Understanding Dielectric Absorption

Many electronics text books have a shortdefinition of dielectric absorption, andmany capacitor manufacturers have writtenapplication notes explaining its effects oncertain circuits. Yet, i t was not unti lSencore introduced the LC53 Z Meter in1979 that the importance of dielectricabsorption was fully realized. It was thenlearned that the D/A got worse with age incertain capacitors. This, in turn, gave agood indicator of the capacitor’s futurereliability.

What Is Dielectric Absorption?

Dielectric absorption (D/A) is the inabilityof a capacitor to release all its storedenergy, even if a dead short is appliedacross its leads. D/A is known by manyother names, such as battery act ion,voltage retention, dielectric soak, residualcharge, or soakage. As Fig. 1 shows, if a

capacitor is discharged, and then allowedto sit with the leads open, it will regain apercentage of the original voltage. Allinsulating materials used as dielectricsshow this effect. Some capacitors mayregain a large percentage of the originalcharge. Others have such a tiny amount ofD/A that the effects can only be detectedwi th h igh ly soph is t i ca ted labora toryequipment.

Fig. 2: One way to show D/A is as a tinyThere are two models used to represent this storage battery which recharges thecondition. Fig. 2 shows the D/A as a tiny capacitor from within.storage battery in parallel with the idealcapacitor. Fig. 3 shows D/A as a resistorand capacitor in parallel with the maincapacitance. Fig. 4 shows a more precisemodel than Fig. 3, since D/A usually hasseveral different time-constants, explain-ing why some D/A effects take longer todissipate than others.

Fig. 3: A more common model uses aresistor/capacitor network in parallel withthe ideal capacitor. The D/A portion cannotdischarge instantly because of the timeconstant formed by the series resistor.

How Does D/A Affect Circuits?

Fig. 1: Dielectric absorption causes a capacitor to recover a percentage of its originalcharge after being fully discharged through an external path.

Some circuits are directly affected by theeffects of D/A. The biggest effects are incircuits that use a capacitor to hold aprecise DC voltage level. One example isthe reference capacitor in an analog todigital converter (ADC). ADC's are used inmeters, level detectors, electronic scales,and any other device which converts ameasurement to a digital number. Othercritical capacitors are in sample and hold

What Was Known Before TheZ METER?

Engineering handbooks, magazine articlesand data sheets have been available to helpdesigners choose capacitors with low D/Afor critical applications. These referencesexplain that certain ceramic and mylardielectrics have high D/A, while teflon andpolypropylene show low levels. Designersof oil-filled or oil-impregnated, high voltagecapacitors, know that they have such highlevels of D/A that they are shipped with ashor t ing w i re connec ted across theterminals to prevent a severe jolt whenhandled weeks or months after they weretested at their rated voltage.

Fig. 4: A more accurate representation of D/A than Fig. 3 uses many differentresistor/capacitor combinations to show time constants ranging from seconds to days, insome cases.

circuits, such as AGC, AFT and peakdetecting circuits. In either case, thecapacitor should charge to a DC level andstore the voltage long enough for theremaining circuitry to respond. It shouldthen be discharged, charged to a new leveland tested again. With D/A, some previouscharge adds to the latest value, causingerrors. D/A of 1% may cause trouble inthese circuits.

D/A can also affect some series couplingcapacitors. The inability to charge anddischarge in step with the applied signalcauses distortion. These capacitors willtolerate much higher levels of D/A thanthose in the previous examples, with 5 to15% being acceptable.

A power supply capacitor may toleratemore than 15% before the c i rcu i t i sa f fec ted . Then, f i l te r ing may reducebecause the capacitor cannot dump all itscharge back into the load fast enough tosmooth the ripple.

The best advice is to use the circuit’soperation as a guide about how much D/Ato accept. If you see circuit troubles whichcould be explained by extra DC voltageacross the capacitor, you may need toreplace the capacitor, even though the D/Areadings are fairly low. Then, remember tocheck the replacement capacitor for D/Abefore putting it into the circuit to preventinstalling a bad capacitor.

Fig. 6: Before the Z METER, mostreference data assumed that D/A wouldnot change once a capacitor with low D/Awas selected to use in a circuit.

Most D/A literature assumes that D/A in acapacitor remains at a certain level, so it isonly important in choosing the correct typeof capacitor while designing a circuit. But,Z METER owners know that D/Asometimes increases and causes circuitproblems.

