or 20MΩ). Discharge the capacitor completely by shorting the leads with a 100Ω power resistor.
- Measure Forward Bias: Place the red probe on the suspected positive lead and the black probe on the suspected negative lead. The meter will show a rising resistance as the cap charges, eventually settling at a very high value (often >1MΩ for healthy caps).
- Measure Reverse Bias: Swap the probes. The reverse-biased leakage current in an aluminum electrolytic is significantly higher, meaning the meter will settle at a noticeably lower resistance value.
- Conclusion: The orientation that yields the highest stable resistance reading is the correct forward-bias polarity (Red = Anode/Positive, Black = Cathode/Negative).
Note: This test applies to aluminum electrolytics. Tantalum capacitors have much lower reverse leakage thresholds and can be damaged by the test voltage of some multimeters (which can output up to 9V on the resistance setting). For unmarked SMD tantalums, discard them; the risk of fire is not worth the salvaged value.
Frequently Asked Questions
Why does the electrolytic cap symbol have one straight and one curved line?
In the ANSI/IEEE 315 standard, the straight line represents the anode foil, while the curved line represents the electrolyte and the outer aluminum can (the cathode). The curve was originally drafted to visually mimic the physical construction of early wet electrolytic capacitors and foil rolling. The IEC standard later moved to two straight lines (one thick, one thin) to standardize CAD drawing, but the curved line remains ubiquitous in American textbooks and legacy schematics.
How to read electrolytic cap symbol polarity on a PCB?
Look for the silkscreen outline. A circle with a "+" sign explicitly marks the anode hole. If there is no "+" sign, look for a filled-in or hatched semi-circle along the edge of the footprint outline; this shaded area designates the negative (cathode) terminal. Always verify this against the physical capacitor's stripe before soldering, as PCB designers occasionally use non-standard footprint libraries.
What is the difference between the electrolytic cap symbol and the battery symbol?
While both represent polarized DC components, the battery symbol consists of multiple alternating long and short parallel lines (representing multiple galvanic cells stacked in series), with the longest line being the positive terminal. The electrolytic cap symbol features only a single pair of plates separated by a distinct gap, representing a single energy-storing dielectric field. Additionally, the battery symbol lacks a curved line or a thick/thin plate distinction in standard IEC drafting.
What does the plus sign mean on an electrolytic cap symbol silkscreen?
The "+" sign on a PCB silkscreen explicitly marks the insertion hole for the positive lead (anode) of the capacitor. It is a direct translation of the schematic polarity indicator to the physical board layout. If a board features both a "+" sign and a shaded semi-circle, the "+" sign takes precedence for the anode, and the shading indicates the cathode. If they contradict, the PCB footprint is flawed, and you must refer to the original schematic to determine the designer's intent.
