|
|
| FaradNet The Capacitor Resource |
| Home | Community | Companies | Catalogs | Knowledge | Search | Store |
Book ToC | Book Index | Page Index
As the field strength is a function of applied voltage and anode film thickness, the leakage current can be considered as being determined by the applied voltage, anode film thickness and the conductivity of the electrolyte. Variations in the temperature of the electrolyte have been shown to cause variations in its conductivity. Variations in ambient temperature therefore cause changes in the leakage current of a dry electrolytic capacitor. The effective amount of change in leakage current with a definite variation in ambient temperature depends also on the conductivity of the electrolyte. Stated more clearly this means that a high resistance electrolyte shows a different rate of change of leakage current with temperature variations than does a lower resistance electrolyte. To illustrate this characteristic, reference is made to the following graphic figure.
From a study of this illustration it appears that the change in leakage current is not as great, over a comparatively wide range of temperature variation, as might be expected. The reason for this is that the use of the lower resistance or more conductive electrolytes is confined to the lower voltage capacitor structures while the capacitors intended for operation at the higher yoltages are limited to the use of electrolytes of high resistivities or low conductivities. Thus, the effect of high conductivity of electrolyte is offset in one case by low field strength and in the other case a high field strength is offset by a low conductivity of electrolyte. This fortunate balance of one set of factors against another is not encountered in the effect of temperature change on other characteristics.
Book ToC | Book Index | Page Index
It is known, on the other hand, that the more conductive the electrolyte employed in a dry electrolytic capacitor, the less the change in effective capacity over a given range of temperature variation. As the more conductive electrolytes are limited in application to the lower voltage capacitors it becomes apparent that the lower voltage capacitors have better temperature characteristics than the higher voltage capacitors. It is unfortunate that this is true because such a characteristic may frequently limit the usefulness of the higher voltage capacitor structures. Of equal importance and interest are the effects of time and temperature, on the above mentioned characteristics, under various conditions. Detailed data will be found in the following paragraphs.
The effects of temperature change on effective capacity are shown in the following illustration. In this illustration the per cent change in normal capacity values at room temperature (21°C) is plotted against various temperatures. Three types of capacitors are shown: namely, 50 working volt, 300 working volt and 450 working volt. In each capacitor structure an electrolyte of corresponding conductivity has been employed. That is, the conductivity of each electrolyte employed is as high as can normally be used for each representative voltage value:
Book ToC | Book Index | Page Index
Book ToC | Book Index | Page Index
The main contributing factor to total equivalent series resistance values of dry electrolytic capacitors is the specific resistivity of the electrolyte employed. Thus it is seen that values of equivalent series resistance are subject to change with variations in temperature.
Illustrative of such characteristics are the following graphic comparisons:
As the more conductive electrolytes are limited, in actual application, to the lower voltage capacitor structures it is evident that the lower voltage type capacitor has the better temperature characteristic as compared to the higher voltage structures.
A summary of the collective effects of temperature variation on the characteristics of dry electrolytic capacitors can be outlined as follows:
An increase in temperature causes a minor increase in effective capacity but a large decrease in equivalent series resistance.
A decrease in temperature causes a relatively large decrease in effective capacity but causes a greater increase in equivalent series resistance.
An increase in temperature causes an increase in direct current leakage but a material decrease in temperature causes an extreme decrease in direct current leakage.
Extreme increases in temperature may result in a loss of moisture from the electrolyte (by evaporation) with a resultant permanent decrease in capacity and increase in equivalent series resistance. A shortened effective life would be the final result.
Extreme decreases in temperature may result in a reduction in effective capacity to a value of almost zero and an almost infinite increase in equivalent series resistance. This, however, causes no permanent injury to the capacitor structure and normal characteristics will be again obtained when the temperature is again brought back to normal values.
The temperature characteristics of dry electrolytic capacitors vary with the temperature characteristics of the electrolytes employed and the temperature characteristics of electrolytes, in turn, vary with the conductivity of the electrolyte.
An increase in temperature also causes an increase in the possibility of corrosion taking place in the capacitor structure if, of course, there are any corrosive substances present.
An increase in temperature may also cause the electrolyte to become sufficiently liquid to leak or run out of the capacitor winding. This may, at times, be highly objectionable.
Book ToC | Book Index | Page Index
Maximum permissible leakage current values are to some extent governed by circuit application limitations.
Average values of direct current leakage for various voltage structures are shown in the illustration below.
Book ToC | Book Index | Page Index
Advantage of this sharp increase in leakage current, with increase in applied voltage, is taken frequently to produce the so-called regulating type of capacitor.
The regulating characteristics of dry electrolytic capacitors are very similar to those of wet electrolytic capacitors with the important exception that dry electrolytic capacitor structures cannot have potentials applied to them which approach too closely the scintillating voltage, otherwise a permanent breakdown may occur. This obviously limits the construction of regulating types of dry electrolytic capacitors to a more narrow range of voltage application than the wet electrolytic structures cover.
