What are Capacitors and When Are They Needed?

Capacitors are electronic components that store and release electrical energy in a circuit. The electrical energy stored in the capacitor is called capacitance and is measured in farads (F). Capacitors consist of two conductive plates separated by an insulating material known as a dielectric. When a voltage is applied across the plates, an electric field is created in the dielectric, causing a separation of charge. One plate accumulates positive charge, and the other accumulates an equal amount of negative charge. This ability to hold charge is what allows capacitors to store energy.

Capacitance is influenced by the size of the plates, distance between the plates, and the type of dielectric material used between the plates.

The equation for calculating capacitance of a basic parallel-plate capacitor is:

Where:

C = Capacitance (F)

e_r = Relative permittivity (dielectric constant) of the dielectric material

e_0 = Permittivity of free space (8.854x10^-12 F/m)

A = Area of one plate (m^2)

d = Distance between the plates (m)

The capacitor will continue to charge until the potential difference across the two plates of the capacitor is equal to the voltage of the source applied to it.

Where are Capacitors Used?

Short Term Energy Storage: Capacitors can store electrical energy for a relatively short period of time when connected to a power source and release it later. Power supply circuits utilize capacitors to supply quick bursts of energy to electronic components during demand spikes and in devices like flash cameras.

Filtering: Smoothing out voltage fluctuations in power supply circuits to produce a more stable DC voltage is critical to most electronic devices that can malfunction or become damaged from power supply “noise.”

Signal Coupling: Audio amplifiers and other signal processing devices can use a capacitor to connect two circuit segments and pass AC components of the signal while blocking the DC component for isolated processing.

Decoupling: High speed digital circuits and sensitive circuits use a capacitor to create a low impedance path to ground for AC signals or noise, effectively isolating one part of the system from another.

Timing and Oscillation: Capacitors, paired with resistors, define timing intervals through their charge and discharge cycles, based on the RC time constant (T=RCT=RC). This mechanism allows for precise timing in electronics.

Tuning: Capacitors are used to adjust resonance frequency of antennas, filters, or oscillators to match desired signal frequency in radio and communication circuits.

Power Factor Correction: In industrial settings, capacitors are used to compensate for lagging current due to inductive loads.

Motor Start and Run: Capacitors help start induction motors by creating phase shift for the starting winding, providing a starting torque. They are also used to improve efficiency and power factor while the motor is running.

Sensing: Certain types of capacitors can sense mechanical changes (like pressure, acceleration, or fluid level) due to their capacitance changing with those physical variables. These capacitive sensors are used in touch screens, pressure sensors, and other devices.

Types of Capacitors

Ceramic Capacitors: As the name suggests, these are made with a ceramic dielectric material. The capacitance of these are usually small and they are used in high-frequency applications such as RF circuits. Ceramic capacitors are known for their stability, low capacitance, and low cost.

Electrolytic Capacitors: With a high capacitance to volume ratio, these capacitors are often used in systems needing high capacitance, such as power supply filtering. Electrolytic capacitors can be aluminum or tantalum (better performance and stability but higher cost) and are polarized (must be connected with the correct polarity).

Supercapacitors: Capacitors that have extremely high capacitance values and store large amounts of energy. These capacitors are used in applications where rapid charge/discharge cycles are needed (power backup systems, energy storage, electric vehicles, etc.).

Variable Capacitors: These capacitors allow for adjustment of capacitance values, typically through mechanical means. They are used in tuning circuits, such as radio receivers, to adjust the resonance frequency of the circuit.

Mica and Silver Mica Capacitors: Both capacitors utilize mica as the dielectric material, providing very stable and accurate values. Silver mica capacitors include a silver coating applied to the mica, providing better conductivity for high precision applications where tight tolerances are required