US3509511A

US3509511A - Electrical resistor

Info

Publication number: US3509511A
Application number: US753969A
Authority: US
Inventors: Steven Soroka
Original assignee: TRW Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1968-08-20
Filing date: 1968-08-20
Publication date: 1970-04-28
Anticipated expiration: 1987-04-28

Description

A ril 28, 1970 Filed Aug. 20', 1968' s. sRoKA 3,509,51 1

ELECTRICAL RESISTOR 2 Sheets-Sheet l FIG. 6

INVENTOR STEVE/V SORO/(A ATTORNEY United States Patent O 3,509,511 ELECTRICAL RESISTOR Steven Soroka, Willow Grove, Pa., assignor to TRW Inc., Philadelphia, Pa., a corporation of Ohio Filed Aug. 20, 1968, Ser. No. 753,969 Int. Cl. Hlllc 7/00 US. Cl. 338-308 7 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND In the manufacture of discrete electrical resistors of the type comprising a cylindrical substrate of an electrical insulating material having a film of an electrical resistance material coated on the surface thereof, one problem is obtaining resistors of a desired resistance value. Since the resistance value of such a resistor is a function of the width and length of the path of the resistance material film, it has been the practice to obtain a desired resistance value by adjusting the width and length of the resistance material path.

One method generally used for adjusting the resistance value of a film type resistor by varying the width and length of the resistance material path, a method commonly called spiralling, comprises cutting a fine groove through the resistance film which groove extends helically around the substrate, This cuts the resistance material film into a narrow, long path which extends helically around the substrate so as to increase the resistance value of the resistor. The width and length of the helical resistance material path depends on the pitch of the helical groove and the number of turns of the groove. By measuring the resistance value of the resistor as the groove is being cut, the spiralling can be stopped when the desired resistance value is reached. Although this method of adjusting the resistance value of a resistor permits the achievement of very accurate resistance values, it has the disadvantage that is is difficult to achieve only very small increments of adjustment particularly when adjusting resistors having very low resistance values.

SUMMARY It is an object of the present invention to provide a novel construction of a film type electrical resistor.

It is another object of the present invention to provide a novel construction of a film type electrical resistor which is easily adjusted as to its resistance value by small increments.

It is a further object of the present invention to provide a construction of a film type electrical resistor which can provide a resistor of low resistance value and can be adjusted as to its resistance value by small increments.

It is a still further object of the present invention to provide an electrical resistor which includes a film of a resistance material coated on the cylindrical surface of a cylindrical substrate and which is adjusted as to its resistance value by small increments by a plurality of narrow grooves in the resistance film with the grooves being arranged in spaced relation along a line extending in a helical path around and along the cylindrical substrates.

Other objects will appear hereinafter.

BRIEF DESCRIPTION OF DRAWING For the purpose of illustrating the invention, there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a perspective view of a resistor of the present invention.

FIGURE 2. is a sectional view taken along line 2-2 of FIGURE 1.

FIGURE 3 is a plan view of the surface of the resistor of FIGURE I laid out fiat.

FIGURE 4 is an end view of an apparatus for forming the resistor of the present invention shown in FIGURE 1 in one position of the apparatus.

FIGURE 5 is a view similar to FIGURE 4 but showing another position of the apparatus.

FIGURE 6 is a top view of the apparatus shown in FIGURE 4.

FIGURES 7 through 10 inclusive are schematic views of an apparatus for forming a modification of the resistor of the present invention in different positions of the apparatus.

FIGURE 11 is a plan view of the surface of the modification of the resistor of the present invention with the surface laid out fiat.

FIGURES 12 through 17 inclusive are views similar to FIGURE 11 of still further modifications of the resistor of the present invention.

