Capacitor Construction
Although capacitors are manufactured in many different shapes and sizes, they all operate using the same basic principle. Every capacitor contains two conductive plates separated by an insulating material called a dielectric. The choice of dielectric material, electrode construction and packaging determines the capacitor's capacitance, voltage rating, stability, frequency performance and lifetime.
Basic Construction
A capacitor consists of four main parts:
| Component | Purpose |
|---|---|
| Conductive Plates | Store equal and opposite electrical charges. |
| Dielectric | Insulates the plates while allowing an electric field to develop. |
| Terminals | Connect the capacitor to the external circuit. |
| Protective Housing | Provides mechanical strength and environmental protection. |
Conductive Plates
The conductive plates are usually made from aluminium, copper or thin metal foil. One plate becomes positively charged while the other becomes negatively charged when a voltage is applied.
Increasing the surface area of the plates increases the capacitance because more electrical charge can be stored.
The Dielectric
The dielectric is the insulating material placed between the conductive plates. It prevents direct current from flowing between the plates while allowing energy to be stored in the electric field.
| Dielectric Material | Typical Applications |
|---|---|
| Ceramic | High-frequency decoupling and bypass circuits. |
| Aluminium Oxide | Electrolytic capacitors. |
| Tantalum Oxide | Tantalum capacitors. |
| Polyester | General-purpose film capacitors. |
| Polypropylene | Precision timing and audio applications. |
| Mica | RF and precision circuits. |
| Paper | Older electronic equipment. |
Terminals
Capacitors use terminals or leads to connect to the external circuit.
- Radial leads.
- Axial leads.
- Surface-mount (SMD) terminals.
- Screw terminals for high-power capacitors.
- Snap-in terminals for large electrolytic capacitors.
Protective Housing
The outer casing protects the internal components against moisture, contamination, vibration and mechanical damage.
Depending on the capacitor type, the housing may be made from:
- Epoxy resin.
- Plastic moulding.
- Aluminium can.
- Ceramic enclosure.
- Metal case.
How Construction Affects Performance
| Construction Feature | Effect |
|---|---|
| Larger plate area | Higher capacitance. |
| Smaller plate spacing | Higher capacitance. |
| Higher dielectric constant | Higher capacitance. |
| Better dielectric quality | Lower leakage current. |
| Low-loss materials | Better high-frequency performance. |
Construction of Common Capacitor Types
| Type | Construction |
|---|---|
| Ceramic | Metal electrodes separated by ceramic dielectric layers. |
| Electrolytic | Aluminium foil, oxide dielectric and liquid or solid electrolyte. |
| Tantalum | Tantalum pellet with tantalum oxide dielectric. |
| Film | Plastic film between metal foils or metallised film layers. |
| Mica | Natural mica sheets with metal electrodes. |
| Supercapacitor | Porous carbon electrodes with an electrolyte. |
Polarised vs Non-Polarised Capacitors
| Polarised | Non-Polarised |
|---|---|
| Must be connected with correct polarity. | Can be connected in either direction. |
| Usually electrolytic or tantalum. | Ceramic, film and mica types. |
| High capacitance. | Lower to medium capacitance. |
| Power supply filtering. | Signal coupling, RF and timing circuits. |
Manufacturing Improvements
Modern manufacturing techniques allow extremely thin dielectric layers, high-purity materials and automated production, resulting in capacitors with higher capacitance, lower losses and improved reliability.
Surface-mount technology has also enabled the production of miniature capacitors used in smartphones, laptops and other compact electronic devices.
Applications
- Power supplies.
- Audio amplifiers.
- Motor drives.
- Radio-frequency circuits.
- Switch-mode power supplies.
- Communication equipment.
- Industrial control systems.
- Automotive electronics.
Key Points
- Every capacitor contains two conductive plates separated by a dielectric.
- The dielectric determines many of the capacitor's electrical characteristics.
- Plate area and spacing directly affect capacitance.
- Different capacitor types use different construction methods.
- Construction influences voltage rating, ESR, frequency response and reliability.