Dielectric absorption

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Dielectric absorption is the name given to the effect by which a capacitor, that has been charged for a long time, discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage from time-delayed dipole discharging [1] , a phenomenon that is also called dielectric relaxation, "soakage", or "battery action". For some dielectrics, such as many polymer films, the resulting voltage may be less than 1–2% of the original voltage, but it can be as much as 15% for electrolytic capacitors. The voltage at the terminals generated by the dielectric absorption may possibly cause problems in the function of an electronic circuit or can be a safety risk to personnel. In order to prevent shocks, most very large capacitors are shipped with shorting wires that need to be removed before they are used and/or permanently connected bleeder resistors. When disconnected at one or both ends, DC high-voltage cables can also "recharge themselves" to dangerous voltages.

Capacitor Passive two-terminal electronic component that stores electrical energy in an electric field

A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals.

Volt SI derived unit of voltage

The volt is the derived unit for electric potential, electric potential difference (voltage), and electromotive force. It is named after the Italian physicist Alessandro Volta (1745–1827).

Dielectric electrically poorly conducting or non-conducting, non-metallic substance of which charge carriers are generally not free to move

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in an electrical conductor but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field.

Contents

Theory

The random orientations of molecular dipoles in a dielectric are aligned under the influence of an electric field by applying a voltage to the electrodes. Diel.png
The random orientations of molecular dipoles in a dielectric are aligned under the influence of an electric field by applying a voltage to the electrodes.
Circuit model for explaining a time-delayed voltage build-up by parallel RC-timers Dielektrische Absorption.png
Circuit model for explaining a time-delayed voltage build-up by parallel RC-timers

Charging a capacitor (due to a voltage between the capacitor plates) causes an electric field to be applied to the dielectric between the electrodes. This field exerts a torque on the molecular dipoles, causing the directions of the dipole moments to align with the field direction. This change in the molecular dipoles is called oriented polarization and also causes heat to be generated, resulting in dielectric losses (see dissipation factor). The orientation of the dipoles does not follow the electric field synchronously, but is delayed by a time constant that depends on the material. This delay corresponds to a hysteresis response of the polarization to the external field.

Electric field Vector field representing the Coulomb force per unit charge that would be exerted on a test charge at each point due to other electric charges

An electric field surrounds an electric charge, and exerts force on other charges in the field, attracting or repelling them. Electric field is sometimes abbreviated as E-field. The electric field is defined mathematically as a vector field that associates to each point in space the force per unit of charge exerted on an infinitesimal positive test charge at rest at that point. The SI unit for electric field strength is volt per meter (V/m). Newtons per coulomb (N/C) is also used as a unit of electric field strength. Electric fields are created by electric charges, or by time-varying magnetic fields. Electric fields are important in many areas of physics, and are exploited practically in electrical technology. On an atomic scale, the electric field is responsible for the attractive force between the atomic nucleus and electrons that holds atoms together, and the forces between atoms that cause chemical bonding. Electric fields and magnetic fields are both manifestations of the electromagnetic force, one of the four fundamental forces of nature.

Torque tendency of a force to rotate an object

Torque, moment, moment of force or "turning effect" is the rotational equivalent of linear force. The concept originated with the studies of Archimedes on the usage of levers. Just as a linear force is a push or a pull, a torque can be thought of as a twist to an object. Another definition of torque is the product of the magnitude of the force and the perpendicular distance of the line of action of force from the axis of rotation. The symbol for torque is typically , the lowercase Greek letter tau. When being referred to as moment of force, it is commonly denoted by M.

