What Is an Aluminum Electrolytic Capacitor?
An aluminum electrolytic capacitor uses high-purity aluminum foil as its anode, a nanometer-thin aluminum oxide layer as its dielectric, and a conductive electrolyte as its cathode. It is the most widely used capacitor type for applications needing capacitance above 1 uF.
You will find them in computer power supplies, air conditioner inverters, LED drivers, and factory motor controllers.
How It Is Made
Step 1: Foil Etching
High-purity aluminum foil (99.9%+) is electrochemically etched to create a microscopic tunnel structure, increasing surface area by 100 to 500 times. More surface area means more capacitance.
Step 2: Anodization
The etched foil is immersed in a chemical bath and a precise DC voltage is applied. This grows a layer of aluminum oxide on the foil — the dielectric. Oxide thickness determines voltage rating. Higher voltage needs thicker oxide, which reduces capacitance for the same foil area.
Step 3: Winding
Anode foil (with oxide dielectric), separator paper (soaked with electrolyte), and cathode foil are stacked and wound into a tight cylinder.
Step 4: Electrolyte Filling
The wound element is sealed in an aluminum can and filled with electrolyte under vacuum. The electrolyte also repairs minor oxide defects — a self-healing property unique to electrolytic capacitors.
Step 5: Aging and Testing
Every capacitor goes through controlled aging at rated voltage and elevated temperature to stabilize the oxide layer and identify early failures.
Four Form Factors
| Type | Mounting | Capacitance Range | Typical Application |
|---|---|---|---|
| SMD (V-Chip) | Surface-mount | 0.1 to 10,000 uF | Compact power supplies |
| Radial | Through-hole | 0.1 to 100,000 uF | General-purpose |
| Snap-in | Through-hole | 47 to 68,000 uF | High-power supplies, inverters |
| Screw-terminal | Bolted | 1,000 uF to 1 F+ | UPS, welding equipment |
How to Select
- Capacitance: Calculate minimum needed for ripple and hold-up requirements
- Voltage: Derate at least 20%. For a 24V rail, use a 35V or 50V rated part
- Temperature: Above 85 C ambient needs 105 C rated parts
- Lifetime: Every 10 C below rated temperature doubles the life. For 10-year warranty products, calculate expected life carefully
- Mounting: SMD for automated assembly, through-hole for prototyping
Key Takeaways
- Aluminum electrolytics are the go-to choice for bulk capacitance at a reasonable cost
- The dielectric is grown aluminum oxide — nanometers thick, enabling high density
- Choose the form factor based on power level and assembly process
- Temperature and ripple current determine lifetime — design with margin
- Always observe polarity — the negative terminal is marked with a stripe
Need specifications or samples? Contact Xuansn Engineering Support — 20 years of experience, free samples available.
