Ceramic Capacitors
June 1954 Radio-Electronics

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.

Today's ceramic capacitors are vastly different from most of those from the middle of the last century. While the ceramic capacitor 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; temperature coefficients.

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 tube replacements.

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 will answer.

Ceramic capacitors

Of the three major types of capacitors used in receivers today - ceramic, paper, and mica - ceramics are the most extensively used.

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 tube.

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.

Temperature compensation

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 capacitance.

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 (at 31°C).

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 minus 1.0%.

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 to another.

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