The long leg of a through-hole polarized capacitor—most commonly an aluminum electrolytic or solid tantalum—is the anode (positive terminal). The short leg is the cathode (negative). If you are holding a fresh component out of the anti-static bag, the lead length is your primary physical indicator of polarity. Inserting a polarized capacitor backward destroys the internal dielectric oxide layer, causing rapid gas generation, thermal runaway, and explosive venting or fire.
This guide breaks down how to verify polarity when the legs are already trimmed, how to decode the cryptic markings stamped on the can, and how to safely substitute a part when your exact bill-of-materials (BOM) item is out of stock.
Decoding Capacitor Polarity and Physical Markings
While the long leg indicates the positive anode on fresh through-hole parts, bench work often involves salvaged boards or pre-trimmed leads. You must rely on secondary visual markers to avoid catastrophic reverse-polarity failures.
- The Cathode Stripe: On aluminum electrolytic capacitors, the plastic sleeve features a prominent stripe running down the side. This stripe is adorned with minus signs (
- - -) and explicitly marks the negative (cathode) lead. The long leg will always be on the opposite side of this stripe. - PCB Silkscreen: Printed circuit boards use three common conventions to mark polarity. First, a white stripe or shaded half-circle on the silkscreen indicates the negative side. Second, the through-hole pads may differ: a square pad typically denotes the positive anode, while a rounded pad denotes the negative cathode. Third, a simple
+sign may be silkscreened next to the positive hole. - Tantalum Indicators: Through-hole solid tantalum capacitors (like the classic teardrop shape) also use a long leg for the positive anode. However, their body marking is different: a
+sign is printed directly above the positive leg, rather than a negative stripe on the cathode side.
Reversing polarity on an aluminum electrolytic capacitor usually results in a loud pop and the release of foul-smelling electrolyte as the rubber safety vent ruptures. Reversing polarity on a solid manganese dioxide tantalum capacitor, however, creates a highly exothermic thermite-like reaction. It will catch fire and can ignite surrounding PCB materials. Always verify tantalum polarity twice before applying power.
Capacitor Type Comparison: Which Dielectric for Which Job?
Choosing the right capacitor goes beyond just matching capacitance and voltage. The dielectric material dictates the component's behavior under temperature stress, high frequencies, and DC bias. Here is a selection framework for the four most common types.
| Type | Construction / Dielectric | Typical Tolerance | Tempco (Temp Stability) | Typical Use Case |
|---|---|---|---|---|
| Aluminum Electrolytic | Etched aluminum foil with liquid or polymer electrolyte | -20% to +50% | Poor (High capacitance drift over temp/age) | Bulk power supply filtering, low-frequency coupling, audio crossover networks. |
| Solid Tantalum | Sintered tantalum powder anode with manganese dioxide or polymer cathode | ±10% to ±20% | Moderate (Better than Al-Elec, worse than ceramic) | Compact DC rail decoupling, sample-and-hold circuits, medical/aerospace where volume is constrained. |
| MLCC (Ceramic) | Multi-layer ceramic (Class 1: C0G/NP0; Class 2: X5R/X7R) | ±5% to -20%/+80% (Y5V) | C0G: Excellent / X7R: Moderate / Y5V: Terrible | High-frequency bypass, RF matching, switching converter output filtering, AC line safety (Y/X caps). |
| Film (Polypropylene) | Metallized plastic film wound or stacked | ±1% to ±5% | Excellent (Highly stable, low dielectric absorption) | High-voltage snubbers, motor run capacitors, precision analog filters, audiophile signal paths. |
Reading the Code: Capacitance, Voltage, and Tolerance Markings
Capacitor markings fall into two categories: direct printing and EIA shorthand codes. Knowing how to read these prevents ordering errors when replacing unmarked or faded components.
Direct Printing (Electrolytic and Large Film)
Large through-hole capacitors simply print the value: 100µF 25V. Some Japanese manufacturers use a shorthand where the unit is the letter. For example, 100u 25V or R47µF (where 'R' replaces the decimal point, meaning 0.47µF).
EIA 3-Digit Code (Ceramics and Small Film)
Small capacitors use a three-digit code measured in picofarads (pF). The first two digits are the significant figures, and the third digit is the multiplier (number of zeros).
104= 10 × 10,000 pF = 100,000 pF = 100 nF = 0.1 µF473= 47 × 1,000 pF = 47,000 pF = 47 nF221= 22 × 10 pF = 220 pF
Tolerance and Voltage Letters
A trailing letter indicates tolerance. J = ±5%, K = ±10%, M = ±20%, Z = +80%/-20%. Voltage is often coded using EIAJ standards on older or miniature parts: 1E = 25V, 1H = 50V, 2A = 100V. For modern designs, always cross-reference the KEMET Technical Resources or manufacturer datasheets to confirm voltage codes, as misreading a 16V cap as a 50V cap will result in immediate failure on a 24V rail.
