Section V Electromagnetic and Other Applications of Aluminum

Chapter 15

Capacitor Foil

Capacitors both fixed and variable are used today A common economic consideration in capacitor design in almost every electrical system. From great power is to obtain the largest amount of capacitance per unit vol­ generating and distributing networks to electric organs, ume of material used. This is, of course, obtained by using including telephone and radio systems, computers and the thinnest electrodes and the thinnest insulating material motors, elevators and x-ray apparatus and so on, the possessing the highest dielectric constant. How far one use of a capacitor almost always is a fundamental can go in these directions depends upon the circuit voltage necessity. to be withstood, the conductance and!or dielectric loss Any arrangement of electrodes whatsoever upon which that can be tolerated and the stability of the assembly electric cbarges accumulate or move will exhibit the inci­ under the operating conditions. dence of capacitance. Where the electrode geometry is Equivalent Network of a Capacitor extensive in space such as a wire or cable, the charges are distributed likewise and we speak of such a structure as All capacitors possess a certain amount of series resist­ being a distributed capacitance. When the electrodes are ance and inductance as well as shunt capacitance and con­ deliberately concentrated in space, the charges are con­ ductance. Fig. 15-1 shows a simple, unrolled foil capaci­ centrated and this is termed a lumped capacitance. All elec­ tor and its equivalent electrical circuits. The series impe­ trical components used specifically as capacitors are looked dance (r + jwL) is made up of the resistance and in­ upon as providing lumped capacitance. ductance of the capacitor leads plus those inherent in the Aluminum has been and is the preferred metal for electrode material, shape and extent. Usually the induc­ capacitor electrodes whether used in rigid plate form or tance is negligibly small as compared to the other factors. in varying thicknesses of foil for d-., a-c low voltage, Actually, the inductance is approximately the same as a high voltage, high frequency, high or low power, impulse wire loop equal in area to that formed by the two leads discharge, etc. and the capacitor unit itself. The foil electrodes appear as a uniformly distributed series resistance. The overall impedance of the equivalent network shown in Fig. 15-1 (c) is: Capacitor Design Considerations .,C Under given conditions of electrical, physical and en­ -~~-+jwL vironmental factors, a capacitor may be called upon to pro­ g2 +.,2 C' g2 +w'C' vide a precise amount of capacitance, a required time (Eq. 15-1) constant of charge and discharge (with proper circuit re­ where: sistance) or a specified impulse release of stored energy. w=2".f The selection of electrode and insulation materials and the fin Hz design of their electrical and mechanical arrangements ginmhos can be optimized to produce an economical capacitoI" that Cin farads will perform properly in its intended service. Lin becrys In Section Ill, Chapter 8, the general conditions gov­ r in ohms erning the relation of potential and charge to capacitance or since is usually very small compared to "PC', were discussed as well as the influence of the dielectric g2 medium. The equations for capacitance relating to a wide g j(w'CL-I) variety of electrode geometries were given and the nature Z~(r+---) of the dielectric polarization of the insulation discussed. wC All of this is applicable to capacitor design. (Eq. 15-2)

15-1 electromagnetic and other applications of aluminum

FO'l elECTRODES

lEADS "",....,''''".. . D.elECTRIC

LEAD INDUCTANCE IL) LEAD INDUCTANCE III

lEAD RESISTANCE lEAD RESISTANCE PLUS EFFECTIVE FOIL RESISTANCE ld

IDEAL EQUiVALENT PARALlel CAPACiTANCE RESISTANCE CORRESPONDING 1<1 TO CONDUCTANCE (g) OF DIELECTRIC

Fig. 15-1. Equivalent electrical circuit of a capaciror.

