What is a Capacitor?

A capacitor is a passive electronic component that stores electrical energy in an electric field. It consists of two conducting plates separated by an insulating material called a dielectric. When a voltage is applied across the plates, an electric field develops across the dielectric, causing positive charge to collect on one plate and negative charge on the other plate. Capacitors are used in a variety of applications, including filtering, energy storage, signal coupling, and voltage smoothing.

Capacitor Basics

Capacitance

Capacitance is the ability of a capacitor to store electrical charge. It is measured in farads (F), named after the English physicist Michael Faraday. One farad is defined as the capacitance of a capacitor that requires a charge of one coulomb to establish a potential difference of one volt across its plates.

The capacitance of a capacitor depends on three factors:
1. The area of the plates (A)
2. The distance between the plates (d)
3. The permittivity of the dielectric material (ε)

The relationship between these factors is given by the following formula:

C = (ε * A) / d

where:
– C is the capacitance in farads (F)
– ε is the permittivity of the dielectric material in farads per meter (F/m)
– A is the area of each plate in square meters (m^2)
– d is the distance between the plates in meters (m)

Dielectric Material	Relative Permittivity (εr)
Air	1.0006
Paper	2.3
Glass	4-7
Mica	5-7
Ceramic	6-10000
Polyester	3.3
Polypropylene	2.2
Polystyrene	2.6
Teflon	2.1

Voltage Rating

The voltage rating of a capacitor is the maximum voltage that can be applied across its terminals without causing damage. Exceeding the voltage rating can cause the dielectric to break down, resulting in a short circuit between the plates. It is important to choose a capacitor with a voltage rating higher than the maximum voltage expected in the circuit.

Equivalent Series Resistance (ESR)

Equivalent Series Resistance (ESR) is the resistance in series with an ideal capacitor that models the losses in the capacitor. These losses are due to the resistance of the plates, the leads, and the dielectric. ESR is important because it affects the efficiency and performance of the capacitor, particularly at high frequencies. A capacitor with a high ESR will have a lower quality factor (Q) and will dissipate more power as heat.

Dielectric Absorption

Dielectric absorption is the tendency of a capacitor to retain a residual charge after being discharged. This is due to the polarization of the dielectric material, which causes some of the charge to be trapped within the dielectric. Dielectric absorption can cause errors in precision circuits and can lead to safety hazards if the capacitor is not properly discharged before handling.

Types of Capacitors

Ceramic Capacitors

Ceramic capacitors are made from alternating layers of metal and ceramic dielectric. They are available in a wide range of capacitance values and voltage ratings, and are known for their low cost, small size, and good temperature stability. However, they have a relatively high ESR and are prone to microphonic effects, which can cause noise in audio circuits.

Film Capacitors

Film capacitors are made from a thin plastic film dielectric, such as polyester or polypropylene, with metal foil or metallized electrodes. They offer higher capacitance values and lower ESR than ceramic capacitors, but are larger in size and more expensive. Film capacitors are commonly used in power supply filtering, signal coupling, and high-frequency applications.

Electrolytic Capacitors

Electrolytic capacitors are polarized capacitors that use an electrolyte as the dielectric. They are available in two types: aluminum and tantalum. Aluminum electrolytic capacitors offer very high capacitance values in a small package, but have a relatively high ESR and are prone to failure due to evaporation of the electrolyte. Tantalum electrolytic capacitors have a lower ESR and are more stable than aluminum, but are more expensive and have a lower maximum voltage rating.

Supercapacitors

Supercapacitors, also known as ultracapacitors or double-layer capacitors, are electrochemical capacitors that store energy in an electric double layer at the interface between an electrode and an electrolyte. They offer very high capacitance values, up to several thousand farads, and can deliver high currents with low ESR. However, they have a lower voltage rating than other types of capacitors and are more expensive.

Applications of Capacitors

Power Supply Filtering

Capacitors are commonly used in power supplies to filter out noise and ripple from the rectified AC voltage. The capacitor is placed in parallel with the load, and acts as a short circuit for high-frequency noise while allowing the desired DC voltage to pass through. The size of the capacitor depends on the amount of ripple that can be tolerated and the load current.

Coupling and Decoupling

Capacitors are used for coupling and decoupling signals in electronic circuits. Coupling capacitors are used to pass AC signals between stages while blocking DC, allowing the signal to be referenced to a different DC level. Decoupling capacitors are used to provide a low-impedance path for high-frequency noise, preventing it from affecting sensitive circuits.

Energy Storage

Capacitors can be used to store energy for short periods of time, such as in flash photography or pulsed laser applications. The capacitor is charged to a high voltage and then discharged rapidly through the load, providing a high current pulse. The amount of energy stored in a capacitor is given by the following formula:

E = (1/2) * C * V^2

where:
– E is the energy in joules (J)
– C is the capacitance in farads (F)
– V is the voltage in volts (V)

Tuning and Filtering

Capacitors are used in tuned circuits and filters to select or reject specific frequencies. In a tuned circuit, the capacitor is combined with an inductor to form a resonant circuit that oscillates at a specific frequency. By varying the capacitance or inductance, the resonant frequency can be changed. In a filter, capacitors are used to block or pass certain frequencies while attenuating others.

FAQ

What is the difference between a capacitor and a battery?

A capacitor stores energy in an electric field, while a battery stores energy in a chemical reaction. Capacitors can charge and discharge very quickly, but have a lower energy density than batteries. Batteries can store more energy, but take longer to charge and discharge.

Can capacitors be used in place of batteries?

In some applications, capacitors can be used in place of batteries. For example, supercapacitors can be used in backup power supplies or regenerative braking systems. However, capacitors have a lower energy density than batteries and may not be suitable for long-term energy storage.

What happens if a capacitor is connected backwards?

If a polarized capacitor, such as an electrolytic capacitor, is connected backwards, it can be damaged or destroyed. The reverse voltage can cause the dielectric to break down, leading to a short circuit or explosion. Non-polarized capacitors, such as ceramic or film capacitors, are not affected by reverse voltage.

How do I know what size capacitor to use?

The size of the capacitor depends on the application and the specific requirements of the circuit. Factors to consider include the capacitance value, voltage rating, ESR, and temperature coefficient. It is important to consult the datasheet and application notes for the specific capacitor and to follow best practices for circuit design and layout.

What is the lifespan of a capacitor?

The lifespan of a capacitor depends on several factors, including the type of capacitor, the operating temperature, the voltage and current stress, and the environmental conditions. Electrolytic capacitors have a shorter lifespan than other types due to the evaporation of the electrolyte, and may need to be replaced after several years of use. Other types of capacitors, such as ceramic and film capacitors, have a longer lifespan and may last for decades under normal operating conditions.

Conclusion

Capacitors are essential components in modern electronics, with a wide range of applications from power supply filtering to energy storage and signal processing. By understanding the basic principles of capacitance, voltage rating, ESR, and dielectric absorption, designers can select the appropriate capacitor for their application and ensure optimal performance and reliability. With ongoing research and development in materials science and manufacturing techniques, capacitors will continue to play a critical role in the advancement of technology.