The standard symbol of an electrolytic capacitor features one straight plate (representing the positive anode) and one curved plate (representing the negative cathode), typically accompanied by a plus (+) sign on the anode side. However, schematic representation varies strictly by regional drafting standards, and misreading these symbols—or the physical polarity stripes on the component itself—is a primary cause of catastrophic failure, including venting and explosion. Below is the definitive reference for identifying, interpreting, and testing polarized capacitors across global standards.
The Complete Capacitor Symbol & Polarity Reference Table
Drafting standards dictate how the symbol of an electrolytic capacitor appears on a schematic. North American engineers largely follow ANSI/IEEE 315, while European and global designs default to IEC 60617. Japanese schematics often utilize JIS C 0617. The table below maps these schematic symbols to their physical component markings.
| Component Type | ANSI/IEEE 315 Symbol | IEC 60617 Symbol | Physical Can / SMD Marking | Polarity Rule & Failure Mode |
|---|---|---|---|---|
| Aluminum Electrolytic (Radial) | Straight plate (+), Curved plate (-), '+' sign | Two parallel plates, '+' sign on anode, or open box with '+' | Dark stripe with minus (-) signs and arrows indicating the negative lead. | Reverse bias destroys the Al2O3 dielectric layer, causing electrolyte boiling and pressure venting. |
| Tantalum Electrolytic (SMD/DIP) | Straight plate (+), Curved plate (-), '+' sign | Two parallel plates, '+' sign on anode | Stripe or bar indicates the POSITIVE anode (opposite of aluminum). | Reverse bias causes immediate thermal runaway and short-circuit; highly flammable failure. |
| Bipolar (Non-Polarized) Electrolytic | Two curved plates facing each other, or two straight plates | Two parallel plates with no polarity indicator | No stripe. Leads are identical. Often marked 'BP' or 'NP'. | Can withstand AC voltage (e.g., audio crossovers). No polarity trap, but higher ESR than standard polarized types. |
| Supercapacitor (EDLC) | Four parallel plates (two pairs), '+' sign | Two thick parallel plates, '+' sign | Stripe indicating negative, or explicit '+' and '-' stamped on the can. | Extremely low reverse voltage tolerance (often < 0.5V). Reversing causes rapid gas generation and seal rupture. |
| Ceramic / Film (Non-Polarized) | Two straight parallel plates | Two straight parallel plates | No polarity markings. Symmetrical leads. | Non-polarized. Failure is usually due to overvoltage (cracking) or thermal stress, not polarity reversal. |
Regional Drafting Standards and the "Rows People Get Wrong"
While the basic physics of capacitance remain constant, the way engineers draw the symbol of an electrolytic capacitor changes depending on where the schematic was authored. According to the Electronics Tutorials capacitor guide, confusing these regional variants leads to frequent breadboard and PCB assembly errors.
ANSI/IEEE 315 vs. IEC 60617
In the North American ANSI/IEEE 315 standard, the curved plate explicitly represents the outer foil (cathode/negative), while the straight plate represents the anode (positive). The curved shape is a historical nod to the physical construction of rolled aluminum foil capacitors, where the outer layer wraps around the inner layer. In contrast, the international IEC 60617 standard often abandons the curved plate entirely for basic polarized capacitors, relying instead on two straight parallel plates with a prominent '+' sign next to the anode, or a hollow rectangular box with a '+' indicator. If you are reading a schematic from a European manufacturer like STMicroelectronics or Infineon, look for the '+' sign rather than relying on plate curvature.
The "Rows People Get Wrong" Notes
The most dangerous row in the reference table above is the Tantalum Electrolytic capacitor. Schematic symbols rarely distinguish between aluminum and tantalum electrolytics; both use the standard polarized symbol. However, their physical markings are exact opposites. Aluminum radial cans use a stripe to mark the negative cathode. Tantalum SMD and DIP packages use a stripe or bar to mark the positive anode. Installing a Kemet T491 tantalum capacitor using the aluminum stripe-logic will result in immediate reverse bias, leading to a violent, exothermic short circuit.