DESCRIPTION OF INVENTION Referring initially to FIGURES 1 and 2, the resistor of the present invention is generally designed as 20. Resistor 20 comprises a cylindrical substrate 22 of an electrical insulating material, such as glass, ceramic, or a plastic. A film 24 of an electrical resistance material is coated on the cylindrical surface of the substrate 22. The resistance material of the film 24 may be well known resistance material such as carbon or a metal either alone or in a binder, such as a plastic, ceramic, or glass. Termination films 26 of an electrically conductive metal are coated on the ends of the substrate 22 and extend over the cylindrical surface of the substrate so as to be electrically connected to the resistance film 24. Terminal wires 28 are secured to the end of the substrate 22 and are electrically connected to the termination films 26.

To adjust the resistance values of the resistor 20, a plurality of elongated narrow grooves 30 are cut through the resistance film 24 to the substrate 22. The grooves 30 are arranged so that they all extend along a line which extends helically around and along the substrates 22 and are spaced apart along this helical line. As shown, the grooves 30 are positioned in spaced apart, side-by-side relation along the length of the substrate. The resistance value of a film type electrical resistor is equal to the resitivity of the resistance material in ohms per square times the number of squares of the resistance film. The number of squares of the resistance film is equal to the electrical length of the resistance film divided by the electrical width of the resistance film. As shown in FIGURE 3, which shows the resistance film 24 unrolled and laid out fiat, the electrical length of the resistance film is the distance between the termination films 26, and the width of the resistance film is the circumference of the substrate. The grooves 30 in the resistance film 24 provide small area of the resistance film between the grooves which are in essence electrically isolated. This does not change the electrical length of the resitsance film, but each of the isolated areas reduces the electrical width of the film so as to increase the resistance value of the resistor 20. Since the insolated areas are small, the incerase in resistance created by each of the isolated areas is small. Thus by controlling the number of grooves 30 in the resistance film 24 in the manner which will be explained, the resistance value of the resistor can be adjusted upwardly in small incremental steps until a desired resistance value is obtained.

One method of forming the grooves 30 in the resistance film 24 is by spiralling machine of the type shown in United States Letters Patent No. 2,724,306 to V. A. Woodell, issued Nov. 22, 1955 entitled Automatic Spiralling Machine For Making Electrical Resistors" which machine is molded in the manner shown in FIGURES 4-6 inclusive. In general, the spiralling machine comprises a thin cutting wheel 32 mounted for rotation about its center. The electrical resistor 20 is supported adjacent the edge of the cutting wheel 32 with the longitudinal axis of the resistor being parallel to the axis of rotation of the cutting wheel. The resistor 20 is supported so that it is rotatable about its own longitudinal axis and at the s m time movable longitudinally across the edge of the cutt ng wheel. Also, the resistor is movable radially of the wheel between a position in which it is engaged by the cutting wheel and a position away from the cutting wheel. A cam plate 34 is fixed in a stationary position relative to the periphery of the cutting wheel 32. A cam 36 is mounted to rotate about an axis coincident with the axis of rotation of the resistor 20 and rotates at the same speed as the resistor. A spring 38 holds the cam 36 in contact with the cam plate 34 but allows the axis of the cam to move toward and away from the cam plate according to the shape of the cam. The cam 36 is either connected directly to the drive for the resistor or is connected by suitable linkage to the support for the resistor 20 so that periodic contact of the cam 30 with cam plate 34 moves the resistor 20 away from the cutting wheel 32 in a like periodic fashion. The cam 36 is shaped to have a low portion and a high portion.

In the use of the spiralling machine to adjust the re sistance value of the resistor 20, when the low portion of the cam 36 is facing toward the cam plate 34, the resistor 20 engages the cutting wheel 32 so as to cut a groove in the resistance film 24 (see FIGURE 4). Since the resistor 20 is moving longitudinally as well as rotating, groove 30 will follow helical line. When the high portion of the cam 36 comes into contact with the cam plate 34, the resistor 20 is moved away from the cutting wheel 32 so as to stop the cutting of the groove 30 (see FIGURE When the cam 36 rotates so that the low portion of the cam again faces the cam plate 34, the resistor is brought back into contact with the cutting wheel 32 to cut a groove 30 in the resistance film 24. Since the resistor is moving longitudinally, the second groove 30 is spaced longitudinally along the resistor from the first groove 30 and extends along the same helical line as the first groove. Thus with each revolution of rotation of the resistor 20 and the cam 36 a groove 30 is cut into the resistance film 24 with the grooves 30 being spaced longitudinally along the resistor and all extending along the same helical line. As each groove 30 is cut through the resistance film, the resistance value of the resistor 20 is increased by a small increment for the reason stated above. By continuously measuring the resistance value of the resistor 20 the operation can be stopped when a desired resistance value is reached.

