September 1930 Radio-Craft[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles from Radio-Craft.
Electrolytic capacitors have long been the components that provide the highest capacitance density factor, that is, they have the highest capacitance value for a given volume of space occupied. Anyone familiar with electrolytic capacitors is aware of the polarization indicated on the package (a marking or unique physical feature), indicating that there is required direction for hookup; in fact, a backwards connection can lead to an explosive failure. Yes, I know there are now unpolarized electrolytic capacitors available. While physical construction of electrolytic capacitors have evolved over the decades since this article was published, the fundamental operation has not. It is interesting to note the reference to capacitors as 'condensers,' a name still commonly used with internal combustion engine ignition systems and with some AC motors that use them at turn-on for providing a starting coil phase shift. Also used here is the term 'valve' when referencing the diode action of the aluminum or tantalum anode element.
The how and why of a radio component which has made great changes in filter design
By Sylvan Harris
The value of electrolytic condensers has been increasingly appreciated during the past few years; they have made it possible to obtain at a low cost capacities previously unheard-of, for filter systems, and thereby to handle increasingly large operating current with smoother output than ever before. A fundamental explanation of the subject will therefore be of interest to every Service Man. Before undertaking this, it should be said that the theory of the operation of electrolytic cells as condensers is so closely related to that of their operation as rectifiers, that a simultaneous discussion of both effects will materially aid in understanding either.
Elements of a "Cell"
The electrolytic cell consists of a so-called rectifying or "valve metal," which is immersed in an electrolyte together with an inactive electrode. The "inactive electrode" may be an additional bar or strip of metal, or it may be simply the container which holds the electrolyte. Its electrical function is merely to connect an outside circuit conveniently to the liquid active electrode, the electrolyte. Many metals under certain conditions, and with certain electrolytes, show "valve action"; which is to say they allow a current to flow only one way. The only elements which exhibit this rectifying action to a degree which is satisfactory for commercial purposes are aluminum and tantalum. Of these aluminum is the more widely used; for the reason that tantalum cells require an acid electrolyte, while aluminum cells operate satisfactorily with the less corrosive borates or phosphates.
The form of cell most widely used comprises an electrode of chemically pure aluminum, immersed in a solution of ammonium borate or phosphate, and an additional inactive electrode, which may be carbon, lead, copper, or any other metal which does not exhibit the valve action. Fig. 1 shows the elementary construction of an electrolytic cell; at A the two electrodes are immersed in the electrolyte, which is held in a neutral container, while at B a copper can serves both as the inactive electrode and as a container for the electrolyte. Three active electrodes in a common electrolyte are shown at C; and the Mershon "individual-electrolyte" design is illustrated at D.
Many theories, most of which are unsatisfactory and incomplete, have been proposed to explain the operation of these cells; the most plausible however, will be explained here. We will consider a cell with aluminum and copper electrodes immersed in a borax solution. Also, we will first suppose that the cell is connected to a battery; the aluminum electrode to the positive and the copper electrode to the negative terminal, as shown in Fig. 1A.
Why the Plate "Forms"
It is well known that, on the surface of aluminum, there is always a coating of oxide due to its exposure to the air. Although this aluminum oxide is a poor conductor of electricity, the coating is so very thin that it does not appreciably limit the flow of current through the cell. Consequently, when a voltage is first applied to the cell, there will be a large flow of current which may result in damage, unless regulated.
Again, because of the porosity of the oxide coating, some of the electrolyte may leak through the pores and attack the aluminum, causing the formation of more oxide. The consequent flow of current "ionizes" the electrolyte, and negatively-charged oxygen molecules (or "ions") are liberated at the negative copper electrode. These are attracted to the positive aluminum electrode, where the electrons are neutralized; and oxygen gas is liberated, only to be entrapped in the aluminum oxide on the surface of the electrode. Then, because of the high resistivity of the gas, the current gradually decreases, as more and more gas is entrapped by the oxide; until finally the flow of current ceases entirely.
