Paper 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.

A quick search for paper capacitors turned up no manufacturers currently offering them. They were the capacitor of choice for many decades until other types with superior dielectric material could be produced at competitive prices. A special formulation of Kraft paper of various thicknesses and chemical impregnations serves as the dielectric material between layers of conductive foil. The vast majority of tubular type capacitors found in radios, televisions, etc., prior to about 1960 were paper. One of the biggest disadvantages of paper capacitors is vulnerability to drying out or absorbing moisture either through the encapsulating material or via leaks in the case or around the external radial leads. Most people who restore vintage electronics replace all paper capacitors out of an abundance of caution, particularly because of the high voltages used by vacuum tubes. Author William Robinson does a great job describing how paper capacitors are manufactured, how they are used, and what their electrical characteristics are. He tells the difference between a "wet" and a "dry" paper capacitor, which I've probably known in the past, but had forgotten. This July 1965 issue of Electronics World magazine presents similar articles on many types of capacitors.

Paper Capacitors

By William M. Robinson/ Chief Engineer, Cornell-Dubilier Electronics Div., Federal Pacific Electric Co., New Bedford, Mass.

One of the most widely used types of capacitor, the paper type has characteristics mostly dependent on the type of impregnant used.

Probably the least exotic member of the capacitor family is the paper-dielectric capacitor. It is well known that mica types have much better over-all electrical characteristics, film dielectrics have lower losses and better resistance to humidity, and electrolytics and ceramics are smaller in size and lower in cost. Yet, after seventy years of commercial application, paper capacitors are still the most widely used capacitive component.

Paper capacitors range in physical size from the sub-miniature to huge complex banks - in ratings from a few volts to hundreds of thousands of volts. Paper capacitors are well suited for a.c. or d.c. service, pulse and energy-storage applications, and for use in simple electronic devices or the most complex computers.

Basic Construction

Basic materials for all paper-capacitor elements are kraft paper, aluminum foil, and a liquid wax or resin impregnant. The type of paper and impregnant, as well as vital design factors, determine the rating and establish the over-all performance characteristics of the finished capacitor. Packaging in a metal or insulated housing, together with the type of terminals used, limits or extends the capacitor's capability with respect to environmental performance.

Dielectric Structure

A paper capacitor receives its name from its basic dielectric material, kraft paper. This material is not just a porous separator as found in electrolytic capacitors but is a vital part of the dielectric structure.

Kraft capacitor tissue is manufactured from specially selected unbleached sulfate pulp made from certain species of soft woods. These pulps are chosen for their uniformly long fibers and high degree of purity. The pulps are made to rigid specifications to insure that the fiber length, chemical composition, and purity of each lot will meet exacting requirements.

There are many types and grades of kraft for use as a dielectric. Some of these perform better on a.c., others on d.c., depending upon the dielectric strength and dissipation factor.

Aluminum foil used for paper capacitors ranges from 0.00017 to 0.0005 inch in thickness and has a purity of at least 99.8 %. The surfaces must be free from all traces of rolling lubricants and other sources of contamination. Although tin-lead alloy foil is occasionally used on very small capacitor elements, aluminum is preferred because of its resistance to corrosion and electrochemical action, both of which are apt to severely degrade the capacitor electrical characteristics under certain operating conditions.

Use of impregnants for kraft paper improves the dielectric strength greatly and increases the dielectric constant, consequently reducing both the size and cost for any given capacitor rating. The impregnant also establishes the temperature-capacitance characteristics as shown in Fig. 1 and is an important factor contributing to the insulation resistance and dissipation factor of the finished capacitor. Some impregnants are used exclusively in capacitors for d.c. service, others for a.c. capacitors, depending upon the dissipation factor and dielectric strength they impart.

Design Factors

The voltage rating is established by the selection of the type, thickness, and number of layers of kraft paper, together with the characteristics of the impregnant used. Insulation spacing between metal parts of the element is directly related to the voltage rating. Capacitance is proportional to the area of the aluminum-foil electrodes and inversely proportional to the distance between the electrodes. Design factors determining capacitance are C = (0.225 A K)/(T x 10³) where A is the active electrode area in square inches, K is the dielectric constant of material, T is the dielectric thickness in mils, and C is the capacitance in microfarads.

Winding of the capacitor element is accomplished with automatic or semi-automatic machines to minimize hand contact and to assure uniformity of the product. Fig. 4A shows a wound roll of kraft and aluminum foil together with inserted tinned-copper contact strips. The extended-foil type of winding, shown in Fig. 4B, has the foil protruding so connections may be made externally. Inserted-tab construction is generally used for d.c. or low-frequency a.c. capacitors, the extended foil being used for the smaller sizes, those used at higher frequencies, or where high-current pulses or discharges are required in operation. 

