Reforming Electrolytic Capacitors


claim that most old electrolytics can be saved if the correct procedure is followed, regardless of how long they have been unused. Such capacitors must be "reformed". This process consists of applying rated voltage through a resistance (about 30,000 ohms, five watt) for five minutes plus one minute for each month of storage (see figure 6). As the capacitor reforms, the voltage across the resistor will drop (measured at the Xs in Figure 6). If that voltage will not drop below 10% of applied voltage after one hour, the capacitor is probably beyond help.

Reforming Electrolytic Capacitors

The process of reforming an old aluminum electrolytic capacitor consists of the application of rated voltage, through a resistor, for a period equal to five minutes plus one minute per month of storage.

The electrolytics appearing on the surplus market have often been in storage for a very long period indeed. Some manufacturers use a visible code, of which the first two digits indicate the year of manufacture.

The circuit shown in the sketch above works reasonably well. Apply the rated voltage through a 5W resistor. Anything from 20K-50K will do, as this is far from a precision process. The meter is used to measure the voltage drop across the resistor; when no current is flowing, there will be no drop. Obviously, when there is a large voltage drop (more than 20% of the applied voltage), there must be a significant current flow through the capacitor. The nature of a proper capacitor is to impede DC current flow, so when there is such flow, something must be wrong.

Note: Apply the appropriate D.C. voltage to the capacitor with a D.C. power supply. An old Kepco, Lamba etc. tube regulated lab power supply rig works great. Be sure to observe the proper polarity!

Within an aluminum electrolytic there is a large area of aluminum foil and an electrolytic paste. As the voltage is applied, current flows until aluminum oxide forms on the surface of the foil, because aluminum oxide is a very good insulator. If excess voltage has been applied during the electrolytics lifetime, it is possible that tiny welds exist which the oxide insulator cannot separate. When that occurs, the capacitor cannot "reform", and should be discarded.

If the amount of current flow (voltage drop across the resistor) is great initially, that is not a problem. If it doesn't start dropping within five minutes of application of voltage, a definite hazard exists. The current flow indicated that energy is being dissipated within the capacitor, in the form of heat. Excess heat may "pop" the electrolytic, causing the paste to spit out...a threat to eyes and paint.

It's also worth remembering (one forgets only once) that a good capacitor will store its energy for quite a while, and discharge it through the hand when picked up. It's smart, then, to discharge the unit deliberately, through a resistor equal to about one ohm per volt of charge.

A new capacitor should rapidly take a charge right to rated voltage, in which case only a small voltage drop will appear across the resistor. It is possible to reform capacitors in the circuit, of course, but if rectification is by solid state diodes and there is a large current flow, it is possible to destroy one or more of the diodes, or to damage the transformer.

Electrolytic capacitors can be dangerous. They can be charged to a high voltage and will retain that energy for quite a while. If the terminals of associated circuitry are touched, a severe shock and burn may result.

Another hazard associated with electrolytics is "spitting". Each of these cans is filled with, among other things, a thick fluid which can be extremely irritating. A small rubber safety plug is fitted to most electrolytics of recent manufacture. When the capacitor fails, internal pressure may go too high; the plug will blow and the fluid will spit out.

Electrolytic capacitors of a given capacity and voltage will vary considerably in configuration and size, from one manufacturer to the another. Ideally, there will be chassis space to permit mounting the "twist-lock" variety. Otherwise, the tubulars (such as the Sprague TVL 1720) must be packed, glued or clipped wherever space is available.

NOTE: The preceding information was obtained from an old copy of Tu-Be Or Not Tu-Be Modification Manual by H.I. Eisenson.

The Electrolytic Capacitor

The origin of the electrolytic capacitor or condenser can be traced to the second half of the 19th century when the discovery was made that film can be formed on aluminum electrochemically and that it will exhitbit unidirectional electrical conductance and other peculiar properties. One of the early attemped applications of the electroltyic capacitor was in conjunction with the starting of single-phase induction motors; efforts were also made to utilize it for power factor correction in alternating current circuits. It appears that no extensive use had been made of this device until the early twenties, when its utility in filter circuits supplying rectified plate current to radio tubes was definitely establisehd. These capacitors were"polarized" and of the "wet" type. Several years later "dry" electroltyic condensers of a low voltage rating and large capacitance found a limited application in A-battery eliminators (comprising a rectifier and a filter circuit) which furnished filament current for d.c. radio tubes. By 1929 the high-voltage dry electrolytic was developed and soon found very extensive and diversified applications in several fields. The annual production of dry and wet electrolytic capacitors amounts now to tens of millions and they are used in radio receivers and transmitters, sound systems and other electronic apparatus, in telephone circuits, in conjuction with electric motors and, to a smaller extent, in a number of other applications.

The capacitor may be considered as a device for storing static electricity. The essential parts of a capacitor are two electrodes, which consist of conducting members, closely spaced by a dielectric or insulating medium. The electrodes are usually metallic plates or foils, while the dielectric may be vacuum, gases (for instance, air), liquid (like mineral or vegetable oil), or solids (like mica, glass, wax-impregnated paper, and so on).