The process is called "forming" the cell. An insulating medium (that is, the oxygen gas) is "formed" at the surface of the aluminum and prevents a flow of current from the aluminum into the electrolyte. It must be noted that all this requires that the aluminum electrode be charged positively in order that the negatively-charged oxygen ions shall be attracted to the aluminum electrode. When, however, by the reversal of current the aluminum electrode is negatively charged, the oxygen ions are attracted to the copper. Since the latter has no porous coating with which to entrap the gas, the oxygen escapes out into the air; and current is again allowed to flow through the cell.
This, in general, is the manner in which the electrolytic rectifier or condenser acts.
Actions Inside the Cell
Its great capacity is due primarily to the extreme thinness of the dielectric or gas (oxygen) layer - just as in a paper condenser; the thinner the paper, the greater the capacity. Increasing the size of the plates also increases the capacity.
Now let us see how the cell works in an electrical circuit; of course, we know that its fundamental connections are about as shown in the standard filter circuit (Fig. 2A), which is used in many Crosley sets. A four-section unit is shown in Fig. 2B, as used in the Amrad Model 81 "Bel Canto."
However, let us first connect the cell into an A.C. circuit, such as that indicated in Figure 3A. Suppose that the copper electrode (or container) is charged positively during the first half of the cycle, and that this positive charge increases from zero to maximum; that is, from a to b, in the voltage wave shown at B. In accordance with the explanation given before, current will flow through the cell; and this current will increase with the voltage, as indicated at c on the heavier line. As the applied voltage decreases from its maximum value, the current in the circuit likewise decreases; until when the voltage is zero, the current is also zero, as shown at point d.
Then the polarity of the impressed voltage changes, and the aluminum becomes positively charged. At first there is no gas film on the surface of this electrode; so that,. even though the applied voltage is small, the current transmitted through the cell may be appreciable - being limited mainly by the load resistance and the low impressed voltage - so that we may have a small current peak as indicated at e. (Fig 3B.) However, the gas film forms again very quickly, cutting down the current; so that it tapers off to zero, as indicated by the line e-f-g. This current, indicated below the horizontal line a-d-h, is the "back-current," and detracts from the efficiency of the cell as a rectifier.
At h the cycle begins again. During the negative half of the first cycle, after the film has been formed, (between f and g) some current may be transmitted through the cell, because of its ability to act as a condenser. The magnitude of this effect is of course, dependent upon the capacity of the cell and upon the frequency as well as the value of the applied voltage; and it is likely to give peculiar shapes to the back-current wave, as for example, the curve f-m-h in Fig. 3C. .
If the applied voltage is quite high, there is also danger of breaking down the film during the negative half of the cycle. Such an effect is illustrated by the wave f-g-k-h in Fig. 3C. After the film is formed, as at e, the applied voltage increases until, at v1 the film breaks down. The back-current then increases from f to g; and from g to k it follows the curve we would have had if no film had been formed at all. Then, when the applied voltage drops to a sufficiently low value, at at v1 the film forms again, and the back-current drops quickly from k on.
Forming should, therefore, be begun with a small voltage, which is gradually increased up to the working voltage, or perhaps a little higher. A current indicator should be kept in the circuit, so that the voltage may be adjusted, to ensure that too much current shall not pass through the cells during forming; otherwise the electrolyte will heat, and the film may be destroyed. In aluminum cells this critical temperature is in the neighborhood of 120 degrees Fahrenheit. Sparking at the surface of the aluminum indicates too great a forming rate.
The Cell as a Condenser
Now let us connect the cell into a circuit which is supplied with a current which always flows in the same direction, as in the battery circuit of Fig. 1A. We have seen that, if the aluminum electrode is always held positive, no current will flow through the cell; because the insulating film will always be maintained. Even if we slightly increase or decrease the battery voltage, slowly, no current will be transmitted by conduction; because we always keep the aluminum positive.