Paper capacitors are usually made with two aluminum foils separated by two or more layers of kraft. Higher voltage windings often employ twelve or more papers with as many as six foils.

Many of the smaller, lower-cost paper capacitors for the entertainment field are impregnated at this stage. Assembly in non-metallic molded, tubular, or dipped enclosures follows the operation of attaching lead wires. This is the wet-assembly method of manufacture.

Most military and industrial capacitors, including all those for a.c. and higher voltage ratings, are made by the dry-assembly process. This consists of assembling the windings in metal containers and, after seaming, conducting the impregnation process through a small hole in the case. The hole is then sealed.

Dry-assembly processed capacitors are made in almost an infinite variety of shapes and sizes using squeezed-seam, rolled-seam, and welded-seam containers.

Terminal connections and insulators range from the sub-miniature to extremely large sizes. The capacitor element and internal circuit connections are generally heavily insulated from the case with thick sheets of kraft paper made to the same requirements as for the dielectric paper.

Impregnation

Capacitors are loaded into the impregnation tanks where they are subjected to extended heat and vacuum cycles to remove all traces of moisture and air. This process is conducted at temperatures well below the critical level. which would be harmful to the kraft paper, by extending the time and monitoring the temperature and vacuum frequently. Force-drying the elements at excessively high temperatures to reduce process time is generally detrimental to reliable operation.

After the internal parts of the capacitor element have been thoroughly evacuated and dried, the tanks are flooded with the impregnant. Alternate vacuum and pressure cycles are applied so that every fiber of the paper is thoroughly impregnated and all voids are removed.

Finishing and QC

At the end of the impregnation cycle, the capacitors are removed from the tanks, drained, sealed, and cleaned, and markings are applied over plain, painted, or plated finishes. The QC (quality-control) program includes 100% tests of capacitance, dielectric strength, seal, and mechanical inspection, as well as statistical checks of insulation resistance and dissipation factor.

Test facilities are used to conduct life tests to determine quality-level and service-life capabilities to verify design standards and to accumulate data for reliability studies.

These facilities consist essentially of high-temperature ovens with adjustable a.c. or d.c. power supplies and controls to maintain specified test conditions. Before any new design standard or material is used in a paper capacitor, it is thoroughly evaluated under accelerated life conditions .

Special Constructions

The capacitors previously described are of conventional foil-paper construction. Other types are made using series-wound elements with high-voltage ratings and low-inductance conductors for energy-storage and pulse applications. Such capacitors often are required to deliver discharges of tens of thousands of amperes in intervals as short as a few microseconds.

Plastic films are also used in conjunction with paper for a dielectric having special characteristics of capacitance change with temperature and an extremely high insulation resistance. The mixed dielectric combines the best properties of both materials, frequently resulting in a more reliable product, although at a higher cost.

Metallized paper capacitors with vapor-deposited metal electrodes are available where space will not accommodate standard capacitors. The smaller size is obtained by use of electrodes less than one micron thick and reduced di-electric for a given rating. This type of unit has limited self-healing properties upon voltage breakdown, as the electrode material will fuse and clear by electrical discharge. Metallized capacitors are used in ratings up to 600 v.d.c. They should not be employed in circuits where high insulation resistance is a necessary critical requirement.

Power Losses

Even though paper capacitors normally perform to more than 99.5% efficiency, there are internal power losses that must be considered for units operating in a.c. or pulse circuits. The methods of calculating power loss for a.c. operation is Watts Loss = D.F. (E²/X_c) where D.P. is the dissipation factor of the capacitor, E is the applied r.m.s. sine-wave voltage, and X_c is the capacitive reactance of the device involved.

The method of calculating power loss for pulse operation is Watts Loss = D.F. (C E₁² N) where C is the capacitance in farads, E₁ is the applied peak voltage, D.P. is the dissipation factor of the capacitor, and N is pulses per second.

Approximately 0.06 watt per square inch of case surface area will result in a 10°C rise above the ambient temperature, assuming the capacitor is operating in still air. This temperature rise has a marked effect on capacitor life, as will be described.

When making the calculations for power loss, the dissipation factor may be assumed to range from 0.0025 to 0.005, depending upon the type of impregnant used. The actual temperature rise may, of course, be measured by testing the capacitor under the given conditions.

Effects of Temperature

Paper dielectric capacitors are designed to operate at ambient temperatures of 55°C, 70°C, 85°C, or 125°C, depending upon the design standards and impregnants used. Chlorinated or mineral waxes and oils are generally imited to 85°C. Silicone-fluid, certain resins, and polyisobutylene-impregnated capacitors may be used to 125°C. Higher temperatures result in gradual decomposition of the cellulose fibers of the paper, as well as chemical changes In the impregnants, causing degradation of electrical characteristics of paper capacitors.