Typical regulation characteristics are shown in the following illustration:
Book ToC | Book Index | Page Index
It is somewhat difficult to exactly duplicate the conditions existing in an electrolytic capacitor by setting up an electrically equivalent network consisting of a pure lumped capacity in series with a lumped resistance. A close approximation of actual conditions is, however, obtained and for practical purposes such an equivalent circuit is therefore used.
The equivalent series resistance of a dry electrolytic capacitor thus becomes that value of resistance which, when connected in series with a pure capacity will produce the same impedance at the same phase angle.
The equivalent series resistance of a dry electrolytic capacitor may be also expressed in terms of power factor of the capacitor.
Unless the capacitor is to be used in a tuned or resonant circuit network the equivalent series resistance or power factor is not a relatively important factor. This fact will be substantiated in the chapters devoted to the applications of electrolytic capacitors.
The factors which determine equivalent series resistance values in dry electrolytic capacitor structures are resistivity of electrolyte and resistivity of contact between the electrolyte and the foil surfaces.
As the lower resistive electrolytes are limited in application to certain voltage structures it appears that the lower voltage capacitors will inherently possess lower values of equivalent series resistance. The difference is not, however, as great as might be first anticipated due to the fact that for a given capacity a lower voltage structure employs a reduced total foil surface area which in turn increases the total equivalent series resistance. Lower voltage structures employing more conductive electrolytes do as a general rule possess lower values of equivalent series resistance. This fact becomes of relatively greater importance in connection with multiple capacitor windings containing capacitor sections of widely different voltage ratings. In such cases, the electrolyte employed must be one suited to the capacitor section rated at the highest voltage. This results in a material increase in the equivalent series resistance values of the lower voltage sections.
In actual practice it should be therefore apparent that actual values of equivalent series resistance must, as a general rule, be the result of considerable compromise between voltage breakdown, leakage current and stability of electrical and chemical characteristics.
Average values of equivalent series resistance for various voltage structures are shown in the following illustration:
In this illustration the equivalent series resistance is shown as an inverse relationship. That is, equivalent series resistance decreases in direct proportion to increases in capacity. Actual equivalent series resistance values for a given capacity are determined as follows:
Equivalent Series Resistance = Series Res. per mfd. / Capacity in mfds.
Book ToC | Book Index | Page Index
Book ToC | Book Index | Page Index
It is interesting to note however, that the physical relationship of anode and cathode foils and their connecting tabs, influence the radio frequency characteristics of dry electrolytic capacitors more than factors such as electrolyte conductivity, as might be normally expected. To illustrate this observed characteristic, reference is made to the following figures, which show the effect of relative positions of foils and their connecting tabs:
With a given capacity, of a given voltage structure, the construction shown in figure A will show a radio frequency impedance of as low a value as one-twentieth of that obtained with the construction shown in figure B.
In multiple wound capacitors this relationship is still more pronounced but can be offset by bringing out a tab from each end of the cathode foil. See figures C and D.
In figure C the first anode (or section) will have an impedance from ten to twenty times greater than that of the second anode. This can be offset by using the construction in figure D.
It has been observed that the ratio of difference in radio frequency impedance between anodes 1 and 2, of figure C, is determined to a large extent by the physical length of anode foil number 2. It has been further observed that the ratio of difference is greater, the higher the frequency.
From the above observed phenomena it is reasonably safe to assume that only a small percentage of the anode and cathode foil surfaces, immediately adjacent to the tab connections, is actually effective at radio frequencies. The reason for this would seem to be that the inductive reactance of the foils themselves, at radio frequencies, tends to cancel out a large portion of the capacitative reactance.
It seems doubtful too, that the electrolyte is a conductive medium at radio frequencies as conduction through the electrolyte is primarily by ionization and it is very difficult to picture electrolyte ions moving at the speeds corresponding to frequencies of for example, 500 kilohertz to 10 or 20 megahertz.
It would therefore seem more logical to consider that electrolytes are insulating mediums at radio frequencies. Under such an assumption, the capacity of a dry electrolytic capacitor, at radio frequencies, would be purely electrostatic, with the electrolyte and anodic film constituting the effective dielectric medium.
Whatever the actual effect, it has been further observed that the employment of wider foils decreases the impedance of a given capacitor structure and still more interesting is the fact that a given capacitor structure exhibits very little change in impedance over a wide band of frequencies, generally changing less than ten per cent from 500 kilohertz to 20 megahertz.
Book ToC | Book Index | Page Index
The condition of increased leakage current values is self remedying when rated working voltages are applied to the capacitor until leakage current values are reduced to normal levels. The time required to accomplish this leakage current reduction to normal values is called the leakage recovery time. Leakage recovery time is determined to a considerable degree, by such factors as voltage rating, electrolyte conductivity and purity of anode foil.