From the above it is obvious that the effective capacl­ Reduction of losses in a capacitor is quite important C from the standpoints of both adequate performance and tance seen across the terminals of a capacitor is ( ) stable life. In capacitors carrying heavy currents, the w'CL-l energy loss is a source of heating which, if not adequately and that this will vary with frequency. For low frequencies reduced or carried off by thermal conduction, can cause it will be equal to C; as frequency increases, the effect of rapid deterioration and failure of the insulation. Control the inductance increasingly reduces the capacitance and of heat loss enters into the design and use of capacitors the capacitive reactance. At a frequency where resonance for low frequency operation in connection with power occurs (usually very high), the overall impedance is en­ factor correction and, at high frequencies, in radio trans­ tirely made up of the effective resistance of the leads and mitting capacitors. In radio frequency circuits, effective the foil electrodes at that frequency. Above the resonant resistance becomes important in series coil and capacitor frequency, the capacitor acts as an inductance coil with combinations required to. have low impedance at the some series capacitance. Although theoretically every resonance frequencies or parallel combinations required to practical capacitor will exhibit resonance at some high have high impedance at the anti-resonance frequency. This frequency, it is always possible to arrange the electrode is because resistance may add appreciably to the desired and terminal wires to obtain the effect of a low impedance, low impedance at the resonance frequency or reduce the long transmission line free of apparent resonance over a desired high impedance at the anti-resonant frequency. wide high-frequency band. Fig. 15-2 shows the reactance In electric wave filters intended to pass a single band vs. frequency effect for a waxed paper insulated capacitor of frequencies and suppress others, the transmission loss designed for audio frequency circuits. is ideally zero over the pass-band and rises sharply be­ Effective Resistance and Loss of a Capacitor: At fre­ yond the edge or edges. Parasitic loss in the reactive ele­ quencies below which parasitiC inductance becomes sig­ ments is unwanted loss which varies over the pass-!Jand nificant, the dissipated watt loss in a capacitor is oc­ and reaches a maximum .at the edges resulting in distorted casioned by both the ohmic loss in the foils and leads and transmission. This sOurce of loss is generally objectionable, the dielectric loss in the insulating materiaL However, as the ohmic loss is almost always insignificant it is usually for example, in carrier-telephone systems where the cumu­ ignored in commerCial practice and power Joss is com­ lative loss of many filters in tandem may result in con­ puted with the following formula: siderable distortion which must be compensated for by means of attenuation-equalizing networks. W = E'",C tan B (Eq. 15-3) Where: E = volts In his efforts to limit the losses in capacitors required to w = 2".f (frequency in Hz) pass alternating current in telephone and electronic cir­ tan B= dissipation factor cuits, the capacitor engineer is usually primarily concerned S= Joss angie with the effect of frequency on series and shunt resistance.

15-2 capacitor foil

This is because the effective resistance undergoes large electrodes of wound paper capacitors the effective foil changes with changing frequency and because of the wide resistance is approximately equal to 1/3 of the loop dc frequency-range which circuits are often required to cover. resistance obtained by adding the dc resistance values of Loss in Foil and Leads: At a first approximation, the the two foils. In other words, due to current attenuation along the foils only 33 percent of the total doc foil re­ effective impedance of the foil and leads of a capacitor ap­ sistance is effective with respect to alternating current. pears as a straight-line factor over a wide range of fre­ Fig. 15-3 shows the effect of several laid-in terminals in quency. At higher frequencies, impedance increases due reducing effective foil resistance especiaUy at the higher to eddy-current and other losses including skin effect frequencies. where only the outer portion of the metallic components carry the current. Where, as is more usual in practice, the terminals are From the watt loss (Eq. 15-3) above, it is seen that laid-in at approximately the middle of the foil electrodes, the heat loss in the foil and leads increases as the square the current spreads in opposite directions along the foils. of the frequency for constant applied voltage. In general, The effective resistance of the loop in each direction is this condition applies over the operating frequency-range then R/6 and, since the two loops are in parallel, the total of many capacitors. effective resistance becomes R/12. When "n" terminals are laid-in on a foil of length "L", it may be shown that EIJeClive Resistance of Foil Electrodes the lowest resistance is obtained by spacing the terminals In the case of wound paper capacitors, there is a simple at intervals of L/n, with each end terminal located L/2n relationship between the effective resistance of the foil from the end of the foil. With this arrangement, the effec­ electrodes and their de resistance. With reference to Fig. tive resistance is inversely proportional to the square of 15-1, it is clear that alternating current entering the foil the number of terminals, electrodes at the lead-in wires decreases as it spreads or In the limiting case, the edge of the foil is connected to­ distributes along the foil, and the current flowing at points gether along its entire length. This, kno,,\'n as "extended remote from the lead-in wires may be only a small fraction foil" or "overlapped foil" construction, gives the lowest of the entering current. It may be shown theoretically and attainable effective foil resistance for a foil of given ma­ demonstrated experimentally that for the long, narrow terial and dimensions. In addition, by providing an efficient

