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In the author's opinion, there is only one true explanation of the operation of an electrolytic capacitor and basic theory of operation will be confined to that opinion, not because of any arbitrary attitude or because it is the author's own theory but rather on the other hand, because all points can and have been experimentally verified, and also because all conditions can be met and checked.
For a long time, it has been known. that several metals, such as tantalum, aluminum, magnesium, titanium, niobium, zirconium and zinc, can be coated with an oxide film by electrochemical means.
While the oxides of these metals exhibit different characteristics it was found that the oxides of tantalum and aluminum possessed highly desirable ones. While tantalum was found to possess an oxide of the most desirable characteristics its cost, so far, has limited its economical usefulness in electrolytic capacitors.
Aluminum, on the other hand, being both plentiful and sufficiently economical, has become the most widely used metal in electrolytic capacitor structures. This is the case to such an extent that all subsequent work will refer only to the use of aluminum as the anodic member of electrolytic capacitors.
An oxide film can be formed on aluminum by electrolytic means. This can be accomplished by introducing the metal into a suitable electrolyte, for example, an aqueous solution of boric add and sodium borate, and passing an electric current through it, the aluminum forming the positive pole or anode. Upon electrolysis of the solution, oxygen is evolved at the positive pole which oxidizes the surface of the aluminum.
The thin film of oxide (Al203) formed on an aluminum surface offers a very high resistance to further passage of current and if the applied voltage is kept constant, the current, after a time, will be reduced to a minimum value called the leakage current. A cell of this type, with aluminum as the anode and an electrolyte as a negative electrode or cathode, is used as a capacitor, with the aluminum oxide film separating them, acting as an extremely thin dielectric.
The electrolytic capacitor has a high capacity per unit volume as compared to other types of capacitors.
The thickness of the oxide film covering the aluminum electrode is extremely thin (approximately l0-5 centimeters), and the dielectric constant K of the Al203 produced is high (approximately 10). If the capacity C is calculated per square centimeter, from the Previously mentioned basic formula
the following result is obtained:
From this it can be seen that an aluminum electrode, of 100 square centimeters surface, will produce a capacity of approximately 8.85 microfarads.
The electrolytic capacitor can only be used with a flow of current in one direction. The aluminum electrode must therefore always be connected to the positive side of the applied voltage, and the electrolyte must always be negative. With the current flowing through the capacitor in this direction, the current intensity is small. If the direction of current flow is reversed, a large current will flow through the capacitor and the capacitor as such becomes useless.
From this, it can be readily seen that the system exhibits the characteristics of a rectifier, and an electrolytic capacitor does not then differ in any way from the well known electrolytic rectifier.
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It has already been mentioned that the oxide film is very thin, being of the order of l0-5 centimeters, so that if a potential difference of 100 volts is applied between the aluminum and the electrolyte, the field strength in the dielectric will be approximately 107 volts per centimeter. With such high field intensities, cold electron emission always takes place. That is, the negative electrode emits electrons.
If it is considered that two plane metal surfaces or electrodes between which the field intensity F is so high that the negative electrode emits electrons, it is then found that the electron current I can be represented by an equation of the following form:
Where A and B are constants of the materials.
If it is assumed that plate a emits electrons more readily than plate b, then this means that when an alternating current is applied the current passes through with greater facility on one half wave than on the other half, the greater current flowing when the plate which is more susceptible to electronic emission constitutes the negative electrode.
To rectify a current, a thin layer of insulation is therefore necessary and must be bounded by two substances capable of emitting electrons to widely different degrees. If the substance which emits electrons the more easily is made the negative electrode, a higher current will flow than when it is positive.
Metals emit electrons easily and semi-conductors and electrolytes emit them with difficulty. The electrons in the electrolyte are in fact not free but are bound in ions, although the powerful electric field obtained can detach some of the electrons from the ions and transfer them to the insulating layer.
It is thus seen why the electrolytic capacitor must always be connected in such a manner that the electrolyte is the negative electrode, for then only will a small current flow through the capacitor.
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It, now, should be quite evident why it would be impossible, or virtually so, to use a second metallic electrode in place of the electrolyte. In such a case the separating layer (aluminum oxide film) would be bounded by two substances which would emit electrons with almost the same facility.
A leakage current is generated because the electroIyte is also able to emit some electrons from the ions, under the influence of the powerful electric fields applied, such electrons migrating through the oxide film to the aluminum electrode. This leakage current is determined by the field strength, the thickness of the oxide film and the conductivity of the electrolyte. If in capacitors made of the same materials the leakage currents are the same at equal potential differences, it may be concluded that the oxide films are of the same thickness. The field intensity is then equal to:
If an aluminum electrode is oxidized in an electrolyte and a specific potential difference V1 is applied, the current through the electrolyte will steadily decrease with time. At a small terminal value I, of this current, the oxidation process is considered as having been completed. Now, if a second aluminum electrode of the same dimensions is placed into the same electrolyte and a potential difference V2, which is double the value of V1, is applied until the leakage current has reached the same final value I, it may be then assumed that in the two capacitors the same field strength prevails at the oxide film. Since, however, V2 = 2V1, t2 must be 2t, and hence also the capacity of the second capacitor half as great as that of the first.
From this it can be seen that the capacity of an electrolytic capacitor is a direct function of the area of the anode member and that the thickness of the dielectric film is always automatically matched to the potentiat difference or voltage.
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Electrolytes of low conductivity or high specific resistance exhibit low leakage current due to the lack of or rather the reduced number of ions but such electrolytes have the drawback that they increase the resistance effectively in series with the capacitor. This is illustrated by the following figure:
A compromise is therefore necessary and consists in fixing the permissible equivalent series resistance R and tolerating a corresponding value of leakage current. Fortunately, low values of leakage current can be obtained with comparatively low values of R.
Another compromise must, however, be made in relation to the voltage breakdown of electrolytic capacitors.
Sparking or breakdown occurs in an electrolytic capacitor when a certain critical voltage or potent difference is applied to it. This breakdown is analogous to the breakdown of the dielectric in other types of electrical capacitors.
(Courtesy Cornell-Dubilier Electric Corp.)
It has been observed that this sparking voltage V is determined by the specific resistivity r, of the electrolyte, and for a specific thickness of oxide film may be expressed as follows:
Where a and b are constants.
The increase of V with r may be explained as follows: the greater the concentration of ions in the electrolyte, the greater will be the number of electrons emitted from the electrolyte and hence the greater the number of electrons migrating to the dielectric, and the more readily will breakdown occur.
In theory at least, electrolytic capacitors can be designed to have a very high breakdown voltage by simply making the specific resistivity r, of the electrolyte, sufficiently great. This again results in a large increase in the equivalent series resistance R; so again a compromise must be made.
Thus in the practical design of electrolytic capacitors there is a constant struggle to keep the value of R down, the leakage current at minimum values and the breakdown voltage at high values. It is doubted that the users of electrolytic capacitors ever appreciate the fact that such compromises have to be constantly made.
(Courtesy Cornell-Dubilier Electric Corp.)
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