Typical leakage recovery characteristics of dry electrolytic capacitors are shown in the following graphic illustration:
In ascertaining leakage recovery characteristics as shown in the illustration above, it is customary to use a circuit diagram such as the following:
Where
E = Rated working voltage
MA = Milliammeter
VM = Voltmeter
C = Capacitor
R = Resistor
Also where
R = 20,000 / C
R = Resistance in ohms
C = Capacity in microfarads
Book ToC | Book Index | Page Index
The loss of moisture is from two causes; one being that small amount used for anodic film maintenance and the other, that lost by evaporation. Moisture lost by evaporation depends, in turn, on container construction and operating temperature conditions. Hermetically sealed, metallic containers would naturally offer a longer life expectancy than less effectively sealed containers of cardboard. An increase in operating ambient temperature would reduce the life expectancy as the rate of moisture loss would vary with change in temperature. For obvious reasons therefore, hermetically sealed containers are preferred where operating ambient temperatures are relatively high. Temperatures in excess of 4O° Centigrade are not generally satisfactory as operating ambients.
To illustrate the active life characteristics of dry electrolytic capacitors, the following graphic figure is referred to.
From this illustration it can be seen that a material increase in equivalent series resistance and reduction in effective capacity results from the operation of a dry electrolytic capacitor in an ambient temperature of 4O° Centigrade, especially where the capacitor is housed in a relatively ineffectively sealed cardboard container. In comparison to these results, the following illustration shows the effects of operation at a room ambient or temperature of 20° Centigrade. In both cases, a 5 per cent alternating current potential component is applied in addition to rated value of direct current potential.
Book ToC | Book Index | Page Index
When potentials, in excess of the formation voltage of the anodic film, are applied to a dry electrolytic capacitor, there is not a uniform increase in leakage current concentration over the surface of the anode foil. In fact the leakage current tends to concentrate at certain minute areas. Such concentrations of leakage current may be caused by one or more of a number of factors. For example: such as conducting particles, in the separator medium or electrolyte, which may lie in close proximity to the anodic film and possess high values of emissivity, or the presence of minute foreign partides on the anode surface which prevent a continuity of dielectric film structure, or actual minute breaks in the film, or highly conductive spots in the electrolyte due to small areas where excess water is present.
Whatever the cause, such leakage current concentrations result in the generation of hydrogen and oxygen, by electrolysis, at the cathode and anode foils respectively, and if the separator medium is not impermeable to the passage of these gases, small pockets of the highly explosive mixture of hydrogen and oxygen will be formed.
The increased current concentration is of such magnitude that a material rise in temperature occurs at the point of current concentration which in turn lowers the work function for emission from the ions of the electrolyte still further until the dielectric actually ruptures and a spark occurs. This spark ignites the small pocket of the explosive mixture of gases and a miniature explosion occurs. As a general rule, this explosion is of sufficient force to disrupt either the anode or cathode foils and form small crater-like holes, the projections of which are forced through the separator material causing actual electrical and mechanical contact between anode and cathode foils.
Obviously, dry electrolytic capacitors may be constructed to withstand extremely high values of over voltage if the separator medium employed is completely impermeable to the passage of the hydrogen and oxygen gases so that no explosive mixtures can accumulate. This factor is chiefly responsible for the frequent employment of regenerated cellulose, such as cellophane, for one of the separator materials. For the same reason, highly dense cellulose or paper is also frequently used.
This phenomenon of high voltage breakdown is, of course, confined to those voltage ranges wherein field strengths are sufficiently great to cause sparking or scintillation. The use of more or less gas tight separator materials in no way minimizes the importance of preventing high leakage current concentration areas from occurring by the employment of proper design factors.
Book ToC | Book Index | Page Index
In aging operations, full rated voltage is applied until leakage current values are decreased to desired orders of magnitude. As a rule too, voltage is applied through a resistance which is of such a value that the current is limited to a point where excess heating of the winding might occur. Thus, in actuality, the terminal voltage across the capacitor is automatically adjusted to a certain limited current until desired values of both voltage and leakage current are reached.
For such aging operations, the circuit network on page 183 is usually employed.
The lamp L serves to indicate short circuited capacitors or capacitors which draw an excessive amount of current.
The single circuit jack J affords a means of inserting a milliammeter into the circuit for the purpose of measuring values of leakage current.
The length of time required for the aging operation is determined by such factors as completeness of original anodic film formation, amount of film damage
R = 80OO OHM RESISTOR
L = 1O WATT 110 VOLT INCANDESCENT LAMP
J = SINGLE CIRCUIT JACK
C = CAPACITOR
during processing of capacitors, purity of aluminum in anode foil, type of electrolyte employed and the voltage rating of the capacitor.