A capacitor is a two-terminal electrical component used to store energy in an electric field. Capacitors contain two or more conductors, or metal plates, separated by an insulating layer referred to as a dielectric. The conductors can take the form of thin films, foils or beads of metal or conductive electrolyte, etc.
How Does a Capacitor Work?
The dielectric insulating layer does not allow DC current to flow through as it blocks it, instead enabling a voltage to be present across the plates in the form of an electric charge. As an energy storage device, an ideal capacitor does not dissipate energy. A capacitor stores energy in the form of an electrostatic field between its plates.
An ideal capacitor is characterized by a constant capacitance C, which is defined as the ratio of charge Q on each conductor, to the voltage V between them. Because the conductive metal plates are spaced closely together, the opposite charges on the conductors attract one another due to their electric fields. This enables the capacitor to act as an energy storage device and store more charge than if the conductors were separated.
By applying a voltage to a capacitor and measuring the charge on the plates, the ratio of the charge Q to the voltage V will give the capacitance value of the capacitor. The equation can be given as C = Q/V. The more familiar formula for the quantity of charge on the plates is Q = C x V.
Capacitor Specifications
The basic measurement unit of capacitance is the Farad (abbreviated F). Because the Farad is a large unit of measurement, a number of smaller multiples of the Farad exist to measure capacitance, particularly as many smaller capacitors exist. Other standard capacitance units include the Microfarad (uF), which is equivalent to 10-6 F, the Nanofarad (nF), equivalent to 10-9 F, and the Picofarad (pF), equivalent to 10-12 F.
Capacitor Voltage
The capacitor’s working voltage (WV DC) is the maximum amount of voltage that can be applied to the capacitor without damaging the dielectric. Capacitors have a maximum DC voltage.
The maximum energy that can be stored safely in a capacitor is limited by the breakdown voltage. Due to the scaling of capacitance and breakdown voltage with dielectric thickness, all capacitors made with a particular dielectric have approximately equal maximum energy density, to the extent that the dielectric dominates their volume.
The factor by which the dielectric material, or insulator, increases the capacitance of the capacitor compared to air is known as the Dielectric Constant, k. A dielectric material with a high dielectric constant is a better insulator than a dielectric material with a lower dielectric constant. Dielectric constant is a dimensionless quantity since it is relative to free space.
Dielectric leakage is another key capacitor specification. Dielectric leakage in a capacitor results from unwanted leakage current flowing through the dielectric. Excess leakage current can damage the capacitor.
What is a Capacitor Used For?
Capacitors are used in electronic circuits to block direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits, capacitors are used to tune radios to particular frequencies. Capacitors can also be used to stabilize voltage and power flow in power transmission systems.
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