Instead of using a spiralling machine having a cutting wheel to form the grooves 30 in the resistance film 24 of the resistor 20, any other well-known type of spiralling machine can be used. For example, US. Letters Patent No. 2,743,554 to E. G. Dailey, issued May 1, 1956, en titled Apparatus for Blast Etching Electrical Devices described a spiralling machine which uses a fine blast of sand to cut a helical groove in the resistance film of a resistor to adjust the resistance value of the resistor This type of spiralling machine can be used to make the resistor 20 of the present invention by providing a control which properly starts and stops the flow of the sand blast during each revolution of rotation of the resistor. Also, U.S. Letters Patent No. 2,710,325 to S. A. Johnson, issued June 7, 1955, entiled Method and Apparatus for Making Electrical Resistors describes a spiralling machine which uses an electric arc to form a helical groove in the resistance film of a resistor. This type of spiralling machine can be used to make the resistor 20 of the present invention by either providing the machine with the camming means of FIGURES 4-6 to move the resistor into and out of engagement with the arc forming stylus or providing a suitable control for starting and stopping the flow of electrical current to the stylus.

Referring to FIGURE 11 there is shown in flat, rolled out form a resistor 20a of the present invention having two rows of the grooves 30a with all of the grooves extending along the same helical line. By having two rows of the grooves each increment of change in the resistance value of the resistor created by each groove is smaller so that a desired resistance value can be obtained with cl ser tolerance. The resistor 20a having two rows of grooves 30a can be formed by means of a substantially elliptical cam 36a as shown in FIGURES 7-10 inclusive. The elliptical cam 36:: has two low portions and two high portions. Whenthe low portions of the cam 36a oppose the cam plate 34, as shown in FIGURES 7 and 9, the resistor 20a engages the cutting wheel 32 so as to cut a groove 30a in the resistance film 24a. When the high portions of the cam 36a engage the cam plate 34 as shown in FIGURES 8 and 10, the resistor 20a is moved away from the cutting when so as to interrupt the cutting of the grooves. Thus, during each revolution of rotation of the cam 36a and the resistor 20a two grooves 30a are cut into the resistance film 24a so as to provide the two rows of the grooves.

Referring to FIGURE 12, there is shown in flat, rolled out form a resistor 20b of the present invention having three rows of grooves 30b in the resistance film 24b. The resistor 20b is formed by providing three cuts in the resistance 2412 during each revolution of the resistor. This can be achieved by using a cam having three low points and three high points. The resistor of the present invention can be similarly provided with more than three rows of the grooves.

FIGURE 13 shows in fiat, rolled out form a resistor 200 of the present invention in which the grooves 30c in the resistance film 240 are arranged in staggered relation along the length of the resistor. However, the grooves 300 are all spaced along a line which extends helically around and along the resistor. This staggered relation of the grooves 30c can be obtained by a spiralling machine in which the timing of the cuts is varied during each revolution of rotation of the resistor. For example, if the grooves 30c are being cut by a spiralling machine such as shown in FIGURES 46, the cam 36 would be rotated against a suitably contoured cam plate allowing a high point of a cam to strike incrementally earlier or later each revolution. If the grooves 300 are being cut in the resistance film 24c from left to right as viewed in FIG- URE 13, the cam plate would be contoured to have a downwardly tapered upper edge from right to left so that each cut would be made later each revolution. Instead of contouring the cam plate, the staggered grooves could be obtained by having the cam rotate at a speed different from the speed of rotation of the resistor.