Effects of temperature and capacitance are shown in Fig. 1. The large loss in capacitance for some impregnants is due to change in the dielectric constant of these materials. Such capacitors recover full capacitance when returned to room temperature. Units operating in a.c. or pulse circuits frequently generate sufficient heat through internal losses to recover full capacitance after less than one hour of operation, even though they are exposed to very low ambient temperatures.

Designers must also consider insulation-resistance changes with temperature. Shunt leakages are often 100 times greater at high ambient temperatures than at 25°C. as shown in Fig. 2. Insulation resistance of capacitors may also decrease with life due to the effects of moisture on units made in non-metallic cases and chemical changes in those made in hermetically sealed construction. Circuits should be designed to accommodate ten times the leakage current specified for a given capacitor with an additional adequate allowance for operation at high temperatures.

Dissipation-factor changes with temperature are not significant except for capacitors operating in a.c. or pulse circuits.

Life performance of paper capacitors is related to ambient temperature. A lO°C increase may reduce life as much as 50%, as shown in Fig. 3. Use of capacitors at temperatures below maximum rating is recommended to extend life and lower failure rates. This may also be accomplished by using capacitors at less than rated voltage where higher ambient temperatures cannot be avoided. When designing a mechanical layout for equipment, capacitors should be located as far as possible, or thermally insulated rom, sources of damaging heat such as tubes, transformers, and power dissipating resistors.

Capacitor Precautions

1. Keep all large capacitors, high-voltage or high-capacitance types, fully discharged with a low-resistance or shorting wire connected across the terminals. Many persons have been seriously injured by touching capacitors long after they had been removed from a source of voltage.

2. Install capacitors away from sources of heat such as tubes, transformers, or power resistors.

3. Do not exceed the temperature or voltage ratings unless such service is approved by the manufacturer.

4. Do not discharge a capacitor with a direct short circuit unless the capacitor is rated for this type of service. Direct discharge may result in an exceedingly high current flow from the capacitor or set up an oscillatory discharge which may be harmful to the capacitor. Limit the discharge current to one ampere unless the capacitor is designed for higher current service.

5. Keep non-metallic cased capacitors in a dry storage area, not exposed to a humid atmosphere .

6. Solder connections to capacitors in a manner which will not damage the seal. On wire-lead types, soldering should be done at least 1/4" away from capacitor body.

7. Use a torque wrench when tightening nuts to avoid damage to capacitor seal.

8. Do not subject capacitors to any unusual environmental conditions of vibration, shock, pressure, or immersion without first checking with the manufacturer.

9. Allow a maximum of ventilation for a.c. or pulse capacitors operating at high ambient conditions.

Voltage Effects

A well-designed paper capacitor generally is rated for a life capability of more than 50,000 hours. However, service conditions may shorten or extend this time. In addition to the temperature effects, the applied voltage also has a considerable bearing on the successful performance of a capacitor.

Both industry and the military have accepted in their specifications the use of the fifth-power rule relating to the voltage life of paper capacitors for d.c. operation. The following equation shows the relationship between the applied voltage and operating life: L₂ = L₁(V₁/V₂)⁵ where L₁ is the life at rated voltage V₁, L₂ is the life at voltage V₂, V₁ is the rated voltage, and V₂ is the intended operating voltage.

Fig. 5 shows that operation for 20% under rated voltage will result in almost a 300% increase in capacitor life. This factor applies mainly to hermetically sealed paper capacitors, but it may also be applied to non-metallic cased types if desired circuit operation is not critical with respect to the capacitor insulation resistance.

Frequency Effects

Electrical characteristics of capacitance and dissipation factor remain fairly stable at increased frequencies. There may be a 5% to 10% loss of capacitance at frequencies above 1 mc. as compared with 60-cycle or 100-cycle values. Dissipation factors will also be somewhat higher.

Since all capacitors contain some degree of series inductance as well as shunt resistance, the effect of these on impedance must be considered in circuit design. Inductance is caused by circuit connections to the capacitor element. Series resistance is also contributed by these connections and the foil electrodes. Shunt resistance results from normal dielectric leakage.

At the higher frequencies, conventional paper capacitors can be expected to perform as shown in Fig. 6 with exact characteristics dependent upon capacitance value and type of circuit connections to the element. Feedthrough capacitors of coaxial construction have much better frequency characteristics although they employ paper dielectrics.

When using d.c. paper capacitors in audio- or radio-frequency circuits, precautions must be observed to limit voltage to prevent overheating and greatly reduced life. At 60 cycles, the maximum voltage applied should be less than 20% of rating; at 1000 cycles, 6%; and at 10,000 cycles and higher, only 1%. These values may be exceeded only if a specific capacitor is evaluated under the intended conditions of operation and is found to stabilize within its maximum ambient temperature rating.

Posted September 8, 2022