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The electrolyte being a liquid, a liquid tight container must be employed to house the capacitor.
Wet electrolytic capacitors seem to have found commercial application and use as early as 1892 and were used at that time, in connection with single phase alternating current motors, for starting purposes.
Wet electrolytic capacitors intended for the more common application in power converter filter net works first began to be used, commercially, in 1921. Since that time continual improvements have been made in all essential characteristics but basically there have been few changes in design.
All wet electrolytic capacitors essentially consist of three parts; namely, a container filled with electrolyte into which is immersed an aluminum anode member.
Some six or seven years ago most of the wet electrolytic capacitors were contained in drawn, seamless cans of copper but in more modern practice the copper containers have been replaced with drawn or extruded, round seamless cans of commercial grades of aluminum.
Typical examples of present day constructions are shown in the following illustrations:
(Courtesy Cornell-Dubilier Electric Corp.)
The internal construction of a typical wet electrolytic capacitor is shown in the following illustration.
From this illustration it can be seen that a complete wet electrolytic capacitor consists of the following parts:
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Commercial aluminum, with an aluminum content of approximately 99.3%, is normally used for fabricating the containers.
When wet electrolytic capacitors are used under certain conditions, the inside surface of the containers is coated, by electroplating, with chromium. Sometimes, in place of the chromium plating, the inside surface of the container is roughened by chemical etching. The reasons for this special treatment of the inside container surface will be explained in detail in subsequent chapters.
In present day constructions of wet electrolytic capacitors, the sizes of containers have been fairly well standardized to the following body dimensions:
| Outside Diameter | Overall Body Length |
|---|---|
| 1" | 13/4" to 41/2" |
| 13/8" | 21/2" to 41/2" |
| 11/2" | 21/2" to 41/2" |
Wet electrolytic capacitors, in operation, must always be mounted in a vertical or upright position, in order that the anode member be always submerged under the surface of the electrolyte. In the mounting of the container, two methods are used. The type of container which has a threaded neck portion is mounted with a nut while the type of container without the threaded neck portion is mounted with a ring type clamp or so-called mounting ring.
In most cases the threaded neck portion is an integral part of the container but in some cases this threaded neck is a moulded part of bakelite or hard rubber.
The size of thread, on the threaded neck containers, has been standardized as follows:
| Outside Diameter | Thread |
|---|---|
| 1" | 5/8" - 18 threads per inch |
| 13/8" | 3/4" - 16 threads per inch |
| 11/2" | 3/4" - 16 threads per inch |
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These covers are generally sealed by spinning either the cover into an indented recess in the container rim or by spinning the edge of the container over tight against the cover.
Some of the more common constructions are shown in the following illustrations.
In the operation of a wet electrolytic capacitor a certain amount of gas is generated from the electrolysis of the electrolyte and unless means is provided for the purpose quite high pressures may be built up inside the container. For this reason it has been found necessary to provide the covers with some form of vent. In practically all instances this vent takes the form of a thin sheet of gum rubber which is perforated by a very small hole. The thickness of the rubber and the size of the perforation determine the pressures at which venting action will occur. In practice it is desirable to obtain venting action at the lowest pressure which will prevent actual leakage or escape of electrolyte through the perforation.
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The coiled and accordian pleated types of anodes are the ones in most common use.
Both these types are fabricated from aluminum foil varying in thickness from 0.010" for the coiled type to 0.005" for the accordion pleated type.
The coiled type, circular pleated type and frequently the accordion pleated type anodes employ aluminum foil that has been perforated with either round holes or oblong slots. This is necessary to reduce the mean path, through the electrolyte, from anode surface to container wall. This of course reduces the resistance equivalently in series with the capacitor.
Anode structures are always fabricated of high purity aluminum of a minimum aluminum content of 99.8%. Higher purities of 99.85% to 99.99% are still more satisfactory and desirable but are less frequently used because of economical reasons. The detailed effects of impurities in anodic materials will be fully covered in subsequent chapters.
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The more common type is a round aluminum rod with flattened sections which are perforated with holes through which rivets or eyelets pass for fastening the anode plate.
Riser rods must also be fabricated of aluminum of the same high purity that is required for the anode proper. This same also applies to any rivets or eyelets used for fastening the anode to the riser rod. It is understood why this is necessary when it is mentioned that all parts of the anode assembly, including riser rods, rivets or eyelets are parts of the anode proper and must therefore be equally capable of being coated with the dielectric film of aluminum oxide.
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Figure A shows a construction in which the container neck is swaged or squeezed tight around the stem bushing.
Figure B shows the same type of construction using a container without a threaded neck portion.
Figures C and D are similar constructions but in the case of C the container neck is fluted in order to exert pressure on the stem bushing while D shows the use of small concentric rings or grooves spun into the container neck to accomplish the same result.
In figure E the riser rod is provided with a ring shoulder which in turn seats tightly against the stem bushing when the end of the riser rod stem is pulled outward and the end riveted over. The bakelite washer provides necessary insulation between the solder lug terminal and the end of the container neck.
Figure F shows a construction similar to figure E except the riser rod stem is tapered and the tapered portion fits snugly into a tapered stem bushing. A liquid tight seal results when the riser rod stem is pulledout tight and the end riveted over against the solder lug terminal. A bakelite washer is also used here to insulate between the solder lug terminal and the end of the container neck.
While there are other types of constructions in general use, these which have been described, serve the purpose of giving a clear idea of the present day methods used.
Stem bushings are always made of pure gum rubber of extremely high chemical purity. If it is necessary, for mechanical reasons, to harden the pure gum stock, extreme care must be exercised in the selection of the proper filler materials.
It is imperative that the stem bushing material impart no impurities to the electrolyte, remain neutral to the electrolyte and maintain its original resiliency over a long period of usefulness.
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In the cases where the end of the riser stem is spun or riveted over against an insulating washer on the end of the container neck, a standard solder lug terminal of tin or solder coated brass or copper may be used.
In the cases where the anode riser stem, however, projects straight from the stem bushing, in the form of a round aluininum rod, it is necessary to use a special type of solder lug terminal. For mechanical reasons it is necessary to fabricate this type of solder lug terminal of steel which in turn is hot solder coated to facilitate soldering. A typical type of solder lug terminal of this class and the method of its fastening to the riser stem is shown in the following illustration:
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In past years a thin perforated and corrugated sheet of celluloid was used, generally, as a separator material but in recent years, hard rubber has completely replaced the use of celluloid. Two main reasons for this change have been economy and the fire hazard always present with the use of celluloid.
Hard rubber used as a separator material must be of exceedingly high chemical purity and be initially fabricated with as low a sulphur content as is possible.
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Normally, the aqueous type, consisting of an aqueous solution of boric acid and ammonium or sodium borate, is employed. In special cases where capacitors are required to have better than normal temperature characteristics an electrolyte consisting of distilled water, boric acid, ammonium borate and either a glycerol or glycol is employed. The electrolyte containing either a glycerol or glycol will not freeze at temperatures at which the straight aqueous electrolytes solidify and therefore the capacitors are rendered operative over a much increased temperature range.
More complete details on electrolytes of both types will be given in subsequent chapters.
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