FaradNet

Glossary of Capacitor Terms

-D-

• dielectric
  
• dielectric adsorption
  
• dielectric constant
  
• dielectric strength
  
• DF - Dissipation Factor
  
• double layer, electric
  
• Double Layer Capacitor (DLC)
  

dielectric adsorption

Also called dielectric hysteresis or dielectric soak. A characteristic of dielectrics which determines the length of time a capacitor takes to deliver the total amount of its stored energy. It manifests itself as the reappearance of a potential on the electrodes after the capacitor has been discharged. Its magnitude depends on the charge and discharge time of the capacitor.

Adsorption mostly occurs in imperfect dielectrics, primarily solid dielectrics, such as glass, wax, Bakelite, etc. Positive and negative charges are separated and then accumulated at certain regions within the volume of the dielectric, usually manifesting itself as a gradually decreasing current with time after the application of a fixed direct voltage. Its magnitude depends on the nature, composition and purity of the material. Moisture present in the dielectric reduces the insulation resistance of the dielectric and thus tends to suppress absorption, but it increases the leakage current.

Adsorption occurs from the penetration of the electrical charge into the mass of the dielectric for an appreciable time, following the almost instantaneous charge when a capacitor is connected to a continuous voltage through a very low resistance. The current caused by the absorption is steadily decreasing and when the dielectric is saturated with the charge, only leakage current is flowing in the circuit.

The leakage current is steady and ordinarily of an exceedingly small value. Upon short-circuiting, the free charge is released almost instantaneously, but if the capacitor exhibits absorption and is left open-circuited for a while, it may again be discharged (once or several times) because the residual charge slowly seeps out of the dielectric. The residual charges are of a smaller magnitude than the free charge.

Adsorption effects are accompanied by heat dissipation (only part of the absorbed charge is recoverable) and constitute the main portion of losses in solid dielectrics observed when these are subjected to alternating (or fluctuating) potentials.

The apparent capacitance of a capacitor may be strongly affected by absorption effects. A capacitor measured at low frequencies may appear to have a much greater capacitance than at high frequencies.

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DF - Dissipation Factor

DF is the ratio between the permittivity and conductivity of a dielectric, the reciprocal of the DF is the quality factor, sometimes called the storage factor. In practical terms, the DF can be expressed as the ratio of the loss current to the charging current, or loss tangent:

D = tan (delta) = I_l / I_c = omegaRC,

where omega = 2pif, R is equivalent series resistance, and C is capacitance; all measured at the same frequency f. DF is 100D and is expressed as a percentage.

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double layer, electric

In electrochemistry "electrical double layer" refers to the electrical double layer at the electrode-solution interface. In other words, there is a separation of charges at the interface between two phases, usually a solid and liquid phase where the solid is the electrode and the liquid is the electrolyte.

The charge layers consist of an excess or deficit of electrons on the metal side and of an excess or deficit of ions of the opposite charge on the solution side. In addition to these free charge carriers, there are oriented molecules with dipole moments which also contribute to the overall electrical moment of the layer.

A further contributor is the posibility of a subsidiary dipole layer being formed by ions which are "specifically adsorbed" (i.e. by chemical bonding) to the metal, and whose charge is neutralized (partly or wholly) by ions of opposite sign held by electrostatic attraction nearby in the solution.

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Updated: 07 November 2001

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