The Bipolar Audio Confusion: In audio crossover networks, you will frequently encounter bipolar (non-polarized) electrolytic capacitors. On ANSI schematics, these are sometimes drawn with two curved plates facing each other (like a butterfly) or two straight plates. Hobbyists often mistake the two-curved-plate symbol for a variable capacitor or a trimmer. If the component value is in the microfarad (µF) range (e.g., 4.7µF to 100µF) and it is placed in series with a speaker driver, it is a bipolar electrolytic, not a variable tuning capacitor.
The JIS Grounded Foil Marker: In older Japanese schematics (JIS C 0617), you may see a polarized capacitor symbol where the curved plate has a small arrow pointing to it, or an adjacent ground symbol. This indicates that the outer foil (cathode) must be tied to the lower-impedance node or ground to act as an electrostatic shield, reducing noise pickup in high-gain amplifier stages.
Safe Interpretation When Physical Markings Fade or Fail
Electrolytic capacitors subjected to high ambient temperatures (e.g., near power resistors or in unventilated ATX power supplies) often suffer from sleeve shrinkage and ink fading. When the negative indicator stripe on a radial aluminum can becomes illegible, guessing the polarity will destroy the component. According to Cornell Dubilier's Aluminum Electrolytic Application Guide, reverse voltage causes the dielectric oxide layer to break down, generating hydrogen gas that will trip the pressure relief vent or rupture the can.
If you are repairing a board and encounter an unmarked or faded electrolytic capacitor, use the following bench procedures to safely identify polarity before applying full operating voltage.
Method 1: The Leakage Current Asymmetry Test
Aluminum electrolytic capacitors exhibit asymmetric DC leakage current due to the anodized aluminum oxide (Al2O3) dielectric. You can exploit this using a standard digital multimeter (DMM) and a bench power supply.
- Set a bench power supply (e.g., Rigol DP811) to 5% of the capacitor's rated voltage (e.g., 1.2V for a 25V cap) and strictly limit the current to 5mA.
- Connect the power supply to the capacitor leads in one orientation. Measure the steady-state leakage current after 30 seconds.
- Discharge the capacitor safely using a 100-ohm power resistor.
- Reverse the leads and repeat the measurement.
- The Verdict: The orientation that yields the lowest leakage current (often in the microamp range) is the correct forward-bias polarity (Positive to Anode, Negative to Cathode). The reverse-bias orientation will show significantly higher leakage (milliamp range) as the dielectric begins to degrade.
Method 2: LCR Meter DC Bias Identification
Standard multimeter diode-test modes output an AC test signal that is useless for determining electrolytic polarity. However, advanced LCR meters with a DC bias function (such as the Keysight U1733C or Wayne Kerr 6400B series) can safely identify the anode. By applying a small positive DC bias (e.g., +1V) and measuring the capacitance and Dissipation Factor (D), you can observe the dielectric behavior. A correctly biased capacitor will show a stable capacitance value and a low D factor. A reverse-biased capacitor will exhibit erratic capacitance readings and a sharply elevated D factor as the internal chemistry resists the inverted electric field.
A common internet myth suggests using a DMM's resistance range to watch the "charging curve" to find polarity. This is highly unreliable for modern low-ESR capacitors (like Panasonic FR or Nichicon UHE series) because the initial inrush current easily exceeds the DMM's test current, saturating the meter's ADC and yielding identical readings in both directions. Always rely on controlled leakage current testing or physical schematic tracing.
When in doubt, trace the PCB copper. The cathode (negative) lead of an input filter capacitor is almost always tied directly to the ground plane, while the anode (positive) connects to the rectifier diode or switching regulator input pin. For further reading on schematic standards and component identification, the Lessons in Electric Circuits reference library provides excellent open-source breakdowns of how these symbols translate to real-world circuit behavior.