100 ~APACITANCE. C=1MF ! INDUCTANCE.l =0.2)J.H 10 SERIES ~ INDUCTIVE ~i-" REACTA~C \1­ ./ EFFECT I IVE RESISjANiE '7 , o.1

V) o.I , ~ :x: o I ~y :I 1,'\Q<';'p..\, ~\"';. , ~ 1 ~'tV;, w I U I Z C.\,\°V , ~ c.p..~ ..... u 10 c.'1-0 -< t­ , RESONANCE III ",,6P "fREQUENCY w <,;.~c.~'" I > 100 -~ , ii /" I S I ZIOOO 05 2 5 10 20 SO 100 200 SOO 1000 5000 10.000 fREQUENCY IN kHz Fig. 15-2. Impedance versus frequency of paper capacitors at audio frequencies.

15-3 electromagnetic and other applications of aluminum

0 CAPACITANCE = lMf w 10 ~ Uz \\ -...... ~ 20 u; \~ -- AT 1 kHz '"w 30 a: Z 40 \\. EFFECTIVE Z RESISTANCE 0 50 \\ i= u \\ :::l 60 0 , w \ AT kHz a: 70 3~ I­ Z .... w 80 ..----- U a: .... de RESISTANCE OF w ...... FOIL ELECTRODES "­ 90 ------1--- ______234 5 NUMBER Of LAID- IN TERMINALS PER FOIL ELECTRODE Fig. 15-3. Impedance at high frequency is reduced by adding terminals. conduction path for heat from the inside to the outside of suitable only for direct voltage in a single direction and the unit, extended foil construction is advantageous in the anode terminal is usually marked "positive" to indicate high-power capacitors having large heat dissipation. Fig. in which direction the voltage shall be applied. 15-5 illustrates the relation between heat loss and fre­ ElectrOlytic capacitors are used extensively in low quency in a waxed paper capacitor. voltage ac applications. One type consists virtually of Electrolytic Capacitors two capacitors with their cathodes connected together so that the two capacitors operate in series but in opposite The electrolytic capacitor provides the most capacitance directions. One capacitor absorbs the applied voltage on in a given space at the lowest cost per microfarad. Pri­ one half ofthe ac cycle and the other capacitor comes into marilya filtering capacitor, this type is largely used in con­ nection with de circuits at working voltages less than 500 volts. For example, at low voltage, several thousand microfarads may be contained in a one cubic inch elec­ trolytic capacitor using etched aluminum foil electrodes. Fig. 15-4 shows a typical eleCtrolytic capacitor design. The high capacitance per unit votume of electrolytic capacitors comes from the extreme thinness of the dielec­ ANODE tric which is an anodic oxide film previously built up by an CATHODE electrolytic process on one of the foil electrodes, known as the anode (capacitance per cubic inch is inversely pro­ ..J.. portional to the thickness of the dielectric). The thick­ ~ 'i k ness of this insulating film is but a few millionths ,':, h .. ~ ;', of an inch and the working voltage gradient can be of the .. .'. J' order of 10 million volts per inch. Etching the anode in­ ? . creases the effective area so as to increase the capacitance •!, c­ as much as 7 to 30 times. '-', "f' With the voltage applied in one direction. the film has FOIL a high resistance to the flow of current and behaves like a PAPER dielectric. With the voltage reversed, the film behaves like a relatively low resistance and.. if the voltage is high enough, it passes large currents, heats 1!tp and soon breaks down. Because of this unidirectional property, the film is Fig. 15-4. Cross-sectional view oj a typical capacitor. 15·4 capacitor foil

10 I.,IAMP N =1.35 ! 5 C= lMF I ..... DIELECTRIC lOSS 2 ~ r..... 1290 fn-2 I"­ ~...... J2lTC) 2 ..... <.f)...... ~ 0.5 z FOil lOSS, I2r I­ <.f) -_. - -:s..; '-, . <.f) 0.2 -­ --- -­ - ---- ­-- -­ - -­ -­ 0.... , .... !'-...... « 0.1 w ...... J: 0.05 ..... "" ...... 0.02