Dipole Electromagnetic phenomenon

In electromagnetism, there are two kinds of dipoles:

When the capacitor is discharging, the strength of the electric field is decreasing and the common orientation of the molecular dipoles is returning to an undirected state in a process of relaxation. Due to the hysteresis, at the zero point of the electric field, a material-dependent number of molecular dipoles are still polarized along the field direction without a measurable voltage appearing at the terminals of the capacitor. This is like an electrical version of magnetic remanence. The oriented dipoles will be discharged spontaneously over time and the voltage at the electrodes of the capacitor will decay exponentially. [2] The complete discharge time of all dipoles can be days to weeks depending on the material. This "reloaded" voltage can be retained for months, even in electrolytic capacitors, caused by the high insulation resistance in common modern capacitor dielectrics. The discharge of a capacitor and the subsequent reloading can be repeated several times.

In the physical sciences, relaxation usually means the return of a perturbed system into equilibrium. Each relaxation process can be categorized by a relaxation time τ. The simplest theoretical description of relaxation as function of time t is an exponential law exp(-t/τ).

Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. Colloquially, when a magnet is "magnetized" it has remanence. The remanence of magnetic materials provides the magnetic memory in magnetic storage devices, and is used as a source of information on the past Earth's magnetic field in paleomagnetism.

Exponential decay probability density

A quantity is subject to exponential decay if it decreases at a rate proportional to its current value. Symbolically, this process can be expressed by the following differential equation, where N is the quantity and λ (lambda) is a positive rate called the exponential decay constant:

Measurement

Dielectric absorption is a property which has been long known. Its value can be measured in accordance with the IEC/EN 60384-1 standard. The capacitor shall be charged at the DC voltage rating for 60 minutes. Then the capacitor shall be disconnected from the power source and shall be discharged for 10 s. The voltage regained on the capacitor terminals (recovery voltage) within 15 minutes is the dielectric absorption voltage. The size of the dielectric absorption voltage is specified in relation to the applied voltage in percent and depends on the dielectric material used. It is specified by many manufacturers in the data sheets. [3] [4] [5] [6]

Type of capacitorDielectric absorption
Air and vacuum capacitorsNot measurable
Class-1 ceramic capacitors, NP00.6%
Class-2 ceramic capacitors, X7R2.5%
Polypropylene film capacitors (PP)0.05 to 0.1%
Polyester film capacitors (PET)0.2 to 0.5%
Polyphenylene sulfide film capacitors (PPS)0.05 to 0.1%
Polyethylene naphthalate film capacitors (PEN)1.0 to 1.2%
Tantalum electrolytic capacitors with solid electrolyte2 to 3 %, [7] 10 % [8]
Aluminium electrolytic capacitors with non-solid electrolyte10 to 15% [9]
Double-layer capacitor data not available

Design considerations and safety

The voltage at the terminals generated by the dielectric absorption may possibly cause problems in the function of an electronic circuit. For sensitive analog circuits such as sample and hold circuits, integrators, charge amplifiers or high-quality audio circuits, Class-1 ceramic or polypropylene capacitors instead of Class-2 ceramic capacitors, polyester film capacitors or electrolytic capacitors are used. [10] For most electronic circuits, particularly filtering applications, the small dielectric absorption voltage has no influence on the proper electrical function of the circuit. For aluminum electrolytic capacitors with non-solid electrolyte which are not built into a circuit, the dielectric absorption voltage generated can be a personnel safety risk. [11] The voltage can be quite substantial, for example 50 V for 400 V electrolytic capacitors, and can cause damages to semiconductor devices, or cause sparks during installation in the circuit. Larger aluminum electrolytic capacitors and high-voltage power capacitors are transported and delivered short-circuited to dissipate this unwanted and possibly dangerous energy.

Sample and hold Digital Control System

In electronics, a sample and hold circuit is an analog device that samples the voltage of a continuously varying analog signal and holds its value at a constant level for a specified minimum period of time. Sample and hold circuits and related peak detectors are the elementary analog memory devices. They are typically used in analog-to-digital converters to eliminate variations in input signal that can corrupt the conversion process. They are also used in electronic music, for instance to impart a random quality to successively-played notes.

A charge amplifier is an electronic current integrator that produces a voltage output proportional to the integrated value of the input current, or the total charge injected.

