Electrolytic Capacitors
July 1965 Electronics World

July 1965 Electronics World   Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Electronics World, published May 1959 - December 1971. All copyrights hereby acknowledged.

The July 1965 issue of Electronics World magazine contained articles on many types of capacitors being used at the time. As of this writing I have posted the articles on paper and plastic film capacitors. Still to come are mica, ceramic, and glass. Newer dielectric formulations have been developed since then, with some being improvements on existing types and others being either rarely or never used back then. Notably missing in the capacitor formats are distributed element on substrate, semiconductor, and air (vacuum). Construction and parameters for both polarized and non-polarized electrolytic capacitors are addressed by Mr. H. Nieders, of the Mallory Capacitor Company (now Duracell).

Electrolytic Capacitors

By H. Nieders

Mallory Capacitor Co., Div. of P.R. Mallory & Co., Inc.

Widely used where large capacitance and small size are required, electrolytics are among our most important capacitor types. Aluminum and tantalum units are covered.

In 1964 some 2,305,000,000 units of all types of capacitors were produced and sold. Of this total, electrolytic capacitors accounted for 249,000,000 units. In terms of dollars, this amounted to about $344,000,000 for all capacitors and $127,000,000 for electrolytic types. These figures would seem to indicate that electrolytic capacitors are expensive - but this is not true.

Electrolytic capacitors actually save money, space, and weight when used properly within their limitations and the limitations of the circuitry employed. The "why" of electrolytic capacitors is economy; the "when" is a matter of application.

Generally, electrolytic capacitors can be divided into two basic types by the nature of the base-oxidizable metal used. This is usually either aluminum or tantalum. There are other usable metals, but these two yield relatively large capacitance per unit of volume at an economical price. A tantalum capacitor costs more money but saves more space and weight than an aluminum unit. There are other technical differences, but the two types are more easily understood when reviewed separately.

Aluminum Electrolytics

Fig. 1 is a simplified representation of an aluminum electrolytic capacitor. Both polarized and non-polarized types are shown. Normally a piece of aluminum foil is etched to increase its surface area, then it is treated electro-chemically to form aluminum oxide on its surface. The oxide is the dielectric of the capacitor - the thickness of this oxide determines the voltage rating. Since this oxide film is so thin and the surface area of the electrodes is so great, the capacitance produced is very large.

Paper spacers are applied next to the aluminum oxide surface. These spacers prevent direct shorts between anode and cathode foils. Thus, for higher voltages, more and thicker spacers are used. The spacers also absorb the electrolyte, allowing it to be retained in the correct place and providing intimate contact with the surfaces of the anode and cathode foils as required for proper operation.

The cathode foil serves only as an electrical connection to the electrolyte. The electrolyte is the true cathode and it has the ability to oxidize any imperfections in the aluminum oxide dielectric.

Non-polarized capacitors use two anodes and have one-half the capacitance of an equal-size polarized unit of the same voltage rating.

When electrolytic capacitors are used as energy-storage devices where the load resistance is small compared to load inductance, current reversals may occur for a short time. Accordingly, the voltage will reverse for the same time, making the use of a non-polarized unit mandatory. Electrolytics for a.c. motor starting and for audio cross-over networks should be non-polarized types. It is also wise to consider using a non-polarized unit where large pulse signals are encountered. For most applications, however, electrolytics are used across a d.c. voltage so that the ordinary polarized units are employed. The designer must avoid voltage reversals on polarized units.

Leakage Current and ESR

All electrolytic capacitors allow a small amount of d.c. leakage current to pass through when rated polarized d.c. voltage is applied. This current will go up with rising temperature and voltage. In general, the amount of leakage current is indicative of the immediate quality of an electrolytic capacitor - the lower the better. Long shelf life or storage will cause the leakage current to go up. Leakage can also be detected by the time it takes to charge the unit to rated d.c. voltage.

ESR (equivalent series resistance) is a measure of the internal resistance of a capacitor. This resistance is responsible for the heating effects associated with a capacitor. The ESR of an electrolytic varies inversely with temperature. It is also related to capacitance and frequency.

The dissipation factor (D.F.) of a capacitor is the ratio of the ESR to the capacitive reactance. The power factor (P.F.) is the ratio of the ESR to the total impedance of the capacitor. For low losses, power factor is equal to dissipation factor up to 12%. Beyond this, dissipation factor increases without limit as losses go up. Naturally, as the capacitor deteriorates, the ESR increases. If all these factors are taken together, it can be concluded that ESR is a most important factor when judging a capacitor for its use and end-life characteristics.

ESR also has an effect on the ripple-current rating of an electrolytic capacitor. The a.c. ripple current combined with the ESR causes internal heating which must be dissipated. A small additional loss caused by the d.c. leakage current is also present, but is usually negligible.

