|
|
| FaradNet The Capacitor Resource |
| Home | Community | Companies | Catalogs | Knowledge | Search | Store |
The aging of wet electrolytic capacitors is generally necessary because, although the anode assembly may leave the formation tank in perfect condition, it must be subjected to considerable handling in subsequent mechanical assembly operations. This handling invariably causes some breakage or cracking of the anodic film and the application of an aging or reforming potential is necessary to repair this damage. Breaks or cracks in the oxide film would, obviously, increase the leakage current values of the capacitor. In fact, an excessive amount of film breakage might increase the leakage current to such a value that it would be impossible to repair the damage due to the fact that the capacitor structure, as a whole, would be incapable of radiating the heat generated as the result of the passage of sufficient current to repair or reform the film. If the heat could not be radiated then ultimately the temperature of the electrolyte would be increased to the boiling point and subsequently lost by evaporation.
To minimize such a condition aging potentials are applied through limiting resistors in order that the cur-rent passing through the capacitor will be limited to a maximum value consistent with the amount of heat a given capacitor structure is capable of radiating, without reaching a temperature in excess of a safe equilibrium value. At the termination of the aging period, capacitors are complete except for the measurement of essential electrical characteristics.
(Courtesy Cornell-Dublilier Electric Corp.)
Book ToC | Book Index | Page Index
Detailed data on the various methods of measuring or determining these various electrical characteristics will be found in the chapter devoted to electrical measurements.
Other characteristics of equal interest are the following:
Book ToC | Book Index | Page Index
In these two illustrations, the indicated effect of temperature variation is limited to the range of temperatures which normally may be expected to be encountered under operating conditions. Changes in capacity, leakage current and equivalent series resistance are indicated in per cent of increase or decrease of nominal values at a temperature of 20 degrees Centigrade.
The following illustration indicates, graphically, the reduced effect of temperature variation on the three essential characteristics of wet electrolytic capacitors when the non-freezing or polyhydric alcohol type of electrolyte is employed. It is immediately apparent,from a study of this graph, that the useful range of the capacitor is materially extended over a wider range than that obtained with aqueous types of electrolytes. This refers especially to the lower temperature ranges.
Book ToC | Book Index | Page Index
Other factors which have a detrimental effect on idle shelf characteristics are: purity of aluminum used in the entire anode structure, amount of impurities occluded in the anode material, separator, vent and stem bushing and the amount of impurities contained in the electrolyte.
Anode assembly surfaces may be initially clean and free from contaminating substances but if any such substances are occluded in the aluminum they may later work out to contaminate the electrolyte. This in particular applies to etched foil type anode structures. For the same reasons it is imperative that hard rubber separators, stem bushings, vents and other parts of the capacitor structure, coming in contact with the electrolyte, be not only cleaned on exposed surfaces but that no contaminating materials be occluded beneath the surfaces, to later work out into the electrolyte.
If the required degree of cleanliness and the required degree of chemical purity of the electrolyte are maintained, and the anode film has been properly formed initially, the capacitor will rapidly reform to a low value of leakage current in a relatively short period of time.
It has been also noted that the relative acidity or alkalinity of the fill electrolyte has a bearing on the rate of anodic film deterioration. An increase in the deterioration rate takes place with an increase in pH value of the electrolyte. The increase in rate of deterioration does not, however, present any serious difficulties unless a pH value of 7 or above is encountered. The fact that the rate of deterioration does increase with increase in pH value of the electrolyte presents further proof that it is a hydration process which reduces anode film effectiveness.
Illustrative of the variation in pH value of the electrolyte with ammonia concentration is the following graph which is based on a boric acid content of 5 grams of boric acid to 100 c.c. of water and a temperature of 25° Centigrade.
Since all wet electrolytic capacitors are subject to some weakening or deterioration of the anodic film during idle shelf periods, it is important that the rate of such deterioration be kept as low as possible. The importance of this is readily recognized by radio engineers because the resulting high initial values of leakage current caused by a high rate of film deterioration may possibly result in damaged rectifier tubes, power transformers or even destruction of the capacitor itself.