The capacitance of a condenser is a measure of the quantity of electricity that can be stored in it at a given potential (voltage). The unit of capacitance is the farad and it corresponds to a charge of one columb at one volt pressure across the terminals of the device. These relations are expressed by the formulae: Q=CE, C+Q divided by E or where C=farads, E=volts, Q=columbs.

As the farad is too great a unit for practical purposes, the microfarad (MF)-one millionth of a farad-or its subdivisions are ordinarily used. The energy in joules, stored in a capacitor, equals one half CE squared.

The capacitance of a condenser is directly proportional to the area of the electrodes spaced by the dielectric and is inversely proportional to the thickness of the latter. The nature of the dielectric is the third determining factor for the capacitance. If this dielectric medium is a mineral oil, the capacitance of the condenser may be, for instance, twice as great as it would be with air, everything else being the same. With castor oil the capacitance will be about five times as great as with air. The ratio of the capacitance of a condenser with a given dielectric medium between the electrodes to the capacitance of the same condenser with air (or, more accurately, vacuum) as a dielectric is designated the dielectric constant of the medium or its specific inductive capacitance.

Electroltyic capacitors consitute one of the several classes of capacitors. To differentiate between the electroltyic and all other classes we shall designate the latter in this text as "nonelectroltyic".

The structure of the electroltyic capacitor comprises the fundamentally important component parts present in any capacitor-the electrodes and the dielectric between them. It also performs the characteristic function of storing and releasing electrostatic charges. However, the electroltyic capacitor possesses in addition some very distinct structural and functional features which justify placing it in a class of its own.

The essential difference between the electrolytic and nonelectroltyic capacitor resides in the nature and thickness of the respective dielectrics and in the presence or absence of an ionic conduction medium (the electrolyte) between the metallic electrodes. In the nonelectroltyic capacitor the thickness of the dielectric is usually not less than the gauge of a thin sheet of paper, while in the electroltyic capacitor the dielectric is many times thinner. In the former class of capacitors the dielectric is made of materials of well-known compositions, like mica, wax- or oil-impregnated paper, glass, oil and the like, while in the latter class the true nature of the effective dielectric has not been definitely established. We know, however, that the dielectric in electrolytic capacitors is intimately associated with surface of the electrode and that its existence is correlated with the formation of an oxide film on the latter.

Another important characteristic of the electroltyic capacitor is the marked ionic conductivity of the medium positioned between the metallic electrodes, in contrast to the highly insulating material (mica, oil, and the like) between the electrodes in nonelectroltyic capacitors.

From the point of view of usage, it is typical of the electroltyic capacitor that it combines some remarkably valuable advantages for...(certain specific)...applications. The most outstanding advantage of the electroltyic condenser resides in the great, in some cases even enormous, capacitance per unit of the electrode area which it exhibits at moderate (100-600V), but particularly at low voltages (down to a few volts).

The following example may illustrate the great compactness of the low-voltage electroltyic capacitor as compared with an equivalent wax-paper capacitor. A unif of the former class designed for use in a unidirectional circuit and rated for 2,000 MF at 5 to 10 volts (depending on the ripple voltage) can be housed in a container of about 10 cubic inches, while the capacitance of the lowest voltage condenser of the latter class, occupying the same space, will be of the order of only a few MFs. Consequently, the bulk, weight and also the cost of the electroltyic capacitor are in this case less to a very great degree.

The cause of this impressive difference between the two kinds of capacitors will be discussed in the following paragraphs. As already stated, for a given type of dielectric, the less its thickness the greater the capacitance per unit electrode area. The limit for increasing the capacitance by this expedient in nonelectroltyic condensers is determined by the gauage of the thinnest available insulators which will provide the required dielectric strength. For example, it is difficult to make condenser paper thinner than 0.0003", and it is the usual practice in the manufacture of wax- or oil-impregnated paper capacitors to place at least two layers of paper between the foils, to prevent breakdowns due to unavoidable pinholes and conducting particles (metallic specks, and the like) in the paper. Thus, 0.0006" between the elctrodes appears to be the minimum spacing. But even if a chance were taken with a single layer of paper between the foils of a condenser for very low voltages, the ultimate minimum spacing would be 0.0003", which would also set the limit to the capacitance per unit area for this type of dielectric, no matter how low the voltage rating of the unit. Thus, whether the latter were intended for 5 volts or 25 volts, it could not occupy a space less than that required for the housing of a section of the required capacitance wound with a 0.0003" paper betwen the foils.

