Tantalum Capacitor Derating Select Correct Voltage, Avoid Fire Risk

What Is Derating?

Keep the actual operating voltage of a tantalum capacitor significantly below its rated voltage.

Example: Apply only 5V to a 10V capacitor. This isn't waste — it's the core technique for ensuring long-term reliability.

Why Must You Derate?

1. Prevent power-on surge failure

Power-on creates a large inrush current. If the rated voltage is barely enough, the surge will punch through the capacitor.

Reflow soldering heat can cause microscopic internal damage — invisible normally, but fails immediately at power-on.

2. Higher temperature = lower voltage withstand

• 25°C: 10V capacitor handles 10V
• 85°C: May handle only 8V
• 125°C: May struggle with 6V

High-temperature environments require more aggressive derating.

Derating Table for Three Tantalum Types

Type	Analogy	Derating	Reason
MnO₂ Solid	Gasoline tank	50%	Failure may cause fire
Polymer Solid	Water bucket	80%~90%	Failure = harmless short, no fire
Wet	Sponge	80%~90%	Self-healing capability

Example (circuit operating at 5V):

• MnO₂ type: Minimum rated = 5V ÷ 50% = 10V
• Polymer type: Minimum rated = 5V ÷ 80% = 6.25V (select 6.3V)

Low-impedance circuits (battery direct supply, large capacitor regulator output): For MnO₂, use 1/3 derating — 5V circuit needs 16V or higher.

Two Deadly Mistakes

Mistake 1: Using a 6.3V capacitor in a 5V circuit

Consequence: Shorts instantly at power-on. Severe cases = smoke and fire.

Correct approach:

• MnO₂ type: Select at least 10V
• Polymer type: 6.3V is fine

Mistake 2: Ignoring circuit impedance

Low-impedance circuits have massive inrush current. Even 50% derating may fail.

Correct approach: Add a 1Ω series resistor to limit inrush current.

One Sentence Mnemonic

"Old type, cut in half; new type, a tenth off"

• Old type (MnO₂): Operating voltage ≤ Rated voltage × 50%
• New type (Polymer): Operating voltage ≤ Rated voltage × 80%~90%

Quick Selection Table

Circuit Voltage	MnO₂ Minimum Rated	Polymer Minimum Rated
1.8V	3.3V~4V	2.5V
3.3V	6.3V~10V	4V
5V	10V~16V	6.3V
12V	25V	16V
24V	50V	30V

Five Practical Tips

1. Prefer polymer type: Pricier but looser derating (80%~90% vs 50%) and safer failure mode.
2. Check manufacturer datasheet: Different brands (KEMET, AVX, Vishay) have slight variations.
3. More conservative at high temperature: Above 85°C, consult temperature-voltage curve in datasheet.
4. Mind polarity: Tantalum capacitors are polarized. Reverse = guaranteed failure.
5. Add series resistor for low-Z: Add 1Ω resistor when possible — dramatically reduces failure risk.

 

 

The following is a Tantalum Capacitor Cross-Reference & Equivalent Series Guide, covering our UF series and corresponding product lines from AVX, Kemet, Vishay, and Panasonic.

 

Should you have any questions or interest in our offerings, please do not hesitate to contact us.

Carey

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