FIGURE 15 shows a resistor 20d of the present invention having the grooves 30d in staggered relation similar to the resistor 20c of FIGURE 13 except that there are more than one groove cut in the resistance film 24d during each revolution of rotation of the resistor. FIG- URE 14 shows a resistor 20e of the present invention similar to the resistor 20d shown in FIGURE 15 except that the grooves We in the resistance film 24s are staggered in one direction over one-half of the length of the resistor and staggered in the opposite direction over the other one-half of the resistor. This arrangement of the grooves can be obtained by using a cam having a plurality of high points and low points and either a suitably contoured cam plate or by suitably varying the speed of rotation of the cam.

FIGURES 16 and 17 show resistance 20 and 20g respectively of the present

invention having grooves

30 and 30g in the resistance films 24 and 24g respectively which vary in length. In the resistor 20 shown in FIG- URE 16 the grooves 30 are relatively short at the ends of the resistor and gradually increase in length toward the center of the resistor so that the grooves at the center of the resistor is of the longest length. This provides a resistor having a varying current density along the length of the resistor with the highest current density being at the center of the resistor. This type of resistor can be used as a heater resistor. In the resistor 20g shown in FIGURE 17, the grooves 30g at the ends of the resistor are of the largest length and the length of the grooves decreases to the shortest grooves at the center of the resistor. This provides a resistor having high current densities at the ends of the resistor so as to provide a uniform heat across the entire length of the resistor. The resistors 20 and 20g can be formed by a spiralling machine with a cam having a single high-point and a cam plate Which is contoured so that the cam engages the cam plate for varying lengths of time during each revolution of the rotation of the resistor.

Although various arrangements of the grooves in the resistor of the present invention have been shown, it should be understood that any desired arrangement of the grooves can be used. However, no matter what arrangement of the grooves are provided, the grooves in the resistance film must be spaced along the length of the resistance film and must be positioned in spaced relation along a line which extends helically around and along the resistor.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. An electrical resistor comprising a cylindrical substrate of an electrical insulating material, a film of an electrical resistance material coated on the cylindrical surface of said substrate, and a plurality of discrete unconnected elongated grooves in said resistance film, said grooves being spaced along said substrate and extending along a line which extends helically around and along said substrate, the length of each groove being less than the circumference of the substrate.

2. An electrical resistor in accordance with claim 1 in which the grooves are arranged in a longitudinal row along the substrate with the grooves being in spaced apart side-'by-side relation.

3. An electrical resistor in accordance with claim 1 in which the grooves are in circumferentially staggered relation.

4. An electrical resistor in accordance with claim 1 in which the grooves are of non-uniform lengths.

5. An electrical resistor comprising a cylindrical substrate of an electrical insulating material, a film of an electrical resistance material coated on the cylindrical surface of said substrate, and a plurality of narrow elongated grooves in said resistance film, said grooves being spaced along said substrate and extending along a line which extends helically around and along said substrate, said grooves being arranged in a plurality of longitudinal rows along the substrate with the grooves in each row being in spaced side-by-side relation.

6. An electrical resistor comprising a cylindrical substrate of an electrical insulating material, a film of an electrical resistane material coated on the cylindrical surface of said substrate, and a plurality of narrow elongated grooves in said resistance film, said grooves being spaced along said substrate and extending along a. line which extends helically around and along said substrate, the grooves at the ends of the substrate being the shortest in length and the length of the grooves progressively increasing to a groove of longest length at substantially the center of the substrate.

7. An electrical resistor comprising a cylindrical substrate of an electrical insulating material, a film of an electrical resistance material coated on the cylindrical surface of said substrate, and a plurality of narrow elongated grooves in said resistance film, said grooves being spaced along said substrate and extending along a line which extends helically around and along said substrate, the grooves at the ends of the substrate being the longest in length and the length of the grooves progressively decreasing to a groove of the shortest length at substantially the center of the substrate.

References Cited UNITED STATES PATENTS