But if we increase and decrease the applied voltage very quickly, we have then a condition such as we obtain in rectifier circuits; an alternating voltage superposed on a constant voltage. The equivalent circuit is Fig. 3D, where we have a source of alternating voltage in series with a source of constant voltage. The alternating voltage is small compared with the constant voltage; so that the aluminum electrode is always sufficiently positive to retain the gas film at its surface.
Under these conditions there will be no conduction current through the cell. Actually, there will be a small leakage current; but this is usually so small (about 0.2-milliampere per microfarad) that it does not detract from the usefulness of the cell as a condenser. However, the large capacity of the cell makes it act as a large condenser, and it therefore offers little opposition to the flow of the alternating component through it. It is in this manner that the electrolytic cell acts as a condenser of large capacity in rectifier-filter circuits. Since the cell will not pass direct current when connected in this way, the D.C. component is not short-circuited; on the contrary, it may be used on any load, such as a radio receiver.
A well-formed condenser will remain formed indefinitely, even with only occasional use. A properly-formed aluminum electrode will not be shiny, like new aluminum sheet, but will have a dull whitish surface, which can be easily scraped off with a knife, showing bright aluminum beneath.
Aluminum condensers, employing ammonium phosphate as electrolyte, "break down" at about 360 volts; when ammonium borate is used, the breakdown voltage is about 500. Tantalum cells, using dilute sulphuric acid, break down at about 460 volts; using hydrochloric acid, they break down at about 210 volts. Commercial aluminum condensers, employing borate solutions, are rated at 400 volts maximum, or thereabouts.
Popular applications of the electrolytic condenser are found in D.C. and battery-operated receivers, where the voltages are always much lower than the breakdown voltage of the condenser.
It is possible, and perfectly practicable, to use electrolytic condensers at high-voltage points in the circuit by connecting several cells, in series, across the voltage to be filtered. For example, if it is desired to filter the output of a rectifier which delivers a peak of 700 volts, the arrangement indicated in Fig. 2E may be employed; this comprises two electrolytic condensers in series, connected across the output of a full-wave rectifier tube of the '80 type. With a maximum of 700 volts applied, the drop across the terminals of each electrolytic cell is only 350 volts, which is well below the breakdown figure. Actually, the two in series could stand a potential of 800 volts; since they are rated at 400 volts each. The capacity in combination of the two is of course, half the capacity of either alone - assuming them to have the same value. With a rated capacity of 8 microfarads each, the two in series would be equivalent to one of four micro-farads.
There has recently been evolved the so-called "dry" electrolytic condenser. This is dry in the same sense that dry batteries are dry; that is to say, dry in the sense that the electrolyte cannot be poured out of the container. A strip of cloth, or similar material, is rolled up with the aluminum, the cloth being saturated with the solution and holding it much as a sponge holds water.
Ventilating the Cell
The objection to the liquid electrolyte has been overcome to a large extent by proper design of the containers, by the employment of rubber gaskets and by adding a gas vent. This vent is usually a small "nipple," which is inserted into the container at one end; it is made of rubber, and contains a minute hole. When the gas pressure within the container becomes great enough, by reason of either increased temperature or excessive evolution of gas, the nipple swells like a miniature balloon; the vent hole then opens and permits the gas to escape. When the pressure is relieved, the nipple contracts, thus closing the hole, and preventing evaporation or leakage of the liquid.
This is the sole purpose of the "nipple." Although some Service Men are of the opinion that it is necessary to remove it, in order to ensure the proper operation of the condenser, it is evident that the presence of the nipple has absolutely nothing to do with the operation of the cell except to prevent leakage and evaporation of the electrolyte. In fact there are known instances where the electrolyte has spilled through this hole, unnecessarily opened by a Service Man, and caused damage to the radio set, when the owner had need to move the set.
(However, this rubber may harden, as some Service Men have found, and require replacements; or salts may so solidly fill the tiny hole as to necessitate re-opening it with a .needle. Another condition, which may be encountered in isolated instances, is lack of any opening, because the perforating machine failed to pierce the rubber.- Editor.)
