Electrolytic Capacitors
Electrolytic capacitors are high-capacitance, polarised capacitors commonly used in power supplies, audio amplifiers, motor drives and many other electronic circuits. They provide excellent energy storage in a compact size, making them ideal where large capacitance values are required. Unlike ceramic or film capacitors, electrolytic capacitors must always be connected with the correct polarity.
What Is an Electrolytic Capacitor?
An electrolytic capacitor uses a thin oxide layer as its dielectric and an electrolyte as part of its internal construction. This allows extremely high capacitance values compared with many other capacitor types.
Because the oxide layer forms only on the positive electrode, electrolytic capacitors are polarised and must be installed with the correct polarity.
Construction
| Component | Purpose |
|---|---|
| Aluminium Foil (Anode) | Positive electrode. |
| Aluminium Oxide Layer | Dielectric. |
| Electrolyte | Acts as the negative electrode connection. |
| Separator Paper | Holds the electrolyte and separates the foils. |
| Cathode Foil | Completes the electrical circuit. |
| Aluminium Can | Protects the internal components. |
| Rubber Seal | Prevents electrolyte leakage. |
How They Work
When a DC voltage is applied with the correct polarity, the oxide dielectric stores electrical energy while the electrolyte provides electrical contact to the opposite electrode.
Because the dielectric is extremely thin, electrolytic capacitors achieve very high capacitance in a relatively small package.
Polarity
Electrolytic capacitors must always be connected correctly.
| Lead | Identification |
|---|---|
| Positive (+) | Usually the longer lead on new radial capacitors. |
| Negative (-) | Marked by a coloured stripe on the capacitor body. |
Reversing the polarity may damage the capacitor, cause overheating, leakage or, in severe cases, rupture of the pressure relief vent.
Typical Specifications
| Specification | Typical Values |
|---|---|
| Capacitance | 0.47 µF to 100,000 µF or more |
| Voltage Rating | 6.3 V to 600 V+ |
| Tolerance | Typically ±20% |
| Operating Temperature | 85°C or 105°C (common ratings) |
| Polarity | Polarised |
Advantages
- Very high capacitance.
- Compact for their capacitance value.
- Excellent energy storage.
- Available in many voltage ratings.
- Cost-effective for power supply applications.
Disadvantages
- Polarised.
- Higher ESR than ceramic capacitors.
- Limited operating lifetime.
- Electrolyte can dry out with age.
- Not suitable for very high-frequency applications.
Common Types
| Type | Typical Application |
|---|---|
| General Purpose | Power supplies and general electronics. |
| Low ESR | Switch-mode power supplies. |
| Audio Grade | Audio amplifiers and preamplifiers. |
| High Temperature | Industrial and automotive electronics. |
| Snap-In | High-power power supplies. |
| Screw Terminal | Large industrial power equipment. |
Applications
- Power supply smoothing.
- Energy storage.
- Audio amplifier filtering.
- DC link capacitors.
- Motor drives.
- Battery chargers.
- Voltage regulators.
- Switch-mode power supplies.
- UPS systems.
- Inverters.
Testing Electrolytic Capacitors
- Inspect for bulging, leakage or corrosion.
- Measure capacitance using a capacitance meter.
- Measure ESR using an ESR meter.
- Check leakage current if specialised equipment is available.
- Replace capacitors with abnormal readings or visible damage.
Common Faults
| Fault | Possible Cause |
|---|---|
| Bulging top. | Gas build-up due to overheating or ageing. |
| Electrolyte leakage. | Failed seal or ageing. |
| High ESR. | Electrolyte drying out. |
| Reduced capacitance. | Ageing. |
| Open circuit. | Internal connection failure. |
| Short circuit. | Dielectric breakdown. |
Safety Precautions
- Always observe polarity markings.
- Discharge large capacitors before handling.
- Never exceed the voltage rating.
- Use capacitors with appropriate ripple-current ratings.
- Replace damaged or leaking capacitors immediately.
Real-World Examples
| Equipment | Typical Capacitor Use |
|---|---|
| Class AB Amplifier | Power supply filter capacitors. |
| Bench Power Supply | Output smoothing. |
| UC3842 SMPS | Primary and secondary filtering. |
| IR2153 Power Supply | Bulk DC storage capacitor. |
| Battery Charger | Output filtering. |
| Motor Controller | DC bus capacitor. |
Key Points
- Electrolytic capacitors provide high capacitance in a compact package.
- They are polarised and must be connected correctly.
- They are widely used for power supply filtering and energy storage.
- Ageing can increase ESR and reduce capacitance.
- Regular inspection helps prevent equipment failures.