What Is a Capacitor?
A capacitor is a passive electronic component that stores electrical energy in the form of an electric field. It consists of two conductive plates separated by an insulating material called a dielectric. Unlike a battery, which stores energy chemically, a capacitor stores energy electrostatically and can charge and discharge very quickly. Capacitors are found in almost every electronic device, from smartphones and computers to televisions, audio amplifiers and power supplies.
How Does a Capacitor Work?
When a voltage is applied across a capacitor, electrons accumulate on one plate while an equal number of electrons leave the opposite plate.
This creates an electric field within the dielectric, storing electrical energy without allowing direct current (DC) to flow through the insulating material.
When the capacitor is connected to a load, the stored energy is released as the capacitor discharges.
Basic Construction
| Part | Purpose |
|---|---|
| Conductive Plates | Store equal and opposite electrical charges. |
| Dielectric | Insulates the plates and stores energy in the electric field. |
| Terminals | Connect the capacitor to the circuit. |
| Protective Casing | Provides mechanical and environmental protection. |
Capacitor Symbol
Most circuit diagrams use one of two symbols:
- Non-polarised capacitor – two parallel plates.
- Polarised capacitor – one straight plate and one curved plate, or marked with a positive (+) terminal.
Polarised capacitors, such as electrolytic capacitors, must always be connected with the correct polarity.
What Does a Capacitor Store?
A capacitor stores:
- Electrical charge.
- Electrical energy.
- An electric field between its plates.
The amount of energy stored depends on the capacitance and the applied voltage.
Capacitance
The ability of a capacitor to store charge is called capacitance.
Capacitance is measured in farads (F), named after the scientist Michael Faraday.
Because one farad is a very large value, most electronic circuits use:
- Microfarads (µF)
- Nanofarads (nF)
- Picofarads (pF)
Common Capacitor Types
| Type | Typical Applications |
|---|---|
| Ceramic | High-frequency decoupling and filtering. |
| Electrolytic | Power supply smoothing and energy storage. |
| Tantalum | Compact electronic equipment. |
| Film | Audio circuits and timing applications. |
| Mica | RF and precision circuits. |
| Supercapacitor | Backup power and energy storage. |
Applications
- Power supply filtering.
- Energy storage.
- Signal coupling.
- Power supply decoupling.
- Noise suppression.
- Timing circuits.
- Oscillators.
- Audio crossover networks.
- Motor starting circuits.
- Radio-frequency tuning.
Advantages
- Fast charge and discharge.
- Long operating life.
- No moving parts.
- Wide range of capacitance values.
- Available in many sizes and voltage ratings.
Limitations
- Cannot store as much energy as a battery of similar size.
- Stored charge gradually leaks away.
- Polarised capacitors must be connected correctly.
- Capacitance may change with temperature, voltage and age.
Real-World Examples
| Equipment | Capacitor Function |
|---|---|
| Audio Amplifier | Power supply smoothing and signal coupling. |
| Switch-Mode Power Supply | Energy storage and output filtering. |
| Radio Receiver | Frequency tuning. |
| Camera Flash | Rapid energy storage and discharge. |
| Arduino Circuit | Power supply decoupling. |
Key Points
- A capacitor stores electrical energy in an electric field.
- It consists of two conductive plates separated by a dielectric.
- Capacitance is measured in farads (F).
- Capacitors can charge and discharge rapidly.
- They are essential components in power supplies, amplifiers, filters and timing circuits.