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Why Z METERS Read Electrolytics Differently Than Bridges

It might seem that all methods ofmeasur ing capac i tance va lue shou ldproduce the same readings. This is not,however, true when testing electrolyticcapacitors. The capacity of an electrolyticcapacitor changes with applied frequency.Therefore, an electrolytic should not beused to compare the accuracy of different

testing methods.

Most digital capacitor testers find value bymeasuring the time needed to charge theunknown capacitor through a precisionresistor. The t ime-constant gives thecapacitor value. Since electrolytics showtheir highest value at DC, an R-C timeconstant tester shows a value higher thanmarked on the capacitor. This value,

however, is accurate. It shows how thecapacitor works in DC circuits.

As we will see later, the variation in valuedoes not cause testing problems if youapply the capacitors tolerance to yourZ METER readings.

Why The Readings VaryElec t ro ly t ics show the la rges t va luevariations. By comparison, you will not seea difference if you test a parallel bank offilm capacitors totalling severalmicrofarads. They will read the same on abridge or on the Z METER. An electrolyticcapacitor of the same value, shows a largevariety of readings, depending on the test

Fig. 1: The basic parts of an electrolytic capacitor.

frequency used. To find out why, Sencorturned to Dr. J. A. Tunheim, Dean of thePhysics Department at South Dakota StatUniversity, Brookings, South Dakota.

Dr. Tunheim published a 98 page repoentitled, Relationships Between Values oCapacitance Measured by the Sencor

LC53 Z Meter and Other S tandarTechniques. The study explored severapossible causes of the frequency effectsCalculations showed that the variationwere not the result of series inductance oresistance, frequency changes in thdielectric constant of the aluminum oxideor effects of the paper spacer. None othese causes had enough effect (below megahertz) to explain why values changes o m u c h a t f r e q u e n c i e s o f s e v e r akilohertz.

The one remain ing var iab le was th

relaxation effects of the molecules in thelectrolyte solution. The abundance owater played the largest role. The repo

Fig. 2: The electrolytic contains water inthe paper spacer and part of the oxidelayer. This water causes the value tochange with frequency.

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explains: Water molecules, however, areknown to give a relaxation frequency in theone kilohertz region (Hasted, 1973). Sincethe relaxation frequency is the frequency atwhich the molecules cannot follow thealternating electric field, this would resultin a drop of the dielectric constant in theobserved frequency range.

The experimenters used the Debye

relaxation equation for dipolar moleculesalong with information published by K. S.Cole in 1941. The theoretical values fromthese formulas agree closely with themeasured values, as shown in Fig. 3. Thesolid line is the predicted value change,while the crosses mark lab results.

Apply The ManufacturersTolerance

The researchers tried to find a way to usethese formulas to convert readings madewith a bridge to those made with a ZMETER. This didnt work because, Therelaxation time would be dependent on theamount of water and the specific com-ponents of the capacitor. These variables

are not published.

However, the Sencore Application Engineer-ing Depar tment found that is notnecessary to calculate a conversion.Simply apply the capacitor s ratedtolerance to the Z METER readings.

Fig. 3: Test results (+ 's) from the SDSU study compared to theoretical values (solid line).

The graph shows the results on a capacitormarked 22 F. The first point to notice is at120 Hz. Most electrolytics are marked withthe value determined with a 120 Hzimpedance bridge. The 120 Hz value (22.2F) is close to the marked value.

Next look at the lowest frequency on thegraph. It agrees directly with the Z METERreading of 23.1 F. By comparison, noticethat a 1000 Hz bridge shows the value(21.2 F) lower than the marked value bynearly the same amount as the Z METERreads it higher. In each case, you get adifferent value. But all readings are correctfor the test methods used. The problem isin the component being tested, not in thetest method.

Electrolytics have a w ide var ie ty o f tolerances. Many have a tolerance of-20%,+80%. Thus, a capacitor marked100 microfarads could have a value as lowas 80 F (100 minus 20%) or as high as180 F (100 plus 80%) and still be withintolerance.

We wanted to find out if the difference inreadings between the Z METER and a 120Hz bridge would ever cause the Z METERto read a good capacitor outside its ratedtolerance. We tested 512 electrolytics ofdifferent values and voltage ratings withthe Z METER and the 120 Hz reading on aHewlett Packard 4192A impedanceana lyzer . The 4192A app l ies tes tfrequencies from 5 Hz to 13 MHz, and sellsfor $13,900.

None of the capacitors read outside thetolerance. We found capacitors with loosespecs (such as -20%, + 100%) showed wider variation than those with tightespecs (such as +/-20%). Plotting thefrequency responses showed that thcapacitors with the wider tolerances hadmuch wider frequency changes than thoswith lower percentages. The manufacturers tolerances predict the amount o

change you should allow when testingcapacitor value. Simply remember to applthese tolerances to your Z METERreadings.

Value Rarely Changes

Most electrolytic problems do not affecvalue. Be sure to use the Z METER to tesfor leakage, dielectric absorption, andequivalent series resistance every timeThese failures are much more commonAnd, these failures are more likely to affecthe circuits operation.

Z METER and LC53 Z Meter are trademarks of Sencore, Inc. Form 4213 Printed in U.S.A.