US2347795A - Carbon-composition resistor - Google Patents

Description

y 1944- PODOLSKY 2,347,795

CARBON- COMPOS ITION RE SI STOR Filed Feb. 19, 1945 2 Shee tsShee.-t l

3,00 W l I.

TOTAL L\FE \N HOURS TANCE T .l .2 A .5 .6 7.8.91 2 a 4 5 e so IE.Z a z o I ,k g or 4 g, 2 Ct Q0 0 3 3 I 0 \Q C! Q I I E g 9 I 6 7 a Z Q) Q0 m g Q 1 1 1 c: 6 E w I/ B LU U \O Q o r 0 a0 20 30 40 5o so no no CURRENT DENSITY M\LL|- AMPERES PER SQUARE INCH 'NVENTOR Leon, P0 dais/1y Patented May 2, 1944 CARBON-COMPOSITION RESISTOR Leon Podolsky, Pittsfield, Mass, assignor to Sprague Products Company, North Adams, Mass, a corporation of Massachusetts Application February 19, 1943, Serial No. 476,382

3 Claims.

My present invention relates generally to electrical resistors, and has particular reference to carbon-composition resistors, i. e., those in which the conducting matrix or composition contains carbon as the sole conducting agent.

While the broader phases of my invention have a wider scope, it is primarily applicable to resistors of very high ohmic value for use in highvoltage circuits, such resistors having particular utility in radio communication ,and signalling apparatus.

Despite the long-standing need and currently increased demand therefor, no resistor has ever before been produced, so far as I am aware, which embodies a resistance value of the multimegohm order, and which is capable not only of operating for prolonged periods under impressed voltages in the multi-kilovolt range, but also of withstanding high operating temperatures. By the term operating temperature" I mean to refer to the actual temperature to which the resistor is subjected during use, part of which is the ambient or air temperature surrounding the equipment, and part of which is the temperature developed within the resistor itself by virtue of its operation. By high operating temperatures I refer generally to temperatures in excess of approximately 80 C.

A general object of my invention is to produce a resistor having these highly desirable and heretofore unattained characteristics.

Attempts heretofore made to achieve high ohmic values in resistors of relatively compact practical size have usually involved the use of either resistance wire or carbon compositions.

Quite apart from the fact that the supply of v resistance wire, and of the necessary dies, drawing equipment, and winding machine capacity are extremely limited, at the present time, the use of resistance wire for achieving the desired result is relatively expensive, inefficient, and beset with difficulties. For example, to produce even a 5-megohm resistor, using .0015" diameter resistance wire, approximately 16,000 feet of wire are required, and the winding of this wire into the many thousands of turns which are necessary is a costly time-consuming procedure. Moreover, in order to prevent short-circuiting, it is necessary to insulate the wire, and this becomes a difficult problem when the insulation is to withstand operating temperatures exceeding even the relatively low temperature of 70 C. Where sufficiently high ohmic values are not achieved, because of space limitations. the problem is further complicated if the resistor is to be operated on high voltages of the order of 5 to 30 kilovolts, because under such circumstances very high internal temperatures are developed by the relatively great amount of power which is dissipated in a confined space. For example, notwithstanding a recent demand for a resistor of 25 megohms to operate on a 15 kilovolt circuit, the maximum resistance which could be placed in the allowable space, employing the best available wire-wound resistor technique, was only 8 megohms and as a result the wattage dissipation was so great that the generated operating temperature was too high for nearby equipment to withstand, not to mention its destructive effect upon the resistor itself.

Similarly, molded carbon resistor bodies, consisting generally of an inert filler, a finely divided carbon conducting material, and a thermoor chemo-setting binder, though widely known and used at relatively low temperatures and voltages, have never been successfully adapted for application at high temperatures or voltages.

By means of my present invention, however, molded carbon composition resistor bodies may now be so designed and constructed that the difficulties heretofore encountered are overcome. Employing the principles hereinafter set forth, I have succeeded in producing resistors with resistance values as high as megohms which are not only capable of operating continuously at temperatures up to C. and under applied voltages as high as 50 kilovolts, but which maintain stable resistance values after long periods of operation under these conditions.

inability of carbon-compound resistors heretofore to operate successfully under severe conditions of this character has not been due 1 merely to the well-known susceptibility of such bodies to the effects of absorbed moisture (which causes an increase in resistance value) nor to the general failure of protective impregnating or coating media, such as organic waxes and varnishes, to withstand temperatures in excess of approximately 80 C. For, even the provision of humidity protection by enclosing the resistor within a hermetic seal has failed to overcome the basic inability of carbon-composition resis tors, as heretofore designed, to comply with the high-voltage, high-resistance-value, high-temperature requirements currently demanded.

