Dielectric absorption

The dielectric absorption (Latin absorbere " suck, suck " ) describes the losses or deviations from the simple ideal behavior of a dielectric in an electric field. In capacitors, the effect is on the one hand as a dielectric loss in AC operation and on the other hand, as a time-dependent voltage on the capacitor in spite of a constant load, for example in the form of a recurring after the discharge voltage (and possibly dangerous) in the condenser. In time determining circuit parts or hold circuits, this can lead to errors. In high-frequency lines, the dielectric absorption may contribute to attenuation.

Explanation

In a real dielectric can not immediately follow a variable electric field polarization. It takes some time to adjust their average orientation to a changing field to a permanent electric dipoles in the dielectric by polarization. The dielectric absorption leads to a conversion of energy from the alternating field to heat both unwanted in non-ideal capacitor, or specifically in the microwave oven.

The resulting between the electrodes alternating electric field aligns the disordered permanent electric dipoles in the dielectric by polarization for each field direction., The polarization of the dipoles by dielectric heating results in losses and is accompanied by heating of the capacitor. The dielectric absorption sucks, figuratively, the energy required for polarization in on. Dielectric absorption and dielectric losses are synonymous in itself. In many capacitors, the dielectric absorption is thus the major source of electrical losses and determined - while neglecting the feed losses - the loss factor, or quality factor of the capacitor ESR.

These properties are in the frequency range from about 0.01 Hz to 1 GHz, the area are usually driven in the commercial capacitors, determined by a material-dependent relaxation time. This results from the space charging process of the capacitor in a time-delayed direction of the polarized permanent molecular dipoles in the dielectric. The time constant is explanation of the frequency dependency of the relative permittivity of many dielectrics for capacitors. Because in said frequency range, the dipoles are mainly polarized by a polarization orientation, wherein the dielectric relaxation substantially matches the dielectric absorption, dielectric absorption is often referred to as dielectric relaxation.

The material-dependent relaxation time also causes after a complete discharge of a capacitor, a material-dependent number of molecular dipoles are polarized in the field direction, without first applying a voltage across the terminals is still measurable. The residual polarization in the dielectric but relaxed over time, which then is formed on the electrodes of the capacitor a voltage of the polarity of the previously applied voltage, so to speak, " loaded " is. In previous releases, the dielectric absorption is therefore described as a recharging effect.

The voltage across the recharging effect slowly builds up, similar to an exponential function. Up to unload all dipoles may take days to weeks depending on the material. The " reloaded " voltage can be in the high insulation resistance capacitor dielectrics are common today - even in Electrolytic Capacitors - keep for months. The unloading and subsequent reloading can be repeated several times.

Measurement

The procedure for measuring the recharging effect of the dielectric absorption is defined in EN 60384-1: The capacitor is charged 60 minutes with rated voltage, then discharged through a resistance of 5 Ω for 10 seconds. After removal of the discharge resistor, the resulting voltage is measured after a 15 minute recovery time. The size of the resulting voltage by the dielectric absorption is indicated in relation to the initially applied voltage in percent and depends on the dielectric used. It is specified by many manufacturers in the data sheets.

For double-layer capacitors, there are no safe levels of manufacturers to the size of the dielectric absorption, so no numerical value can be specified in the table above.

Effects in circuits

In today's conventional capacitors, this effect has two implications. The force generated by the dielectric absorption voltage at the terminals may under certain circumstances in the function of a circuit to cause problems. For sensitive analog circuits such as sample-and- hold circuits, integrators, or amplifiers then Class 1 ceramic or come polypropylene capacitors instead of class 2 Kerkos, polyester film capacitors or electrolytic capacitors are used. In the overwhelming number of most circuits, particularly when the capacitors for filtering unwanted frequencies are used, but these are often minimal electrical Nachladespannung has no effect on the electrical operation.

For aluminum electrolytic capacitors with non-solid electrolyte, however, the object created by the recharging effect voltage for components not yet built by sparking during installation can be an environmental hazard. It can through this voltage at 400 V electrolytic capacitors may well be 50 V, caused when installed in the circuit damage to semiconductors or other components. Larger aluminum electrolytic capacitors, but also high-voltage capacitors, power capacitors, must therefore be shorted transported and delivered.

The second implication of the effect of dielectric absorption is known only recently and is due to the significantly improved properties of modern capacitors. A closer examination of the time course of the leakage current of tantalum electrolytic capacitors polymer electrolyte after the voltage application has been found that the current increases to a value greater than the actual leakage current. The difference will be explained with the energy requirements needed to match the delayed spontaneous alignment of the molecular dipoles in the dielectric of the field direction. This power is thus a part of the residual current, but can also be separated by special considerations of it.