It has been observed that the approximate rate of film deterioration or "shelf life" of a wet electrolytic capacitor can be ascertained by a comparatively short accelerated test. Such a test is made possible by the observed fact that, on idle shelf life, the rate of film deterioration increases with ambient temperature increases. Thus, an accelerated test for idling shelf life characteristics can readily be obtained by subjecting the capacitor to some temperature, below the boiling point of the electrolyte, for a definite period of time. Tests have shown, for example, that the subjection of a wet electrolytic capacitor to a temperature of 85°C for one hour produces effects equivalent to one month of idle shelf life at normal room temperature.
The effects of this test are manifold because in addition to an increase in the rate of actual film deterioration a check is obtained on the quantities of impurities occluded in the various parts of the entire capacitor structure.
The following graph serves to illustrate the "recovery" leakage current characteristics of capacitors after an idle shelf period of 6 months.
Curve A shows the leakage recovery characteristics of a capacitor with a high rate of anodic film deterioration. Curve B shows the effects of a lower rate and Curve C the typical leakage recovery characteristics of a satisfactory wet electrolytic capacitor correctly designed and fabricated. Curves A and B represent poor leakage recovery characteristics because curve A represents a capacitor incorporating the use of an electrolyte of a pH value of 7.1 and curve B represents a capacitor incorporating the use of an anode with a film improperly formed initially.
Book ToC | Book Index | Page Index
Book ToC | Book Index | Page Index
In a plain can construction the alternating current compound must be maintained at a relatively low value; otherwise, the interior can surface may be formed or filmed by the alternating current voltage. Such a formation effectively produces a capacitance in series with the main capacitance of the capacitor which results in a reduction in the capacity of the unit as a whole. This change does not always become immediately effective, but is a function of several variables such as the reverse (alternating current component) current density at the inside container surface; temperature of electrolyte, ion concentration of electrolyte, and time of application of the reverse potential. For a given capacitor the alternating current component voltage will determine the reverse current density and hence the change in total capacity.
By experience, it has been found that the total capacity reduction, from nominal values, can be limited to a reduction of approximately 10 per cent if the following values of alternating current component are considered as the maximum allowable:
| Can Diamter (inches) | Can Length (inches) | Alternating Current (mA) |
|---|---|---|
| 1 | 4.5 | 35 |
| 1.375 | 4.5 | 65 |
| 1.5 | 4.5 | 75 |
If cans of a length shorter than the nominal length of 41/2" are used, then reductions in the indicated maximum alternating current component values must be made in proportion to the reduction in area of the inside container surface.
The desired object in coating or plating the inside surface of the can or container with chromium is to prevent the formation of an anodic or-dielectric film on that surface. This object is obtained to a considerable degree due to the fact that chromium is not film formmg as it does not readily oxidize. Chromium is also used because it can be satisfactorily plated on aluminum and remains reasonably inactive in relation to the normally employed electrolytes.
If a can is therefore chromium plated on the inside surface, the alternating current component is limited only, as a general rule, by the amount of heat a given capacitor will satisfactorily dissipate, without causing an increase in the leakage current to an extent that a self destructive heat cycle results.
Based on a maximum operating equilibrium temperature of 50°C and with normal rated working voltages applied, the following maximum values of alternating current component can be satisfactorily used:
| Can Diamter (inches) | Can Length (inches) | Alternating Current (mA) |
|---|---|---|
| 1 | 4.5 | 125 |
| 1.375 | 4.5 | 200 |
| 1.5 | 4.5 | 225 |
In the use of chromium plated containers one undesirable feature has been noted and that is, direct current potentials cannot be applied to the capacitor in the reverse direction. It is found that if the can is made positive and sufficient current flows through tho structure, the chromium will be removed from the can surface and will go into chemical solution with the electrolyte forming a chromate salt. Out of the resulting solution, chromium may be plated on the anode surface thus rendering the capacitor useless for further operation. It has been found that this entire cycle of destruction requires only 15 seconds if the direct current, in the reverse direction, is of the order of 100 milliamperes.
Book ToC | Book Index | Page Index
It is, however, general practice to rate regulating capacitors at an operating voltage at or slightly above the potential used in initially forming the anodic film. It is also common practice to specify the degree of regulation by specifying a maximum leakage current at the rated operating voltage and a minimum value of leakage current at a higher value of voltage termed the regulating voltage. Usually the regulating voltage specified is 75 volts more than the rated operating voltage. The magnitude of increase in leakage current varies somewhat with the capacity and general type of capacitor.
The following illustration shows a series of curves wherein voltage variation and corresponding variations in leakage current values are noted for various film formation potentials.
Book ToC | Book Index | Page Index