GB2161327A

GB2161327A - Laser trimmed plate resistor

Info

Publication number: GB2161327A
Application number: GB08416881A
Authority: GB
Inventors: Herman R Person; Douglas E Johnson
Original assignee: Dale Electronics Inc
Current assignee: Dale Electronics Inc
Priority date: 1984-07-03
Filing date: 1984-07-03
Publication date: 1986-01-08
Also published as: DE3425718C2; FR2567311B1; DE3425718A1; FR2567311A1; GB8416881D0

Abstract

An electrical resistor comprises a flat metal plate having a thickness from 8 mils to 50 mils. A pair of electrical leads are operatively attached to the ends of the metal plate and the side edges of the metal plate are provided with a plurality of notches extending inwardly in spaced apart relation to one another, with slots of one of the sides of the plate being staggered with respect to the slots of the other side of the plate. Each of the slots extend completely through the thickness of the plate and are formed by a laser beam cut from a computer controlled laser cutter 22 which anneals the metal of the plate and imparts stability to the electrical characteristics of the metal plate. Once formed, the metal plate may be bent into different shapes to achieve the desired geometric configuration. Furthermore, the plate can be imbedded in molded material so as to provide a protective covering for the plate. <IMAGE>

Description

SPECIFICATION Electrical resistor The present invention relates to high power, low resistance value resistors.

Normally such resistors are wire wound resistors which are made by winding a fine wire made from some resistance material around a ceramic cylinder with the ends of the wire welded to caps which are swaged to each end of the cylinder. Copper leads are normally welded to the caps and extend axially therefrom. Usually such wire wound resistors are then coated with some form of insulating material to protect the resistance wire.

The foregoing method of manufacturing a wire wound resistor requires numerous parts and is very labour intensive. The number of parts in such resistors is usually six (two end caps, two leads, one ceramic core, and one resistance wire). In addition to this, a coating of some sort must be provided.

An object of the present invention is the provision of an improved power resistor which has high stability, relatively low resistance value and a low temperature co-efficient.

The present invention utilises a rectangularly shaped flat plate, having opposite ends and opposite side edges. The plate is preferably metal such as a nickel aluminum alloy, and it is preferably of a thickness from 1 to 50 mils. Electrical leads are spot welded or otherwise attached to the opposite ends of the rectangular plate. Then a laser beam is used to cut a plurality of notches or slots in the side edges of the metal plate. The laser beam cuts completely through the metal plate and the notches are arranged in staggered relationship to one another so as to increase the effective length that a current will have to pass in order to pass from one end of the plate to another.

The process of cutting the resistance material with a laser beam automatically and naturally anneals the resistance material so that the electrical characteristics of the resistor are stable after the part is completely made. Other methods of cutting the resistance material such as sawing, abrading, etc. introduce stresses into the resistance material which later cause the resistance and the stability of the resistor to change with time. The present invention which utilizes the laser beam to cut the slots automatically results in the resistor having substantial stability over extended periods of usage. It is believed that this results from the fact that the laser beam provides a smooth, straight cut, and that the laser beam also anneals the metal during the cutting process.

Laser beams have been previously used to cut and trim what are referred to in the art as thick film resistors. These resistors include a substrate having a metal film imprinted thereon. The thickness of the metal film is usually less than one mil.

These film resistors are of a different type than the high powered, low resistance value resistors commonly manufactured by the wire wound process.

Heretofore, the high powered resistors have conventionally been made by the wire wound technique, because of the difficulty in providing stability to a slotted metal plate having a thickness such as the thickness required for high powered resistors. Normally, this thickness ranges from 8 mils to 50 mils. The present invention, however, is possible because of two results not heretofore recognized, i.e., that the laser beam provides a smooth cut to the groove and further, because the laser beam anneals the metal during the cutting process.

As a result, the resistors made by the present invention are stable throughout extended periods of use.

The invention will now be described further, by way of example with reference to the accompanying drawings, in which Figure 1 is a perspective view of the machine for producing the resistance element of the present invention; Figure 2 is a partial perspective view showing the laser cutting tool in place over a resistance element to be cut; Figure 3 is a plan view of a resistance element which has been cut by the present invention; Figure 4 is a top plan view of a modified form of the present invention; Figure 4A is a sectional view on the line 4A-4A of Fig. 4; Figure 5 is a plan view of a modified form of the present invention; Figure 5A is a section on the line 5A-5A of Fig. 5; Figure 6 is a plan view of a modified form of the present invention;; Figures 6A and 6B are sections on the lines 6A6A and 6B-6B of Fig. 6; Figure 7 is a plan view of a modified form of the present invention; and Figures 7A and 7B are sections on the lines 7A7A and 7B-7B of Fig. 7.