My invention is based upon an extensive study and a resultant fuller understanding and ap preciation of certain relationships, not heretofore recognized, between the voltage gradient impressed upon a carbonaceous resistor body, the density of the current passing through it, and the maximum operating temperature to be reckoned with when it is used. My invention achieves the desired objectives by a practical harnessing of certain principles, heretofore unknown, revealed by these relationships.

The accompanying figures are graphic representations of certain of these relationships, as indicated in each figure.

Heretofore, designers of resistors, in attempting to develop higher and higher resistance values, have resorted to the traditional principle that greater resistance results from (a) an increase in the specific resistance of the composition itself, (1)) an increase in the length of current travel, and (c) a decrease in the cross-sec tional area through which the current is to pass. In the zeal for creation of high ohmic value in as small a space as possible, stress has naturally been laid upon the last-mentioned factor, and the effective length of the resistor has always been shortened to the maximum possible degree commensurate with the total ohmic value desired.

Thus, resistors have been designed consisting of films of carbon composition coated in helical bands upon ceramic tubes. But the ability of such a resistor to perform satisfactorily and reliably and to withstand prolonged periods of use under high-voltage, high-temperature conditions has never been more than partially successful, and complete failure after premature periods of use has been the most general experience. Sometimes the resistors have failed because of instability in resistance value or by openor short-circuiting. In many cases, the resistors are through after short periods of operation, causing self descruction and creating a fire hazard.

In none of these prior practices has a high operating temperature been considered as anything but an unavoidable and annoying evil. That such temperature represents a factor affecting certain basic relationships in predeterminable manner, has never been realized.

My present studies and research have demonstrated that, for any given temperature, and regardless of the nature of the carbon composition itself or its specific resistance, there is a maximum allowable current density and a maximum allowable voltage gradient (and a resultant maximum allowable power dissipation per unit of volume) above which the resistor will surely fail. By maximum allowable current density I mean that there is a minimum allowable crosssection which the resistor must have, for any given current capacity, so that the quantity of current per unit of cross-sectional area will never exceed a predetermined amount, regardless of the fact that greater ohmic value might be achieved, under traditional concepts, by further reduction of cross-sectional area. Similar 1y, by maximum allowable voltage gradient I mean that there is a minimum allowable effective length which the resistor must have, [or any given impressed voltage, so that the drop of potential per unit of length will never exceed a predetermined amount, regardless of the fact that zreater compactness might be achieved by further shortening the length, or that such minimum length might not appear to he necessary, under traditional concepts, for the creation of the desired ohmic value.

As a preliminary to a more detailed discussion of my invention, attention is directed to Figure 1. I have found, as a result of numerous life tests on a wide range of carbon resistors having different compositions and structures, the tests being conducted in free air and in sealed enclosures, and under various conditions of voltage and current, at temperatures above C., that the useful life of such a resistor can be predicted on the basis of the change in resistance value which has taken place after 200 hours of operation. Figure 1 shows the relationship between change in resistance value after 200 hours of operation" and total life. It will be noted that if a resistor changes its resistance value more than approximately 4% after 200 hours of operation, its useful life will be of relatively short duration. For example, if a resistor suffers a change in resistance value of even slightly more than 5% after 200 hours of operation, its total useful life will be less than 4,000 hours. If it suffers a change of 10% after 200 hours, its total useful life will end after approximately 2,000 hours. If the change in resistance value at 200 hours is over 13%, it is an indication that the resistor is already rapidly deteriorating and will fail completely almost immedi ately. On the other hand, resistors whose ohmic values undergo (after 200 hours) no change greater than approximately 4%, will survive for periods from 5,000 to 7,500 hours.

These preliminary findings enabled me to state the essence of my invention in the following terms:

1. In order that a carbon-composition resistor may operate for a useful life of practical magnitude, its resistance value must not change more than about 4% after 200 hours of operation, and in order that no change greater than this shall take place, then regardless of the 11m pressed voltage or the specific resistance the resistor must be so designed that the current density will not exceed a certain maximum value depending upon the maximum operating temperature to which the resistor will be subjected. 2. In order that a carbon-composition resistor may operate for a useful life of practical mag-- nitude, its resistance value must not change more than about 4% after 200 hours of operation, and in order that no change greater than this shall take place, then regardless of the current capacity or the specific resistance the resistor must be so designed that the voltage gradient will not exceed a certain maximum value depending upon the maximum operating temperature to which the resistor will be subjected.

These maximum values of current density and voltage gradient are proportional in a predeterminable manner to the temperature factor, heretofore ignored, and are given to an adequately approximate degree by the following formulae: 734,4

V=725-2.79T (2) where D is the current density in milli-amperes per square inch, V is the voltage gradient in volts per inch, and T is the operating temperature in degrees centigrade.