Failure Modes and Visual Symptoms of Capacitor Abuse
Capacitors rarely fail without leaving forensic evidence. When troubleshooting a dead power supply or a malfunctioning analog board, look for these specific visual and olfactory symptoms.
| Abuse Type | Visual / Physical Symptoms | Underlying Mechanism |
|---|---|---|
| Reverse Polarity (Al-Elec) | Bulging or domed top cap; vented cross-score on top; black/brown soot around the base; strong fishy odor. | Reverse voltage dissolves the aluminum oxide dielectric, causing massive current flow, electrolyte boiling, and gas pressure buildup until the rubber bung or scored top vents. |
| Reverse Polarity (Tantalum) | Charred, melted, or cracked epoxy body; scorch marks on the PCB; no doming. | Manganese dioxide cathode acts as an oxidizer when the dielectric fails, creating a self-sustaining thermal runaway fire. |
| Overvoltage / Surge | Often no external signs initially; may show a small puncture hole on the side of the can or a shorted reading on a multimeter. | Dielectric puncture creates a direct metallic short. In electrolytics, this eventually leads to thermal venting; in ceramics, it results in a dead short that may blow the upstream fuse. |
| Age / Thermal Drying | Slight dome on top; crusty brown electrolyte leaking from the bottom rubber seal; circuit operates in 'hiccup' mode or has high ripple. | Liquid electrolyte slowly evaporates through the rubber end seal over years of heat exposure. Equivalent Series Resistance (ESR) skyrockets, destroying filtering capability. |
Safe Substitution: What to Do When the Exact Part is Missing
When your exact BOM part is on a 40-week backorder, you can substitute capacitors safely if you follow strict engineering derating rules. Never swap parts based solely on the printed microfarad value.
1. Voltage Rating: Always Derate Upwards
You can always substitute a higher voltage rating for a lower one (e.g., using a 35V cap in a 16V circuit). However, you must account for dielectric-specific derating. According to Cornell Dubilier's Aluminum Electrolytic Application Guide, aluminum caps generally require a 20% voltage derating. Solid tantalum capacitors are far more sensitive to surge currents and require a strict 50% derating (e.g., a 10V rail requires a minimum 20V tantalum rating) to prevent field failures.
2. The MLCC DC Bias Trap
If you are substituting a polarized electrolytic with a multi-layer ceramic capacitor (MLCC) to save space or improve ESR, beware of DC bias. Class 2 ceramics (X5R, X7R) lose massive amounts of capacitance when DC voltage is applied. A 10µF, 10V 0805 X5R ceramic might only provide 3µF of actual capacitance at 10V DC. Always check the manufacturer's DC bias graph before substituting ceramics in power rails.
3. ESR and Ripple Current Ratings
If the original part was a "Low-ESR" series (like Panasonic FR or Rubycon ZLH) used in a switching power supply output, you cannot substitute a standard general-purpose capacitor (like a generic 105°C standard series). The higher ESR of the standard part will cause excessive internal heating from ripple current, leading to premature venting. Match or beat the original ripple current specification.
4. Temperature Rating
Never substitute a 105°C rated capacitor with an 85°C rated one in a power supply. The 105°C parts use a more robust electrolyte solvent formulation that dramatically extends operational lifespan, even if the ambient temperature never exceeds 40°C.
Frequently Asked Questions
What happens if I cut the long leg of a capacitor before soldering?
If you trim both legs to equal length before installing the capacitor, you lose the primary physical polarity indicator. In this scenario, you must rely entirely on the negative cathode stripe printed on the component's shrink sleeve and match it to the shaded silkscreen or rounded pad on the PCB. If the sleeve is also missing or faded, the part is unidentifiable and must be discarded; guessing polarity on an electrolytic capacitor risks a violent failure upon energization.
Does the long leg of a capacitor always mean positive?
For standard through-hole polarized capacitors (aluminum electrolytic and solid tantalum), yes, the long leg is universally the positive anode. However, this rule does not apply to non-polarized capacitors (like ceramic, film, or specific non-polarized audio electrolytics), where lead length is arbitrary and determined purely by the manufacturing taping process. Furthermore, surface-mount device (SMD) capacitors do not have legs; they rely on a printed negative stripe on the top of the can or a darker anode pad on the PCB footprint for polarity identification.
Can I use a non-polarized capacitor instead of a polarized one with a long leg?
Technically yes, but practically it is constrained by size, cost, and capacitance density. Non-polarized (NP) or bipolar aluminum electrolytic capacitors exist, but they are physically much larger and more expensive than their polarized counterparts for the same microfarad rating. You cannot use a standard polarized capacitor in an AC signal path or a circuit where the voltage swings below ground, as the reverse voltage swing will destroy the dielectric. If you need high capacitance in an AC coupling application and only have polarized parts, you can wire two identical polarized electrolytics in series back-to-back (anode-to-anode or cathode-to-cathode), but the resulting total capacitance will be half the value of a single unit.
Why do some surface mount capacitors lack a long leg for polarity?
Surface mount technology (SMT) eliminates wire leads entirely to save vertical space and allow for automated pick-and-place assembly. For SMD aluminum electrolytics (often called V-chips), polarity is indicated by a thick black or white semi-circle printed on the top of the plastic base, which denotes the negative cathode side. For SMD tantalums, a colored band or a beveled edge on the epoxy body indicates the positive anode. Always consult the specific manufacturer's footprint datasheet, as anode/cathode marking conventions for SMD tantalums can occasionally vary between brands like KEMET, AVX, and Vishay.