Before the introduction of the Z METER,nearly all the information written about D/Aignored electrolytic capacitors, probablybecause they have such high levels of D/Athat they cannot be used in the criticalapplications. During early Z METER tests,

Fig. 5: The DC storage capacitor in a servo circuit is a type of “sample and hold” however, Sencore Application Engineersapplication which can cause trouble if the voltage from the D/A adds to the correction found that the D/A level is a very goodvoltage. indicator of electrolytic aging. The D/A

o f t e n g e t s w o r s e b e f o r e a n y o t h e rmeasurable changes appear, such asvalue, leakage, or ESR. Z METER usersuse th is knowledge to p ick a goodreplacement capacitor.

What Causes Dielectric Absorption?

Theory tells us that all the potential energyof a charged capacitor is held in anelectrostatic field which causes the dipolesin the dielectric to orient themselves alongthe lines of force. In real life, however, alldielectrics have some degree of chemicalpolarity, so some energy is also storedas chemical changes in the dielectr icmolecules.

After this f irst polarizing process, thedielectric contains two types of electrons:bound and free. The bound electronssaturate the negative plate and the surfaceof the dielectric. The free electrons movethrough the dielectr ic. Free electronswhich travel from one plate to the othercause leakage. But, some free electrons

Fig. 8: The free electrons passing throughthe dielectric cause leakage. The oneswhich cause dielectric molecules to changestate cause D/A.

only move part way through the dielectric,and store energy chemically, similar to theway a storage battery holds energy. Theseare the electrons responsible for D/A.

D/A is often called “soakage” becausethe capacitor acts like a sponge. Whendischarged through an external path, thissoakage charge releases very slowly,taking minutes or even weeks in somecases.

What Causes D/A To Get WorseWith Time?

This depends on the type of capacitor. Acapacitor with a plastic film dielectric oftendevelops D/A if water or chemicals worktheir way to the dielectric. Thecontamination displaces the air whichnormally fills the voids between the metalplates and the dielectric. Since water has ahigher D/A factor than, say, mylar, theD/A increases.

Fig. 7: Charging a capacitor moves thedipoles of the molecules in the dielectricfrom a random arrangements (A) to apolarized arrangement (B). The dipolesshould return to random when discharged.

Capacitor charging happens in two steps,sometimes called “electrification. " In thefirst instant after application of a voltage,an electrostatic field builds between thetwo plates, causing the dipoles in thedielectric to turn to face the lines of force.These tw is ted d ipo les s to re energy ,compared to the normal uncharged state inwhich all the dipoles are randomly oriented Fig. 9: D/A increase in plastic film capacitors if moisture or chemicals creep in through theto produce a net charge of zero. seals and fill air pockets between the dielectric and the metal plates.

Moisture also affects electrolytic capa-citors, but in the opposite way of thefi lm types. The electrolyt ic capacitorincreases in D/A as the moisture leaves thecapacitor. As the electrolyte loses water,some dissolved chemicals may form saltcrystals between the paper spacer and thealuminum oxide which serves as thedielectric. These crystals have a higherresistance than the normal (wet) electrolytesolution, causing some parts of the oxidesurface to have a series resistance. Thisseries resistance does not cause increasedleakage, since the insulating oxide is stillintact. It may not cause the ESR reading toincrease either, since adjoining sections ofthe paper may still have enough water tokeep the resistance low.

During this drying, the D/A becomes veryhigh because the electrolytic effectivelyhas hundreds of tiny resistor/capacitornetworks, all connected in parallel; somewith short time constants, and some withlong ones. This forms a model very muchlike the one shown in Fig. 4, which causeshigh levels of D/A.

Electrolytics will also increase in D/A ifchemica l changes in the e lec t ro ly tesolution or in the Oxide layer change.Contamination may seep in through theseals and cause the capacitor to have astorage-battery action. This may combinewith the drying action to cause even higherD/A levels.

Multi-section electrolytics may also showhigh levels of D/A i f there is leakagebetween adjacent sections. The leakageacts as a resistor in series with the nextsection, forming the same model as Fig. 3shows. The D/A effect changes as theresistance changes. If each section of thecapacitor has a different value, the smallervalue will show a higher amount of D/Athan the larger value. Fig. 11 shows howthe D/A from the larger section has agreater effect than the smaller section, asone, then the other is tested.

Fig. 11: Leakage between sections of amulti-section electrolytic act like D/A. Thesmallest value section will appear to have alarger amount of D/A than the others.

Fig. 10: As an electrolytic dries, some of the chemicals in the paper spacer cause highresistance salt crystals to form. These, in turn, produce a model similar to the one in Fig. 4.

Z METER and LC53 Z Meter are trademarks of Sencore, Inc. Form 3759 Printed in U.S.A.