These relationships are'shown on the chart of Figure 4, and have been ascertained as a result of a series of tests upon many types of carbon-composition resistor bodies.

Average results of these tests are shown in Figures 2 and 3.

Referring to Figure 2, a number of different resistor bodies were subjected to varying current densities at operating temperatures of 80 C. After 200 hours the change in resistance value was measured in each case. With increased current density, the resistance value suffered a correspondingly greater change, but up to a current density of 120 milli-amperes per square inch no change of resistance value above 4% was noted. Similarly, resistor bodies were operated for 200 hours at operating temperatures of 100 C. Again, it was observed that increased current densities brought about correspondingly greater changes in resistance value. However, changes in resistance value above 4% were relatively rapid when current densities exceeding approximately 92 milliamperes per square inch were employed. In the same way, Figure 2 shows how resistor bodies tested at various higher operating temperatures were caused to suffer correspondingly greater changes in resistance values within the range of the current.densities indicated. For example, resistors operating at 170 C. will undergo less than a 4% change in resistance value (after 200 hours) only if the current density does not exceed approximately 26 milli-amperes per square inch.

The practical significance of these results is obvious. For example, by reading Figure 2, it may be predicted that a resistor operating at 150 C. and subjected to a current density greater than about 31 milli-amperes per square inch will not stand up, since its resistance will change more than 4% after 200 hours, and Figure 1 has shown that resistors which undergo such a change will fail prematurely.

The curves shown in Figure 3 are of similar character. Different resistor bodies were subjected, at different selected operating temperatures, to various voltage gradients up to 600 volts per inch. A reading of Figure 3 indicates, for example, that a resistor operating at 80 C. will suffer a resistance change over 4% (after 200 hours) and will therefore fail prematurely if it is subjected to a voltage gradient exceeding approximately 500 volts per inch; while a resistor operating at 170 C. may not be subjected to a voltage gradient over approximately 245 volts per inch if its change in resistance value at 200 hours is not to exceed 4%.

So far as I am aware, these relationships have never before been studied nor has their existence even been suspected. To the limitations implicit in them, I attribute, in a large measure,the inability of the art heretofore to produce a resistor of high ohmic value capable of operating successfully and continuously over long periods of time at high voltages and under high operating temperatures.

As used in the appended claims, the term high resistance" is intended to refer to an ohmic value in the multi-megohm range, 1. e., to a resistance value higher than 1 megohm; and the term high-voltage is intended to refer to voltages in the multi-kilovolt range, 1. e., to voltages higher than 1,000 volts. While my invention is obviously applicable to resistors of lower ohmic value, for use in circuits of lower voltage, and under operating temperatures below 80 C., no insurmountabl'e difficulties have been heretofore encountered within these lower ranges, and for this reason my invention is primarily useful and valuable in the design and manufacture of carbon-composition resistors intended to embody the higher resistance values mentioned, and intended to operate in the higher voltage circuits and at temperatures exceeding approximately 80 C.

resistors may now be successfully produced regardless of the nature or specific resistance of the composition itself, and regardless of the structural nature of the resistor body, provided only that the dimensions are so chosen that the limitations herein set forth as to current density and voltage gradient (for any contemplated maximum operating temperature) are observed.

In choosing the dimensions to be employed, it should be borne in mind that the resistors may assume any desired shape, and may be designed to provide a path of current travel which is straight, curved, helical, or of any other directional character. To save space and to achieve other manufacturing advantages, it may be found desirable to compose the resistor of separate series-connected segments each of which defines a relatively circuitous current path generally transverse to the resistor as a whole. In limiting the voltage gradient, in accordance with the present invention, the over-all length of current travel is made sufficient in extent, relative to the voltage to be impressed upon the resistor, to keep the volts-per-inch below the maximum allowable value; and in limiting the current density, in

accordance with the present invention, the cross-.

section of the current path (which is preferably uniform throughout) is made sufhcient in area, relative to the current to be carried to keep the milli-amperes-per-square-inch below the maximum allowable value. In case the cross-sectional area may be for some reason or other of nonuniform magnitude, the smallest area governs.

Having thus descrihed my'invention, and illustrated its use, what I claim as new and desire to secure by Letters Patent is:

1. A high-resistance carbon-composition resistor for use in high-voltage circuits under operating temperatures in excess of 80 C., whose di- Such mensions are such that the current density does not exceed the approximate value given by the formula where D is the current density in milli-amperes per square inch, and T is the operating temperature in degrees centigrade.

2. A high-resistance carbon-composition resistor for use in high-voltage circuits under operating temperatures in excess of 0., whose dimensions are such that the voltage gradient does not exceed the approximate value given by the formula where D is the current density in milli-amperes per square inch, V is the voltage gradient in volts per inch, and T is the operating temperature in degrees centigrade.

LEON PODOLSKY.

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