The Basics of Capacitors

The Basics of Capacitors

Capacitors are one of the most useful and versatile components in electronic circuits.capacitor They can block direct current while allowing alternating current to pass through them, they are used in analog filter networks to smooth the output of power supplies and can be tuned by circuits to resonate at specific frequencies. They are even used to store energy for use when needed most in things like digital computers, though their exact nature is a little mysterious to most people who have never worked with them.

The capacitor is essentially a pair of metallic plates with an insulating material between them. The metal plates are conductive while the insulating material is made from a variety of different materials including air, paper, plastic, polystyrene, mylar, ceramic or aluminum electrolytic. The size and arrangement of the plates and dielectric material is varied to achieve different ratings for the capacitor. For small values of capacitance (microfarads and less) ceramic disks are often used with wire leads bonded to the top or bottom of the disc. Larger value capacitors can be made by stacking multiple sets of discs or using a multilayer laminated construction.

When a DC voltage is applied across the capacitor the positive (+ve) charge accumulates on one plate and the negative (-ve) charge accumulates on the other plate. Since the opposite charges attract each other an electric field forms between the plates, with a force perpendicular to the plates. The amount of charge accumulated is proportional to the surface area of each plate and therefore to the overall capacitance of the capacitor. The greater the capacitance of the capacitor the more plate surface area is required.

In addition to the voltage-induced electric field, the polarization of the plates influences the capacitance. The polarization of the plates is related to their distance from each other and also to the amount of charge on each plate. A plate with a greater amount of charge requires a greater distance between it and the next plate with the same amount of charge to avoid electrical shorting due to self-discharge.

The type of insulating material used in the capacitor affects the capacitance because not all materials have the same ability to prevent the transfer of charge. Some materials have a higher capacitance than others because they offer less opposition to the field flux for a given amount of charge and can therefore collect more of it. The capacitance of a capacitor is also influenced by its dielectric constant, which is a number that quantifies how much the insulator increases the capacitance when compared to free space.

Capacitors may have their connecting leads arranged in many configurations, but for most applications they are soldered to the circuit board and are referred to as surface-mount devices. They are available in a wide range of packages and mounting types to suit the specific requirements of an application. The mounting configurations that are most common include axial and radial leaded through-hole types and chassis-mount devices.