A semiconductor device is an electronic component that exploits the electronic properties of semiconductor material, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced vacuum tubes in most applications. They use electrical conduction in the solid state rather than the gaseous state or thermionic emission in a vacuum.

Another effect of dielectric absorption is sometimes described as "soakage". This manifests as a component of leakage current and it contributes to the loss factor of the capacitor. This effect has been known of only recently:[ failed verification ] it is now a proportionately greater part of leakage current due to the significantly improved properties of modern capacitors. [8]

No figures are available from manufacturers for double-layer capacitors.

See also

Related Research Articles

Electrolytic capacitor polarized capacitor whose anode is made of a metal that forms an insulating oxide layer through anodization, acting as the dielectric of the capacitor; a solid, liquid, or gel electrolyte covers the surface of this oxide layer, serving as the cathode

An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel electrolyte covers the surface of this oxide layer, serving as the cathode or negative plate of the capacitor. Due to their very thin dielectric oxide layer and enlarged anode surface, electrolytic capacitors have a much higher capacitance-voltage (CV) product per unit volume than ceramic capacitors or film capacitors, and so can have large capacitance values. There are three families of electrolytic capacitor: aluminum electrolytic capacitors, tantalum electrolytic capacitors, and niobium electrolytic capacitors.

Capacitor plague high failure rate of electrolytic capacitors produced between 1999 and 2007

The capacitor plague was a problem related to a higher-than-expected failure rate of non-solid aluminum electrolytic capacitors, between 1999 and 2007, especially those from some Taiwanese manufacturers, due to faulty electrolyte composition that caused corrosion accompanied by gas generation, often rupturing the case of the capacitor from the build-up of pressure.

Electronic component basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Dielectric heating

Dielectric heating, also known as electronic heating, radio frequency heating, and high-frequency heating, is the process in which a radio frequency (RF) alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.

Filter capacitors are capacitors used for filtering of undesirable frequencies. They are common in electrical and electronic equipment, and cover a number of applications, such as:

Capacitor types Quantitative

Capacitors are manufactured in many forms, styles, lengths, girths, and from many materials. They all contain at least two electrical conductors separated by an insulating layer. Capacitors are widely used as parts of electrical circuits in many common electrical devices.

Ceramic capacitor

A ceramic capacitor is a fixed-value capacitor where the ceramic material acts as the dielectric. It is constructed of two or more alternating layers of ceramic and a metal layer acting as the electrodes. The composition of the ceramic material defines the electrical behavior and therefore applications. Ceramic capacitors are divided into two application classes:

Tantalum capacitor

A tantalum electrolytic capacitor is an electrolytic capacitor, a passive component of electronic circuits. It consists of a pellet of tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounded by liquid or solid electrolyte as a cathode. Because of its very thin and relatively high permittivity dielectric layer, the tantalum capacitor distinguishes itself from other conventional and electrolytic capacitors in having high capacitance per volume and lower weight.

In electronics, leakage is the gradual transfer of electrical energy across a boundary normally viewed as insulating, such as the spontaneous discharge of a charged capacitor, magnetic coupling of a transformer with other components, or flow of current across a transistor in the "off" state or a reverse-polarized diode.

Failure of electronic components Ways electronic elements fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.

Polymer capacitor

A polymer capacitor, or more accurately a polymer electrolytic capacitor, is an electrolytic capacitor (e-cap) with a solid electrolyte of a conductive polymer. There are four different types:

Film capacitor Construction of electrical capacitors

Film capacitors, plastic film capacitors, film dielectric capacitors, or polymer film capacitors, generically called "film caps" as well as power film capacitors, are electrical capacitors with an insulating plastic film as the dielectric, sometimes combined with paper as carrier of the electrodes.

