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Also, in the common cathode, concentrically wound multiple section capacitors, the cathode foil and the electrolyte saturated separator are in continuous strips or pieces. The appearance of a completed winding of this type may be observed by referring to the following illustration of a dual section:
Summarizing the above mentioned data, common cathode, concentrically wound, dry electyrolytic capacitors may be constructed by using one cathode foil which is common to one or more anode foils. In this construction, the individual anode foils may be of any capacity or voltage rating. In other words, in one concentric winding, the individual anode foils do not have to be similar in respect to either capacity or voltage. There are some disadvantages to the employment of multiple section capacitors of this type, in certain circuit applications but this subject will be covered, in detail, in the chapter devoted to the uses of dry electrolytic capacitors.
Common anode, concentrically wound, dry electrolytic capacitors differ from the common cathode type in that the common foil is the anode which is in turn common to one or more cathode foils.
There are definite limitations to this type of construction. The cathode foils are not the other conducting surfaces of the respective capacitors as this is the function of the electrolyte. From this fact it becomes apparent that the electrolyte, in each section of the concentric winding, must be isolated electrically or one of the cathode foils may become positive in respect to the electrolyte. In other words, the electrolyte must not be common.
Another limitation is that the common anode foil must be formed, over its entire surface, to a potential equal to the potential of the highest voltage rating of any one capacitor section in the winding. This fact may entail an economical waste of material in certain capacity and voltage combinations.
The physical and electrical isolation of the electrolyte between sections frequently presents serious mechanical construction problems. For a more complete understanding of why it is necessary to isolate the electrolyte, reference is made to the following schematic diagram.
This schematic diagram shows a typical filter network with an inductance L in the negative side of the circuit and a common anode, concentrically wound, electrolytic capacitor with the negative terminals connected across the inductance L.
For the sake of illustration, assume that the p0tential applied to the network, as indicated by the voltage E1, is 400 volts direct current. Also assume that the resistive load R is of such a value that the current flowing through the inductance L, causes a potential drop in the inductance of 100 volts. This potential drop, of course, is the resultant of the current times the resistance of the inductance L and the voltage at E2 will become 300 volts. The net result is a potential of 400 volts applied to capacitor section C1, and a potential of 300 volts applied to section C2. If the electrolyte is common to the two cathode foils corresponding to C1 and C2 then a positive potential of 100 volts will be applied between the electrolyte and one of the cathode foils. As the cathode foil is unfilmed the result is a relatively large current flow between the two cathode foils. Two effects result from this flow of current. Either the positively polarized cathode foil acquires an anodic film or the current flow represents a resistance in parallel with the inductance L. Both effects are equally undesirable. The first will cause an effective reduction in the original capacity of C1 and the second will reduce the effective value of inductance of L. It obviously becomes necessary to form the cathode plate of C1 to a potential equal to the voltage difference between E1 and E2 and add to the areas of both anode and cathode foil surfaces of C1 to compensate for the reduction in desired capacity or completely isolate the electrolyte in one section from the electrolyte in the other. Both methods are frequently employed but the latter is by far the more satisfactory.
One method of isolating the electrolyte between sections, in a concentric winding, is shown in the following illustration:
In this method it is customary to wind one capacitor section (C1), then wind a section of insulating material such as varnished paper or cambric of sufficient width to provide a projecting barrier and of sufficient length to provide from one and a half to two complete turns. The other capacitor section (C2) is then wound. The barrier of insulating material keeps the electrolyte in section C1 separate from that in section C2.
Common anode, concentrically wound, dry electrolytic capacitors are seldom fabricated in multiple units of more than two capacities.
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The simplest construction employed is a cardboard tube, from each end of which projects connection terminals. In this construction the terminals usually take the form of bare leads of tinned copper. The capacitor winding proper is sealed or potted into the cardboard tube with an asphalt or asphalt base sealing compound of high chemical purity. The cardboard tube is either impregnated in a wax dr varnish to make it moisture-proof. Such a cardboard tubular construction is shown in the following illustration:
Tubular cardboard encased units of the above construction are generally confined to capacitors whose voltage and capacity ratings represent small physical sizes. Larger sizes are also encased in cardboard, both in single as well as multiple section constructions. Such a construction is shown in the following illustration:
Dry electrolytic capacitors are also commonly supplied in rectangular shaped cardboard containers. In fact both shape and size are practically unlimited, being arranged to meet each specific application.
Metal containers, both round and rectangular, are also in common use. Typical illustrations follow.
In the above illustrated construction, the capacitor is placed into a round aluminum can. In the open end of the can a moulded bakelite or hard rubber stud is fastened into place by crimping or turning over the can edge; the stud resting in turn on a shoulder or bead indented into the can wall. This stud is threaded in order that the entire assembly may be mounted vertically on a metal plate or radio chassis. The anode tab has a hole in it and through this hole passes an aluminum rivet, the other end of which is riveted or swaged over to hold in turn a solder lug for external electrical connection. The cathode foil tab is usually connected to the can by crimping it under the rim of the can.
This same construction is also employed for multiple section capacities with either solder lug terminals or flexible wire leads.
Other container types are shown in the following photographic illustrations:
When the container is made of aluminum it is not necessary to insulate the capacitor proper from the container, provided that circuit requirements are such that the container can be the negative or common negative terminal, in the case of a multiple section. If the container is not made of aluminum, however, it is necessary to insulate the capacitor from the container and make provisions for preventing the electrolyte from coming in contact with it. Otherwise problems of corrosion will arise. Where circuit requirements are such that the capacitor must be insulated from the container or where the container is of some metal other than aluminum, it is customary to seal or pot the capacitor into a liner box, or inner container, of insulating material such as waxed or varnished paper or cardboard.
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