June 1954 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
capacitors are vastly different from most of those from the middle of the last century. While the
in your modern electronic circuit is likely in the form of a thin circular or
rectangular package, or of a tiny surface mount outline, those described in this
Radio-Electronics magazine article were rather large tubular devices
that had the appearance of a wirewound power resistor. The advantages of ceramic
capacitors over other types available in the 1950 (paper and mica) are basically
the same as today - high voltage handling and tight tolerances of down to ±1%.
Table I lays out a comparison. Wikipedia has an extensive
comparison of modern capacitor types, illustrating how far technology has
advanced since then. Author Jesse Dines delves into detail about positive and
negative temperature coefficients.
Ceramic Capacitors: Composition of the ceramic capacitor;
By Jesse Dines*
Too long ago, in the days when television was still a cloud on the horizon,
replacing a capacitor was no problem to the service technician. He determined
the capacitance of the defective unit and ordered a new one of the same value,
specifying a voltage rating high enough to make sure that it would stand up
under normal and abnormal voltages in the circuit where it was intended to work.
The problem which faces the service technician who replaces a capacitor today
is a very different one. He has to know about a number of characteristics of
capacitors beside the capacitance value, tolerance, and voltage rating. TV and
FM circuits are more critical, and the wrong capacitor can produce serious results-or
no results at all.
For example, yesterday's technician would not hesitate to replace a faulty
0.05-μf blocking capacitor with any type of 0.05-μf unit, whether mica,
paper, or ceramic. If today's repairman tried to replace a defective capacitor
in a TV set with the first one of the same capacitance picked up around the
shop, he would be courting trouble. For instance, a TV receiver is brought into
a service shop. There is no video - everything else is normal. After trouble-shooting
the video detector bypass capacitor - a 5-μμf N100 ceramic with a tolerance
of ± 0.05-μμf - is found to be faulty. The service technician replaces
it with a 5-μμf mica capacitor with a tolerance of ± 20%. Although
present, the video is now weak in spite of complete realignment and several
What is the explanation? The replacement capacitor could have had any value
from 4.0 to 6.0 μμf. But the set was so designed that the value should
lie between 4.5 and 5.5 μμf. Perhaps this alone would not have made that
much difference, but what about the N100 ceramic? What does N100 signify? Must
a ceramic be replaced with only a ceramic? These are questions that this article
Of the three major types of capacitors used in receivers today - ceramic,
paper, and mica - ceramics are the most extensively used.
Fig. 1 - The schematic and table below it illustrate the
widespread use of ceramic capacitors in modern equipment.
Table I - Characteristics of paper, mica, and ceramic capacitors.
The years preceding television saw the predominant use of paper capacitors.
Mica capacitors were used only in special applications, such as tuned and critical
time-constant circuits. Ceramic capacitors were not used to any great extent
until recently. Their increasing popularity stems from their newly discovered
advantages over the other types. Ceramic capacitors have a low power factor,
high dielectric and high mechanical strength; they are impervious to moisture
(0.007% or less); they are easily fabricated in a multitude of shapes (discs,
plates, and tubes); they can be made durable; and they are relatively small
(many are one-seventh the size of the other types of capacitors). Unlike other
types, the ceramic body itself will stand temperatures exceeding those found
in electronic apparatus, without causing any changes in capacitance. Also, ceramics
permit capacitances of unusually close tolerance at modest costs.
To illustrate the widespread use of ceramic capacitors, Fig. 1 shows the
tuner and video i.f. amplifier of Philco chassis 84, H-4 code 103, as well as
a table giving the capacitance, capacitance tolerance, purpose, and type of
capacitors used. All capacitors shown are ceramics. An analysis of the schematic
of the entire television chassis reveals that only a small percentage of paper
and mica capacitors are used throughout. The predominance of ceramic capacitors
in this chassis is typical of the majority of newly released TV receivers. Ceramics
are composed of the natural mineral, rutile, which has a very high dielectric
constant. In the past decade, titanate bodies, made from rutile, were mixed
experimentally with oxides of barium, calcium, and magnesium; ceramics with
varying characteristics were obtained. From these groups, dielectric constants
from 18 to 12,000 (there are indications that 12,000 is not the maximum value)
have been obtained. The higher the dielectric constant, the smaller will be
the resulting capacitor for a given capacitance.