1.01 0.1 0.2 0.5 1 2 5 10 20 50 100 200 500 1000 FREQUeNCY IN KILOCYCLES PER SECOND Fig. 1.5-.5. Heat loss versus frequency at constant current in paper capacitor

play during the succeeding half-cycle when the voltage face and edge treatment. High purity aluminum capacitor reverses. The most common type is a capacitor wound as foil is produced by a number of manufacturers and is a non-polar capacitor utilizing two anodes instead of an available in a wide range of thicknesses, widths and alloys. anode and a cathode. Such capacitors are used extensively Precision processing assures control of impurities, foil uni­ on low ac voltages; for example, as motor-starting capaci­ formity and continuity, freedom of sticking during un­ tors when the fuil voltage is of short duration. Where the winding, excellent gauge control, perfectly slit burr-free voltage is very low, they can be used continuously; for example, to filter audio frequencies in radio sets. In edges, and tightly wound compact coils. A full range of general, they are limited on ac with respect to voltage purities are available-from 99,35% to 99.99% pure because of the high power factor. aluminum. Improvements made in the design and manufacture of Foil is produced either in the dry condition, or with a aluminum electrolytic capacitors allow for use in a wide so-called slick finish, or etched and anodized. variety of commercial and industrial precision circuit applications. Common usage today is in the telecom­ Dry Foil: Specialized annealing technique provides a munications industries. The highest aluminum purities, for surface free from residual oil contamination. A thor­ example 1199, allow for the manufacture of high reli­ oughly wet surface will show no droplet formations in­ ability capacitors with extended life and temperature dicative of oil residue. characteristics and long, stable storage life. Slick Foil: A slightly lubricated surface developed in Capacitor Foil Availability combination with annealing practices overcomes fric­ Precise capacitor design begins with high-purity alumi­ tion generated by winding equipment, but will not con­ num capacitor foil of precise thickness and desired sur- taminate the dielectric. electromagnetic and other applications of aluminum

TABLE 15-1 Chemical Composition-Maximum Allowable Impurities in Weight Percent

I Iron & ! r Minimum Alloy Silicon Iron Silicon Copper Manganese Titanium Magnesium i Zinc Other Aluminum i

I .65 1235 ! .05 I .05 99.35 1145 .55 .05 .05 .03 99.45 1180 .09 .09 .01 .02 .02 99.80 1188 .06 .06 .005 .01 .01 .01 .02 .01 99.88 1193 .04 .04 .006 .01 .01 99.93 1199 .006 .006 .006 .006 i i .006 .002 99.99

Anodized Foil: High purity aluminum foil is specially Table 15~1 gives chemical composition of the aluminum alloys most used in condenser foil production. It is to be recalled, in this treated to provide a very thin oxide film on its surface. connection, that the addition of other metals to aluminum usually This film acts as a dielectric and results in high capaci­ lowers its electrical conductivity, Also that heat treatment putting tance as compared to paper capacitors. It can be etched other metals in solid solution with the aluminum also lowers con~ to increase the surface area 1 to 30 times. thereby pro­ ductivity, viding even greater capacitance in a given volume. Table 15-2 gives typical properties. Table 15~3 gives thickness and width limitations, A typical group of product data tables of one aluminum TabJe 15-4 gives welght-area conversion factors. foil manufacturer is reproduced here. Table 15~5 gives typical splice data. Table l5·6 gives foil toll sizes and weights.

15-6 capacitor foil

TABLE 15-2 TABLE 15-4 Typical Physical Properties-O Temper Weight·Area Conversion Factors