Commercial Condenser Design
The capacity of an electrolytic condenser or of any condenser, for that matter, is proportional to the area of the active electrodes - in this type the aluminum electrode and the liquid electrolyte. Consequently, a large surface of aluminum is required in order to obtain a large capacity; so various ingenious arrangements are found in commercial condensers by means of which these large surfaces are provided. In most makes an aluminum sheet is coiled spirally about a central post, or "riser," to which the sheet is welded. The arrangement is shown clearly in Fig. 4A. A cap of insulating material is mounted on the end of the riser, together with the required fastening nuts and soldering lug.
Another method of giving the anode a large surface is that of "extruding" the aluminum into the form shown in Fig. 4B. In any case, a sheet of insulating material around the anode is required; so that the metal may not come into conductive contact with the surrounding container, which is a lead to the cathode or negative element. This insulator is generally a sheet of celluloid, which is perforated to permit good circulation of the electrolyte.
General Service Notes
A service hint for reducing motorboating in some amplifiers is to change the filter wiring as shown progressively in Figs. 2D and 2C. If the output is to feed the plates of tubes that require over 300 volts, such as the type '10 and '50, it is necessary to insert condensers in series with the high-voltage leads and poled as shown, at the points marked X. (This principle is incorporated in transmitter design.) Of course, only one negative post can be grounded, and care must be taken so to place the other series condenser units that the cases cannot make contact with each other or with the ground.
In one of the Amrad receivers incorporating type '99 tubes in a series-filament circuit, there is provided a single 60-mf. condenser across the high-voltage D.C. output of the rectifier (two 216B's in parallel), and also a dual unit having capacities of 15 and 30 mf. In connection with these units it may be noted that the electrodes are widely spaced, and can seldom short. Occasionally, the center electrode or negative terminal does not seat flat against the rubber gasket, and it may jar against the positive anode. If this occurs, the '99 tubes will not light and the "B" voltage between the black and brown, red or green wires will be zero. To locate definitely a short circuit in these units it is necessary to disconnect them entirely, as in testing a filter in which paper condensers are used. A short in these electrolytic condensers may be remedied by loosening the clamping nuts on the negative post (cathode) and straightening the post.
Instead of using one container and several anodes, the makers of the "Acracon" unit recommend the use of individual single-anode condensers for each capacity required; this advice is based upon the contention that considerable cross-current leakage will exist between multiple anodes at different voltages in a common electrolyte.
The capacities commercially available may reach 72 mf., at 400 volts, as in the case of the Polymet "E." The capacity limit, however, is almost solely a matter of convenience and necessity.
A service hint concerns the Crosley D.C. sets which are provided with a lamp socket on the attachment cord. If a lamp, placed in this socket, burns brightly, reverse the plug connection to the D.C. line; the lamp should then light dimly or not at all. When the set is thus connected properly, the lamp is to be replaced by a fuse.
The Amrad "Model 1-5" "B" eliminator uses a dual 4-8-mf. and a dual 15-30-mf. Mershon condenser. The following table should be filed by the Service Man as a reference for the electrolytic condenser capacities used in Crosley receivers:
Twin 8-mf. condensers: 608 A.C. ("Gembox"),704A ("Jewelbox"), 705 D.C. ("Showbox"), 706 A.C. ("Showbox"), 609 A.C. ("Gembox," "Gemchest"), 610 A.C. ("Gembox," "Gemchest"), 41-A A.C., 42 A.C.
Triple 8-mf. condensers: 704B ("Jewelbox"), 40-S, 41-S, 42-S, 82-S, 30-S A.C., 31-S A.C., 33-S A.C., 34-S A.C. 60-S D.C., 61-S D.C., 62-S D.C., 63-S D.C.
Four 8-mf. sections: 804 A.C. ("Jewel("Jewelbox").
Triple 10-mf. condensers: 704 A.C. box").
Posted September 28, 2015