Ceramic capacitors are made in various forms, such as tubulars, discs, and
plates; each manufacturer builds capacitors in his own way, yet all are basically
of the same construction. Two cross-sectional views of typical tubular type
ceramic capacitors are shown in Fig. 2.
A capacitor consists of two conducting surfaces separated by a dielectric
material. This condition is met in Fig. 2 where the dielectric material is a
ceramic and the conducting surfaces are homogenous silver plates. The style
CN capacitor has radial leads and is coated with a moisture-proof, non-hydroscopic
plastic. The CI style is insulated with a special end seal compound which allows
the wax impregnant to enter and thoroughly fill all the voids inside the steatite
Ceramics serve the same purposes as paper and mica capacitors, such as coupling,
bypassing, and d.c. blocking. Table I gives the characteristics and functions
of these three types. However, ceramics have an exclusive function as temperature-compensating
capacitors. Noticeable changes in temperature produce changes in capacitance.
The amount of capacitance is dependent upon the actual composition of the capacitor:
that is, whether titanium dioxide, barium titanate, or any other compound is
combined with rutile. A temperature-compensating capacitor may serve its normal
function in a circuit, or it may have a dual purpose. For example, it may be
used for d.c. coupling and at the same time for temperature compensation.
Fig. 2 - The cross-section drawings show how tubular ceramic
capacitors are made.
Fig. 3 - Capacitance change vs. temperature.
Table II - Temperature coefficient tolerances.
Changes in capacitance due to temperature changes can be detrimental since
they can cause changes in frequency. A tank circuit should be designed so that
its natural resonant frequency does not change with temperature changes. As
you can imagine, if the tank circuit of the r.f. oscillator in a television
receiver drifted, both sound and video reception would be impaired. While a
small amount of drift can be tolerated in intercarrier-type television receivers,
it still must be kept within certain limits. Reception will be impaired also
by the detuning of any of the several traps, and the circuits associated with
the horizontal automatic frequency control.
Transformers, coils, wires, sockets, component leads, etc.,
increase in inductance as the ambient temperature increases, lowering the natural
resonant frequency. This undesirable effect may be minimized, or even eliminated,
by using coils of special construction, such as hermetically sealed ones. However,
the most economical method is to insert a negative temperature coefficient (TCN)
capacitor across the inductive circuit. TCN indicates that temperature and capacitance
vary inversely; that is, an increase in temperature' causes a decrease in capacitance.
In the case of the positive temperature coefficient (TCP), the temperature and
capacitance vary directly - an increase in temperature causes an increase in
The resultant temperature coefficient (TC) of all the component parts, leads,
and connections, etc., of conventional electronic equipment is positive. To
counterbalance this effect, a TCN of equal magnitude must be inserted in the
circuit, leaving a zero temperature coefficient (TCZ). It is difficult to pre-determine
the positive temperature coefficient of a circuit. In most cases, a circuit
is first built, then TCN type ceramics with different coefficients are inserted
in the circuit. Only through trial and error is the point of best frequency
stability reached. It has been found that positive temperature coefficients
are fairly constant and uniform in television sets of similar design. This enables
the design engineer to approximate beforehand the amount of TCN capacitance
necessary. The compensating capacitor is placed at that point in the circuit
where maximum compensation is needed.
There are several sections in a television receiver where compensation is
necessary; they are, mainly, the r.f., oscillator, and i.f. sections. Either
the oscillator or the i.f. circuit may be compensated for, since both circuits
work hand-in-hand and thus the compensating of one will take care of the other.
If possible, the oscillator is compensated for, since it offers less engineering
difficulty. Sufficient compensation can usually be obtained by the addition
of one ceramic capacitor to the oscillator circuit. A separate compensating
capacitor must be used in the r.f. section as well.
Ceramic coupling capacitors, generally NPO's (N-negative, P-positive, and
O-zero, which means that they have neither a negative nor a positive temperature
coefficient) are used from the audio frequencies through the ultra-high frequency
range. These capacitors are also finding increasing use in test equipment, where
careful shielding frequently causes frequency drift through overheating.
The temperature coefficient
Temperature coefficient is expressed in parts per million per degree C (p/m/°C).