Double-layer capacitance is the important characteristic of the electrical double layer which appears, for example, at the interface between a conductive electrode and an adjacent liquid electrolyte. At this boundary two layers of charge with opposing polarity form, one at the surface of the electrode, and one in the electrolyte. These two layers, electrons on the electrode and ions in the electrolyte, are typically separated by a single layer of solvent molecules that adhere to the surface of the electrode and act like a dielectric in a conventional capacitor. The amount of electric charge stored in double-layer capacitor depends on the applied voltage. The unit of capacitance is the farad.

KEMET Corporation was set up in 1919 and now is based in Fort Lauderdale, Florida. The company produces many kinds of capacitors, such as tantalum, aluminum, multilayer ceramic, film, paper, polymer electrolytic, electromechanical devices, electromagnetic compatibility solutions and supercapacitors. Capacitors are electronic components that store, filter, and regulate electrical energy and current flow. As an essential passive component used in most circuit boards, capacitors are typically used for coupling, decoupling, filtering, oscillating and wave shaping and are used in communication systems, data processing equipment, personal computers, cellular phones, automotive electronic systems, defense and aerospace systems, consumer electronics, power management systems and many other electronic devices and systems. The company also manufacturers a variety of electronic components such as, AC line filters, EMI cores and filters, flex suppressors, relays, metal composite inductors, ferrite products, and transformers/magnetics. The product line consists of nearly 5 million distinct part configurations distinguished by various attributes, such as dielectric material, configuration, encapsulation, capacitance, voltage, performance characteristics and packaging.

SAL electrolytic capacitor

SAL electrolytic capacitors are a form of capacitor developed for high capacity in a small package, with a long and robust service life. They are aluminum electrolytic capacitors with anodic oxidized aluminum oxide as dielectric and with the semiconducting solid manganese dioxide as electrolyte. They are made of etched and formed aluminum anodes, which are folded for the dipped pearl types or wound into a roll for the axial style. The solid manganese dioxide electrolyte is formed onto this roll in a pyrolytic process, similar to that for solid tantalum capacitors.

Aluminum electrolytic capacitor

Aluminum electrolytic capacitors are polarized electrolytic capacitors whose anode electrode (+) is made of a pure aluminum foil with an etched surface. The aluminum forms a very thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode (cathode) (-) of the capacitor. A second aluminum foil called “cathode foil” contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.

Niobium capacitor

A niobium electrolytic capacitor is a polarized capacitor whose anode electrode (+) is made of passivated niobium metal or niobium monoxide on which an insulating niobium pentoxide layer acts as the dielectric of the niobium capacitor. A solid electrolyte on the surface of the oxide layer serves as the second electrode (cathode) (-) of the capacitor.

References

  1. "Modeling Dielectric Absorption in Capacitors, by Ken Kundert" (PDF).
  2. "Elliot sound products, 2.1 - Dielectric Absorption".
  3. WIMA, Characteristics of Metallized film capacitors in Comparison with Other Dielectrics "Archived copy". Archived from the original on 2012-11-05. Retrieved 2012-12-14.CS1 maint: archived copy as title (link)
  4. "Film Capacitors, TDK Epcos, General technical information" (PDF). Archived from the original (PDF) on 2012-02-01. Retrieved 2012-01-23.
  5. AVX, Dielectric Comparison Chart
  6. "Holystone, Capacitor Dielectric Comparison, Technical Note 3" (PDF). Archived from the original (PDF) on 2012-02-01. Retrieved 2012-01-23.
  7. "Kemet, Polymer Tantalum Chip Capacitors" (PDF). Archived from the original (PDF) on 2014-11-23. Retrieved 2012-01-23.
  8. 1 2 R. W. Franklin, AVX, Analysis of Solid Tantalum Capacitor Leakage Current, PDF, PDF
  9. CDE, Aluminum Electrolytic Capacitor Application Guide, PDF
  10. National Semiconductors, Understand Capacitor Soakage to Optimize Analog Systems Archived 2010-01-23 at the Wayback Machine
  11. What's all this Trapped Charge and Dielectric Compression stuff anyhow?

Further reading