July 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
I learned a new word in
this issue of Electronics World magazine's special report on ceramic
It looked like a made-up word to describe something that looks like a disc.
Anyway, the July 1965 issue contained a collection of articles on the various
sorts of capacitors in use at the time. Other types of dielectrics covered were
tantalum, glass, plastic-film, mica, paper, and metallized-dielectric. Ceramic
and electrolytic capacitors were by far the most widely used capacitors during
the vacuum tube era since they were relatively inexpensive to manufacture and
could handle high voltages. Hobbyists who service and/or refurbish vintage
electronic equipment still need this information if for no other reason than to
verify component values based on color codes. It also helps to know how the
electrical properties vary over temperature, frequency, applied voltage, etc.,
when deciding which type of modern capacitor will best serve as a suitable
Ceramic-Capacitor Color Code
By Engineering Dept./Capacitor Products, Centralab
The Electronics Div., Globe-Union, Inc.
Varying composition of the ceramic dielectric can produce a broad range of temperature-compensating,
Ceramic dielectrics are among the most outstanding and versatile of all capacitor
dielectric materials. They can duplicate, and in many instances far surpass. the
characteristics of other dielectrics. Their wide range of dielectric constants (K)
gives them a notable advantage in microminiaturization. The K advantage which ceramics
possess is only one of the inherent superiorities of this dielectric. The ability
to operate over wide frequency ranges, to be functional over large temperature excursions,
and to possess either linear or non-linear characteristics gives a heretofore simple
capacitor giant capabilities. The ability to mold ceramic materials into numerous
shapes, sizes, and thicknesses gives them both mechanical and electrical design
possibilities beyond those of any other dielectric material.
The development of ceramic capacitors can be traced to the German discovery in
the 1870's of Rutile (titanium dioxide). Experiments with this material enabled
production of ceramic capacitors with K's less than 25 in the early 1920's, but
these efforts were scarcely more than a laboratory curiosity until the advent of
higher dielectric constant materials.
In 1936, Centralab commercially produced the first ceramic capacitor in a tubular
configuration, having a dielectric constant of 100. World War II gave tremendous
impetus to the development of higher K ceramic dielectric materials and implementation
of the use of the disc configuration. Capacitor usage in disc form grew by leaps
and bounds until the mid-50's when their usage far surpassed that of any other dielectric
Virtually all of the ceramic used for the dielectric in ceramic capacitors can
be divided into two broad categories: temperature-compensating (or low K) and high
dielectric constant (high K). This latter category can be expanded into a conventional
ceramic dielectric type and into a semi-conductor type, represented by such trade
names as "Ultra-Kap," "Hypercon," "Magnacap," and "Transcap."
Since glass and porcelain capacitors possess electrical characteristics similar
to temperature-compensating ceramic, these materials will also be included in the
following discussion on capacitor types.
Fig. 1 - As the proportions of the capacitor material are altered,
the slope of the temperature curves are changed.
Fig. 2 - Curves show the variation in capacitance and dissipation
factor with frequency, for high-K type dielectric.
Fig. 3 - (A) The d.c. voltage coefficient of capacitance at 25°C
and 1 kc. (B) The a.c. voltage coefficient.
Fig. 4 - The a.c. voltage coefficient of dissipation factor at
25°C and 1 kc. (B) The d.c. voltage coefficient.
Fig. 5 - Leakage resistance of semiconductor capacitor rated
at 25 v.
Ceramics possessing this characteristic have a predictable capacitance change
vs temperature. This change is called temperature coefficient (TC) of capacitance
and is expressed in parts per million per degree C (PPM/°C). It indicates the
number of picofarads of capacitance change per degree change in temperature (centigrade).
These ceramics are available with a TC from P120 (or +120) to N5250 (or -5250)
and with dielectric constants from 5 to 570, generally increasing as TC becomes
more negative. Standard coefficients and tolerances are available as specified in
EIA Specification RS-198, and Military Specification MIL-C-20D should be consulted
Temperature compensation for commercial applications is specified from +25°
to + 85°C. MIL Specs require measurements from -55° to +85°C, or in
some cases to + 125 °C, or to the maximum operating temperature.
Magnesium and calcium titanates are two of the compounds used for the temperature-compensating
types. The various K levels are achieved by mixing magnesium titanate (exhibiting
positive TC) with calcium titanate (having negative TC). The material proportions
alter the slope of the temperature curves (see Fig. 1).
These titanate materials and other compositional additive types provide a K range
from 5 to 110, with temperature coefficients from positive 120 to negative 750 PPM/°C,
while the extended TC series have K ranges from 100 to 570 with temperature coefficients
from N 1000 to N5250.
The temperature-compensating ceramics possess little or no voltage-coefficient
characteristics, that is, a change of capacitance or dissipation factor with applied
voltage. The effect of frequency on these materials indicates that change of capacitance
and dissipation factor over the frequency range of 60 cps to 1 mc. is minimal.
Glass and porcelain capacitors exhibit general characteristics similar to the
temperature-compensating ceramic and also possess high insulation resistance, excellent
stability, and high "Q". They are limited to temperature coefficients in the NPO
to P140 range and are not available in a large variety of configurations as are
TC ceramics. They are also widely used in applications involving adverse environments
and where cost is not a large consideration.
High Dielectric Constants
Ceramics in this class are characterized by a dielectric constant ranging from
600 to 10,000, a dissipation factor less than 2.5%, and insulation resistance greater
than 20,000 megohms. They do not have the stability and high "Q" of the TC type.