It is written as a number which is preceded by the letters N (negative) or P
(positive) in order to indicate whether it has a negative or positive coefficient.
Thus N750 has a negative temperature coefficient and P100 has a positive one.
Let us analyze the expression N080 ceramic. This means that the capacitor has
a negative temperature coefficient whose capacitance changes 80 parts (out of
every million parts) for every degree (C) change in temperature.
For a better understanding, consider a 1,000-μμf capacitor which has a temperature
coefficient of N1,400. The 1,400 indicates that it changes 1,400 p/m/°C
or 1,400/1,000,000) or 0.0014. Thus every time the ambient temperature increases
1°C, the capacitance decreases 0.0014 x 1,000 μμf or 1.4 μμf.
To illustrate this effect on circuit capacitance, consider the case where
the temperature rises to 31°C, only 60 above room temperature (25°C).
The total decrease in capacitance is 1.4 x 6, or 8.4 μμf. This amounts
to an 8.4/1,000 or 0.84% change. Note that it is common for temperatures to
rise as high as 85°C in standard electronic equipment.
To illustrate the seriousness of an 8.4-μμf change, take the case of
an i.f. circuit operating at a center frequency of 40 mc. Using the relationship
F2 = (F1 x C1) / C2, where
F1 = frequency at room temperature,
F2 = frequency at any other temperature,
C1 = capacitance at room temperature,
C2 = capacitance at any other temperature,
F1 = 40 mc, C1 = 1,000 μμf, and C2 = 1,000 - 8.4 or 991.6 μμf
Therefore at 31°C,
F2 = (40 x 1,000) / 991.6 = 40.33 mc.
Undoubtedly, a 0.33-mc (the difference between 40.33 mc and 40 mc) change
will seriously impair the video of the television receiver.
The temperature coefficient of capacitors on the market today varies from
about P120 to N4,700, but there is actually no limit to how high or low it may
go. A set of curves showing percentage capacitance change vs temperature for
coefficients, ranging from P100 to N1,400 is shown in Fig. 3. The KPO curve
has a zero percentage capacitance change; that is, the capacitance of an NPO
ceramic does not change with variations in temperature. The higher the temperature
coefficient, the greater the percentage capacitance change. To illustrate the
use of a curve, take the case of an N150. At minus 55°C the percentage capacitance
change is only slightly more than 1.5%; at 25°C (room temperature) its capacitance
change is zero; and at 85°C its capacitance change is slightly less than
Although the curves are illustrated as straight lines, there is a slight
curvature which manufacturers consider negligible and therefore choose to ignore.
Temperature coefficients of ceramic capacitors have a tolerance which is also
expressed in p/m/°C. Thus an N1,400 with a tolerance of ±250 p/m/°C
may range anywhere from N1,150 to N1,650. The tolerance is dependent upon not
only the coefficient but also the capacitance. The greater the coefficient,
the greater the tolerance; the larger the capacitance, the closer the tolerance.
The specific tolerances for different coefficients vary from one manufacturer
Table II (page 81) indicates the tolerances of different capacitance ranges
for various temperature coefficients. An N030, for example, changes plus or
minus 250 p/m/°C when its capacitance is from 0.5 to 2 μμf but changes
only plus or minus 30 p/m/°C when it is over 10 μμf. This indicates
that closer tolerances and therefore more reliable capacitances can be realized
for relatively higher capacitance values. A capacitance cannot always be obtained
for a desired coefficient. The range of capacitances given for any coefficient
varies with different manufacturers. Generally, the larger the coefficient,
the greater the availability of capacitances.
In many cases, a capacitor is known as a general-purpose ceramic. Such a
capacitor can have any TC within the temperature range shown in Fig. 4. These
capacitors are used in circuits where changes in capacitance, due to changes
in temperature, are insignificant.
An N330 ± 500 p/m/°C capacitor is a general-purpose ceramic, although
it appears to be a TCN. The set manufacturers generally use that designation
when referring to general-purpose ceramics; this is shown in Fig. 1. Such capacitors
are commonly known as SL types and may be replaced with general-purpose ceramics.
* Educational Director, Ram Electronics Sales Co., Irvington, N. Y.
Posted March 12, 2021