Have you ever noticed a marking such as 40/70/21 on a capacitor and wondered what it means?
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| Capacitor marking. |
This marking is not the capacitance value and not the voltage rating.
Instead, 40/70/21 is a climatic category marking used to indicate the environmental conditions associated with the capacitor.
Breaking Down 40/70/21
❄️ 40 — Lower Temperature Category
The first number indicates the lower temperature category:
−40°C
This means the capacitor is classified for operation at temperatures down to −40°C, subject to its applicable specification.
🌡️ 70 — Upper Temperature Category
The second number indicates the upper temperature category:
+70°C
This means the capacitor is classified for operation at temperatures up to +70°C, subject to its applicable specification.
💧 21 — Damp-Heat Test Duration
The third number indicates:
21 days
This refers to the duration associated with the relevant damp-heat environmental test.
What Is a Damp-Heat Test?
A damp-heat test evaluates how a component performs under high temperature and high humidity conditions.
Depending on the applicable standard and test procedure, the capacitor may be evaluated for characteristics such as:
Insulation performance
Leakage current
Resistance to moisture
Capacitance stability
Electrical performance after environmental exposure
This helps determine whether the component can maintain acceptable performance after exposure to demanding environmental conditions.
A Common Misunderstanding
❌ Incorrect:
“It is tested at −40°C for 21 days.”
✅ Correct:
The numbers represent the capacitor's climatic category, including its lower and upper temperature categories and the relevant damp-heat test duration.
In Short
40/70/21 =
❄️ −40°C → Lower temperature category
🌡️ +70°C → Upper temperature category
💧 21 days → Damp-heat test duration
Understanding capacitor markings can help you select components suitable for their intended operating environment.
⚡ Always check the manufacturer's datasheet and the applicable standard when selecting a capacitor for a specific application.
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