A rule-of-thumb for determining maximum permissible ripple current conditions is that the capacitor case temperature shall not exceed + 10°C above the maximum rated temperature for the capacitor. It is also common to allow a 5°C increase in case temperature rise for each 10°C decrease in ambient operating temperature.

Variations and Types

The capacitor designer can vary the cost, life, electrical parameters, temperature range, and mechanical factors by changes in anode foil, cathode foil, paper spacers, electrolytes, terminations, and packaging.

For example, using an anode with a high ratio of formation voltage to rated capacitor voltage in combination with heavy paper separators and a conservatively activated electrolyte (close to neutral ph), will result in a capacitor having long life, -20 to +65°C temperature range, and medium changes in electrical parameters. The choice of packaging can affect even this conservatively designed unit. Generally the package can be judged by its ability to keep the electrolyte from escaping, leaking, or diffusing from the container. The better the package is sealed, the better the capacitor will retain its initial characteristics.

Capacitors are made and sold by intended end use in three grades: (1) commercial, (2) industrial/instrument, and (3) premium. The commercial generally offers the most capacitor for the lowest price. The industrial/instrument is a conservatively designed capacitance section in the lowest priced package. The premium grade is the finest capacitance section and best package for optimum electrical performance in all respects.

Within each of these three categories, the manufacturer offers a very large variety of sub-types that differ from each other in certain characteristics and applications. Table 1 generalizes the importance of the various characteristics and ranks them for the three main grades of capacitor. In addition, Figs. 2, 3, 4, and 5 show some of the important characteristics of two specific types of aluminum electrolytics, one in the industrial/instrumentation grade and one in the premium grade.

Tantalum Electrolytics

Tantalum is the second most popular oxidizable-base metal for use in electrolytics. Tantalum oxide has almost twice the dielectric constant of aluminum oxide and is exceptionally stable with temperature. It is also very inert to chemical attack and this property allows the use of highly ionized acid electrolytes not possible with aluminum. Tantalum is available in a high-purity form both as a foil and a powder, thus more diverse physical arrangements are possible than with aluminum. All these properties of tantalum make it possible to produce tantalum capacitors with these advantages: higher μf./volt per unit volume, wider operating temperature range, better temperature stability characteristics, longer life, more rugged construction features, better electrical parameters, and excellent shelf life. The main drawbacks  are the greater cost and the lower operating voltages of the tantalum types.

Of all the plus features, the stable shelf life at temperatures of 30-40°C for periods of time up to 5 and even 10 years without harmful parameter changes is the most outstanding. This factor alone makes it possible to use electrolytics widely in military gear. Thus, the expense of this rare metal was justified and has led to the almost explosive development and use of several types of tantalum electrolytics in the past ten years. In 1964, for example, 64,000,000 units, worth $51,000,000, were sold.

Three major categories of tantalum electrolytics are being made and used in quantity. They are the solid electrolyte types, foil types, and wet anode types with solids outselling the other two by a ratio of 5 to 1 or more.

The tantalum foil types are similar in construction to the aluminum electrolytics. The oxide on the anode foil is the dielectric, the electrolyte is the cathode, and the cathode foil is primarily a connector. They are cased in aluminum with various end seals made of rubber or Teflon. Generally, the package construction is more critical because of the emphasis on long life and stable electrical parameters for high reliability end use.

Wet-anode styles are made by pressing tantalum powder and a binder, in a mold or die, to a given shape, usually cylindrical. These pellets are then sintered under high vacuum and temperature to remove the binder and impurities, leaving a rugged, porous metal pellet which is then electro-chemically treated to form a layer of tantalum oxide. These anodes are then assembled in silver outer cases, filled with an electrolyte, and sealed. Again, a rubber or Teflon end seal or a combination of both are used to achieve a nearly hermetically sealed container.

Solid-electrolyte units are made by using a sintered anode which has an oxide formed on it the same as that used for wet-anode types. The porous anode is then impregnated with a liquid solution of manganous nitrate which is then fired in an oven and converted to manganese dioxide. This semiconductor material is the solid electrolyte and true cathode of the capacitor. A layer of carbon followed by a layer of silver paint completes the cathode connection. This complete capacitor is then soldered in a tinned metal container and a glass-to-metal seal affixed to the positive end. Thus we have a rugged, hermetically sealed package with no liquids to leak out.

A brief comparison of these three types of capacitors is given in Table 2. The characteristics are ranked numerically for brevity and ease of interpretation.

A thorough and detailed description of all three styles, along with a quality-control plan and much information on test techniques is available in the MIL-C Specs listed in the table. These MIL-Specs are widely used as industry standards and many suppliers are qualified to furnish products which meet these specifications. All major manufacturers also have special designs and lines in addition to the Military types.

The graphs shown in Figs. 6, 7, and 8 illustrate typical characteristics of a solid-electrolyte tantalum electrolytic capacitor.

Posted September 9, 2022