Polarity Guide

Tantalum Capacitor Polarity (+/-): Markings, Orientation & Failure

Tantalum capacitors are polarized — install one backwards and it can catch fire in seconds. This guide shows you exactly how to read polarity markings on through-hole and SMD tantalum caps, and what to do if you're unsure.

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POLARITY MATTERS

Unlike ceramic capacitors, tantalum capacitors will fail violently if installed with reversed polarity. The failure mode is a short circuit that generates extreme heat — enough to ignite the manganese dioxide cathode. Always double-check polarity before powering on. When in doubt, use a lower voltage test first.

Through-Hole (Dipped) Tantalum Markings

Through-hole tantalum capacitors use a teardrop-shaped epoxy body. Polarity is indicated in two ways:

Lead Length

The longer lead is positive (+), just like electrolytic capacitors. If leads have been trimmed, look for the body marking instead.

Long lead = + (Positive)
Short lead = − (Negative)

Body Marking

A "+" sign is printed on the body near the positive lead. The marking is usually molded into the epoxy or printed in contrasting color.

[ + ] → Positive terminal side

SMD Tantalum Capacitor Polarity Markings

Critical difference from electrolytic capacitors: On SMD tantalum capacitors, the stripe/bar marks the POSITIVE terminal. On SMD aluminum electrolytic capacitors, the stripe marks the NEGATIVE terminal. Mixing these up is the #1 cause of tantalum failures.

SMD Case Size Reference

EIA Code	Size (mm)	Letter	Polarity Mark
3216	3.2 x 1.6	A	Stripe on + end
3528	3.5 x 2.8	B	Stripe on + end
6032	6.0 x 3.2	C	Stripe on + end
7343	7.3 x 4.3	D	Stripe on + end
7360	7.3 x 6.0	E	Stripe on + end

What Happens When Polarity Is Reversed

Reverse-polarity failure in tantalum capacitors is not gradual — it is sudden and often violent. Here is the failure sequence:

Dielectric breakdown (milliseconds)

The thin tantalum pentoxide (Ta₂O₅) dielectric layer breaks down under reverse bias, creating conductive paths.

Short circuit forms

Current flows through the manganese dioxide (MnO₂) cathode layer. The resistance drops to near zero.

Thermal runaway

The short-circuit current generates extreme heat. The MnO₂ is an oxidizer, so the heat is self-sustaining.

Ignition or explosion

The capacitor can catch fire, emit smoke, or pop open. Nearby components and PCB traces may be damaged.

Voltage Derating Guidelines

Tantalum capacitors require more aggressive voltage derating than other capacitor types. Following these guidelines dramatically reduces failure risk:

Application	Max Applied Voltage	Example
General commercial	50% of rated voltage	25V cap → max 12.5V applied
Industrial / automotive	40% of rated voltage	25V cap → max 10V applied
Aerospace / medical	30% of rated voltage	25V cap → max 7.5V applied
High-reliability / MIL-spec	25% of rated voltage	25V cap → max 6.25V applied

Replacement Alternatives for Tantalum Capacitors

Polymer Tantalum

Uses conductive polymer instead of MnO₂. Fails open (safe) instead of short. Lower ESR. Drop-in replacement in most cases.

Recommended upgrade

Ceramic (MLCC)

Non-polarized, very low ESR, excellent for decoupling. Watch for DC bias derating on high-capacitance MLCCs (actual capacitance drops under voltage).

Good for decoupling

Aluminum Electrolytic

Larger footprint, higher ESR, but much cheaper for bulk capacitance. Good for power supply bulk filtering where space allows.

Budget option

Tantalum Capacitor Polarity FAQ

How can I tell which side of a tantalum capacitor is positive?

On through-hole tantalum capacitors, the positive lead is longer and marked with a "+" sign on the body. On SMD tantalum capacitors, the stripe or bar on the case marks the POSITIVE terminal — this is the opposite of electrolytic capacitors, where the stripe marks negative.

What happens if a tantalum capacitor is installed backwards?

A reverse-biased tantalum capacitor can fail catastrophically — often within seconds. The manganese dioxide cathode layer breaks down, causing a short circuit that generates extreme heat. This can lead to thermal runaway, visible smoke, fire, or even a small explosion. Never apply reverse voltage to a tantalum capacitor.

Do all tantalum capacitors have a polarity stripe?

Yes, all standard tantalum capacitors are polarized. Through-hole (dipped) types have a "+" printed near the positive lead. SMD types have a colored bar or stripe on the positive end. Some very small SMD packages use a molded notch or dot instead of a stripe.

Can I replace a tantalum capacitor with electrolytic or ceramic?

It depends on the application. Ceramic capacitors (MLCC) are often a good replacement if the value and voltage match — they are non-polarized and have lower ESR. Aluminum electrolytic capacitors can work for bulk decoupling but are larger and have higher ESR. For timing circuits or precision filtering, stick with tantalum or polymer tantalum.

What voltage derating should I use for tantalum capacitors?

Industry best practice is to derate tantalum capacitors to 50% of their rated voltage. For example, use a 25V-rated tantalum in a 12V circuit. This dramatically reduces failure risk. Some aerospace and medical standards require even more aggressive derating (30–40% of rated voltage).

What is the safest way to test a tantalum capacitor?

Use a multimeter in capacitance mode with the capacitor removed from the circuit. Check that the reading is within tolerance of the marked value. Then switch to resistance mode: a good tantalum should show high resistance (megaohms). Low resistance indicates a short. Never apply voltage higher than 50% of the rated voltage during testing.

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