With electroltyic capacitors, however, such limitations do not exist as the minimum thickness of the dielectric is not determined by the gauge of the layer or spacer placed between the electrodes. The almost intangible dielectric of the electroltyic capacitor is formed electrochemically on the electrode surface, and the accurate control of its thickness, down to less than one millionth of an inch, can be readily effected. Thus, by the simple expedient of varying the filming voltage, which governs the thickness of the dielectric, one may produce condensers, within wide voltage and capacitance ranges, which can be housed in containers of the same dimensions. For example, a container of 10 cubic inches will be suitable for the assembly of a capacitor rated for 100 MF and formed at 100V or of a capacitor of 400 MF at 25V. Hence, the electroltyic capacitor exhibits a remarkable adapabiliy to the operating voltage.

Finally, the unilateral properties of the film (or its conductance in one direction only) make it possible to use electrolyitc capacitors for the blocking of direct current in the undesired direction. The unilateral feature makes the electroltyic capacitor also more adaptable to circuits in which unidirectional pulsating currents are flowing; in such cases, by making the condenser polarized or asymmetric in its action, its bulk and cost can be substantially halved without reducing its capacitance. Contrasting with this, nonelectroltyic condensers are always symmetric and no analogous economy is feasible when they are used on pulsating current, nor can they be employed to block or restrict the flow of d.c. in one direction only.

Most of the electroltyic capacitors are of the polarized or asymmetric type and can be used only if the voltage impressed across their terminals is unidirectional. Furthermore, they must be connected with the proper polarity, or damage to the capacitor and associated apparatus may result. The unidirectional feature, however, may be turned into an advantage in special cases as has pointed out in the forgoing. The electroltyic capacitor can even be made semipolarized to block the flow of d.c. in one direction and to restrict it to a predetermined value in the opposite direction.

Nonelectroltyic capacitor sections or units can be built for much higher operating voltages than is practicable for the electrolytic type. Furthermore, the nonelectrolytic capacitor can be operated continuously on alternating current at its rated value.

The conventional dry electroltyic capacitor comprises a winding very similar in its appearance to that of a wax- or oil- paper nonelectrolytic capacitor. The winding consists of two foils of which at least one must be of a film-forming metal; in commercial electroltyic capacitors both foils are practically without exception of aluminum. They are interleaved with paper layers impregnated in a suitable electrolyte. The latter is known as nonaqueous, as it contains not more than a few percent of water. As a rule, no unabsorbed electrolyte is present in the container and it is due to this fact that these spacer-wound capacitors are designated as "dry", regardless of the fluidity or solidity of the impregnant.

Since the foils are very closely spaced, usually between 0.0001" and 0.0006", the resistivity of the electrolyte may be comparatively high without unduly increasing the resistance of the path for the current flow.

Because of the close spacing of the electrode foils and the thin gauge of the latter (down to 0.0005"), the dry capacitor can be built very compactly, saving space and weight. Furthermore, two or even more sections of the same or of a widely different voltage ratings can be assembled in one container, without interference among the sections, as the latter can be effectively insulated from one another.

The Electrodes of the Electroltyic Capacitor

The true cathode of the electrolytic capacitor is the electrolyte (or, more accurately, its ions, which thus play the part of one of the electrodes), the film is the dielectric and the anode is the second electrode represented by the metallic member on the surface of which the film is formed. However, to establish a good contact between the electrolyte and the external circuit, a second metallic member is required; the latter is in intimate contact with the electroltye and is called in practice the cathode, although it serves primarily to distribute the current over, or pick it up from, the electrolyte. This arrangement is characteristic of polarized or asymmetric capacitors, intended for operation with unidirectional potentials. For such use a single dielectric film is sufficient and the second electrode-the cathode-does not have to be necessarily of a film-forming metal. It is essential, however, to select for its construction a material which is not attacked by the electrolyte and which will not contaminate the latter. It must, of course, meet the usual strucrual requirements and be of moderate cost. The cathode foils of dry electroltyic capacitors are made of aluminum, though of a lower grade than required for the anodes, a purity of about 99% being satisfactory. Aluminum has been preferred for cathode foil because of its comparatively low price, light weight and consequently great coverage, ease of fabrication and winding. It is not apt to corrode while in storage and is not attacked by the usual electrolytes, nor does it contaminate them.

In nonpolarized or symmetric capacitors, used in a.c. circuits, the electrolyte is again the true cathode, but in this case two aluminum electrodes, both provided with dielectric films, must be used. Each of the films, however, is fully effective only while it is subjected to a positve potential, i.e. during the respective positive half cycles. On the negative half cycles the dielectric properties of the films are alternately reduced to a small value and during that time the foils on which they are formed serve to distribute the current over the electrolyte by conduction.. The arrangement in this capacitor is equilvalent to the series-oppositon connection of two polarized units. With this combination the two units become alternately effective and substanially ineffective as the polarity is reversed. In both analogous cases, as the voltage alternates, the electric charges are being shifted from the one film to the other back and forth. It must be noted, however, that the symmetric electrolytic capacitor differs basically from the series combination of two nonelectroltyic capacitors. In the lattter case both dielectrics remain fully effective at all times and the electric charges in the two units rise and fall simultaneously.

The above material was taken largely from THE ELECTROLYTIC CAPACITOR by Georgiev, 1945.