I Alloy Gauge Tensile-psi % Elongation Thickne.. lin.) I Sq In./lb Sq Ftllb !lb/432,000 Sq In." 1199 .003" 5,000 3.2 .00017 60,300 418.75 7.16 1193 .003" 8,100 6.6 .0002 51,300 356.25 8.42 1188 .003" 6,300 5.7 .00023 44,600 309.72 9.69 1180 .OO3 tt 6,500 6.0 .00025 41,000 264.72 10.54 1145 .003" 10,000 7.0 .00030 34,200 237.50 12.63 1235 .003" 10,500 8.1 .00035 29,300 203.47 14.74 .00040 25,600 177.78 16.88 .00045 22.800 156.33 18.95 .00050 20,500 142.36 21.07 TABLE 15-3 .00055 18,600 129.17 23.23 Thickness And Width Limitations .00060 17,100 118.75 25.26 .00065 15,800 109.72 27.34 Alloy Gauge Finish" Widths .0007 14,600 101.39 29.59 1235; 1145 .00017 ..·.0002 .. MIS 3/8 tI~26t1 .00075 13,667 94.91 31.61 1235; 1145 .0002"-.00023" MIS 3/8u~31u .00060 12,800 88.89 33.75 1235;1145 .00025" MIS 3/.·43" .00085 12,058 83.74 35.83 11 1235; 1145 .0003 MIS .3 /g "~50u .00090 11,400 79.17 37.89 1235;1145 .00000f~_.OOO4·~ MIS 3Iau~64° .00095 10,789 74.92 40.04 1235; 1145 .00045"·.001" MIS 3/8"~72" .0010 10,250 71.18 42.15 1235; 1145 .0015"·.0059" MIS,2SB 1/4N.72" .0015 6,830 47.43 63.25 1235; 1145 .002 .. ·.0059" 2SB I /4 "~52" .0020 5,130 35.63 84.21_ 1180; 1188 .0004"-.0015" MIS 3/s ".36" .0025 4,100 28.47 105.37 1180; 1188 !.002 ....0059 .. 2SB 1/4 "·36" .0030 3,420 23.75 126.32 1193; 119~ ,.001 .... 0015.. MIS 3/8 "·36'* .0035 2,930 20.35 147.44 1193; 1199 .002"·,0059" 2SB 1/4 u·36" .0040 2,560 17.78 168.75 "MIS des.gn.tes Matte one side. .0045 2,280 15.83 189,47 2SB designates Two sides bright. .0050 2,050 14.24 210.73 .0055 1,860 12.92 232.26 ·432,000 sq. in. signifies one ream (500 sh••ts) of 24 TABLE 15-5 in. x 36 in. sheets. Splices (Annealed Foil-Dry or Slick)

GaB~ Width Splice .00017"-.0004" 23u Maximum Knurl .00017"-.0015" All Widths Foil Tape .002 t1-.OO5" All Widths (Electric Weld) (Electrolytic Foil) (Ultrasonic Splice)

15-7 electromagnetic and other applications of aluminum

TABLE 15-6(a) Roll Size

Width Type of Core Maximum 00

11 114 _ 311 1 SII,' Aluminum 6" n 5 3 -311# 1 / 16 #1 Aluminum 12H

81 1/4 "_3 3" Aluminum 8" 3"_72## 3"" Aluminum 13" 171i_72u 3 U Iron 30"

TABLE 15·6(b) Roll Weight Data-Unmounted Foil

SPOOLED ROLL WEIGHT OF FOIL PER INCH OF WIOTH-(POUNOSI

Outside ~iameter ALUMINUM CORE IRON CORE (lnehes) 10-1-5/16" 10-3" 10-2·1/2" 10-3" 00-1·1/2" 00-3-3116" 00-3" 00-3-1/4" 2" 0.31b - - - 2%'" 0.3 - - - 3" 0.5 3%" 0.7 0.21b 0.31b 0.21b 4" 1.0 0.4 0.5 0.4 4%U 1.3 I 0.7 0.8 0.7 5" 1.7 1.1 1.2 1.1 ! 5%" ! 2.1 1.5 1.6 1.5 6" 2.6 2.0 2.1 2.0 6%" 3.1 2.5 2.6 I 2.5 7" 3.6 3.0 3.1 3.0 7%" 4.1 3.5 3.6 3.5 8" 4.7 4.1 4.2 4.1 8y"" 5.3 i 4.7 4.8 4.7 9" 6.0 i 5.4 5.5 5.4 9%" 6.7 6.1 6.2 6.1 lOU 7.5 6.9 7.0 6.9 10%U 8.3 7.7 7.8 7.7 1,." ~.., 8.5 8.6 8.5 11 Y,," 10.0 9.3 9.4 9.3 12" 10.9 10.2 10.3 10.2 12%" 11.8 11.2 11.3 11.2 131"1 12.8 12.2 12.3 12.2 1314" 13.8 13.2 13.3 13.2 14n 14.8 14.2 14.3 14.2 14%U , 15.9 15.3 15.4 15.3 15" 17.0 16.4 16.5 16.4 15%U 18.2 11.6 17.7 17.6 NOTES: 10 and 00 dimensions represent the Inside and Outside Diameter, respectively, of the metal COre. The above figures are approximate and do not include core weight. For approximate net weight of Foil per roll, exclusive of core weight, multiply the figure under the applicable roll 00 and type of core by the inches of roll width.

15·8