The high-K ceramic dielectrics generally are barium-titanate modified with alkaline
earth titanates, zirconates, and stannates. Barium-titanate dielectric material
has a peak (Curie point) K of approximately 6000 at 120°C. The addition of small
amounts of barium stannate shifts the peak to a lower temperature, and increasing
percentages raise the K level. A barium-zirconate modification exhibits a similar
but less marked effect. Magnesium-titanate additions produce a stabilizing effect,
and although these additions reduce K level, a more linear curve is thereby produced.
Typical capacitor bodies are combinations of these materials and exhibit K vs
temperature characteristics whose peak or Curie point occurs at about room temperature
25°C), with K falling off on either side of this peak. Generally, the magnitude
of this change will be greater for materials with higher dielectric constants. It
is obvious that this change in dielectric constant vs temperature will cause the
capacitors to exhibit capacitance changes which directly follow these curves. Capacitors
are available such that their capacitance vs temperature change characteristics
fall within established limits.
In addition, these materials will exhibit capacitance changes with respect to
time (aging), frequency, and voltage. Fig. 2 shows the variation of capacitance
and dissipation factor vs frequency, indicating a decrease in capacitance as frequency
increases. The effects of applied a.c. and d.c. voltage on capacitance are shown
in Fig. 3 and on dissipation factor in Fig. 4.
Capacitors using these high-K ceramics will decrease logarithmically in capacitance
over a period of time. Most reputable manufacturers take these small changes in
capacitance into account, and units are produced to remain within tolerance (at
room temperature) for one year. It is also well to note that whenever the unit is
heated it will tend to be restored to original capacitance. Therefore, aging is
generally not a problem.
A recent discovery utilizing advanced techniques for processing high-K ceramics
led to the development of the semiconductor-type ceramic capacitor. The "Ultra-Kap,"
first introduced by Centralab in 1955, uses this principle. These capacitors provide
capacitance values as much as 100 times greater than those attainable through the
use of conventional ceramic dielectrics, achieving levels heretofore available only
with electrolytics and film-type capacitors. The economy and compact size of these
semi-conductor units resulted in their widespread usage in transistor circuits for
bypass and coupling applications. They are most suitable for operation in the audio-frequency
range, and the voltage ratings of these devices are from 3 to 25 volts.
All capacitors of this type will exhibit lower insulation resistance than standard
ceramic types, with insulation resistance decreasing with applied voltage. They
are therefore best suited for use in low-impedance circuits where this low resistance
will not cause loading problems. Fig. 5 illustrates the minimum leakage resistance
of this particular type of capacitor rated at 25 volts.
The TC type of ceramic dielectric capacitor is essentially linear and therefore
ideal for use in tuned circuits to compensate for inherent impedance changes. In
a receiver, this compensation can be added to the oscillator circuit to effect drift
compensation throughout that portion of the receiver which is dependent on the oscillator
frequency. The i.f. amplifier drift can be controlled indirectly in this same manner.
The linearity and tolerance of the temperature coefficient are slightly influenced
by the physical configuration and may be noticeably influenced by the metallic mass
in contact with the ceramic.
For this reason, even though the ceramic can be fabricated in a variety of shapes,
the disc or tubular style is most suitable for use in critical temperature-compensating
applications. Standard compensating disc capacitors are available in sizes ranging
from 3/16" to 1" in diameter with voltage ratings up to 6 kv. d.c. Tubular capacitors
are generally available in 1 kv. d.c. or less.
In addition to temperature compensation, the stability, high "Q," and high insulation
resistance (low leakage) associated with this ceramic make it an ideal general-purpose,
high-quality capacitor for use in tuned circuits, oscillators, high-frequency filters,
r. f. power applications, or any other electronic circuit requiring high "Q" stability.
Ceramic-Capacitor Summary Chart
Feedthrough capacitors are a combination of bypass and feed through that enables
the designer to run leads through the chassis and simultaneously bypass. Both TC
and high-K materials are used in the fabrication of these units to provide very
low or very high capacitance as required. They are produced in physical sizes ranging
from 1/8" in length to over 8" in length and voltage ratings ranging from 50 volts
to 60 kv. d.c. Conductor current-carrying capabilities can be as high as those required
for broadcast transmitters.
Feedthrough capacitors are available in either tubular or discoidal forms. These
latter types are considerably more expensive than the tubular configurations. Discoidal
types tend to exhibit more linear attenuation characteristics in the 500- to 1000-mc.
Standoff capacitors are available in disc or tubular configurations. Tubular
standoffs are used in such low-frequency applications as tie points and chassis
bypass. Disc standoffs find use in high-frequency applications where lead inductance
must be minimized.
Special high-voltage capacitors for induction heaters, x-ray, diathermy, and
r.f. applications where high-frequency and high r.f. currents are encountered, such
as plate, coupling, tank, and bypass functions, utilize the desirable characteristics
of TC ceramics.
High-K materials can also be used in those applications where no specific temperature
coefficient is required and where dielectric losses are inconsequential. These devices
are designed for specific applications but in most cases utilize variations of four
basic shapes, which are cup, double cup, slug, and cylinder. Voltage, current, frequency,
and mechanical considerations are some of the factors which influence the final
capacitor design that is to be employed in a given circuit.
Capacitors of either TC or high-K dielectric (as listed by UL) can be used as
a.c. line bypass or for antenna isolation applications in commercial receivers.
Posted September 26, 2022