Referring to Fig. 3 of the drawings, the numeral 10 generally designates the electrical resistor of the present invention. Resistor 10 is comprised of a metal plate 12 having a plurality of grooves 14 extending inwardly from one edge thereof and a plurality of grooves 16 extending inwardly from the opposite edge thereof.

A pair of leads 18, 20 are connected to the opposite ends of plate 12 by spot welding or by any other convenient process for electrically connecting the leads to the plate.

Plate 12 may be comprised of any conductive material, and the preferred material is a nickel chromium alloy. The thickness of the metal alloy can be in the range of 1 mil to 50 mils, and the preferred range is from 8 to 50 mils.

The notches 14 and 16 are staggered and alternating with one another. Furthermore, notches 14, 16 extend inwardly so that they are in overlapping relationship with one another. This effectively increases the path which electrical current must follow travelling from lead 18 to lead 20. The resistance of the device can be varied by increasing the depth of cut of the grooves 14 and 16 and also by changing the distances between grooves 14 and 16.

The closer the grooves 14 and 16 are together, the greater the resistance, and the further apart these grooves are from one another, the less the resistance.

As can be seen in Fig. 3, the slots 14, 16 adjacent the ends of the plate 12 are closer together than the slots 14, 16 are adjacent the longitudinal midpoint of plate 12. This causes the resistance to be greater at the opposite ends of the plate. The reason for this is that with power resistors, a hot spot is normally encountered adjacent the longitudinal center of the resistor. The reason for the hot spot in such conventional wire wound resistors is that the heat can be dissipated more easily from the ends of the resistor than it can from the longitudinal center of the resistor.

By placing the slots 14 and 16 closer together adjacent the ends of the resistor, it is possible to permit them to dissipate a greater amount of heat adjacent the ends of the resistor while at the same time keeping the temperature of the resistor relatively constant across the entire length thereof.

Thus, the tendency to form a hot spot adjacent the center of the resistor is minimized by the relative spacing of the grooves as shown in Fig. 3.

Referring to Figs. 1 and 2, a laser cutting machine 22 is shown for forming the resistor of the present invention. Machine 22 is conventional in construction and is typical of laser cutting machines which are commercially available for the purpose of cutting and drilling steel. Typical of such industrial laser cutting machines is a machine manufactured by Ratheon Company, at its Laser Center at 4th Avenue, Burlington, Massachusetts, 01803 under the trade designation SS-501-3. This machine is a laser welding system capable of 400 watts average power at up to 200 pulses per second. The maximum energy per pulse is 25 joules.

The pulse width variable is from 1/8 mis to 3 m/s.

Machine 22 includes a laser head 24 for directing a laser beam at the object to be cut.

Figure 2 shows the support table 26 of the machine which is used to move the work piece relative to the laser head 24. Support table 26 is capable of moving in a first direction by means of longitudinal sliding of slides 28. Movement of the work piece in a direction transverse to this first direction is caused by virtue of slide blocks 30 which are slidably fitted within channels 32. Thus, slides 28 and slide blocks 30 provide an XY axis upon which the work piece can be moved.

The first step in the manufacturing process involves attaching the leads 18, 20 to the plate 12.

Once these are attached, the plate and the leads are secured between slide blocks 30 in the fashion shown in Fig. 2. Attached to the leads are a pair of electrical connectors 34, 36 which are attached to wires 38. Wires 38 lead to a computer control system 40 which is shown in block diagram in Fig. 1.

Such computer control systems are known in the art as is exemplified by United States patent 4,345,235 dated August 17, 1982. The computer control system 40 senses the electrical resistance value between the two leads 18 and 20. The computer control system 40 is then capable of determining the distances between and the depths required for grooves 14 and 16 in order to achieve the desired electrical resistance value for the completed resistor. Once this is calculated by the computer control system, it directs the machine 22 to cut the grooves in accordance with the calculated pattern. The computer control system continues to monitor the resistance of the device throughout the cutting process so that precision can be obtained in the ultimate resistance value of the device.

The laser beam systematically cuts the various notches 14 and 16 in the plate by forming a groove which extends completely through the thickness of plate 12 and which extends inwardly in overlapping and staggering relationship with the grooves formed on the opposite side thereof.

The slots are cut perpendicular to the longitudinal axis of the resistor so that the electrical length of the current path is lengthened, thereby increasing the effective resistance of the device.

The procedure for cutting the resistance to the desired value includes the following steps: (a) The resistor is placed in position on the XY table by automatic parts handling equipment (not shown).

(b) The initial resistance of the resistor is read.

(c) This initial resistance is fed to the computer system 40 where it is compared to the resistance value that is previously programmed into computer 40 and which represents the desired resistance value.

(d) The computer then calculates the required number of slots, the depth of the slots, and the spacings of the slots so as to provide the ultimate desired resistance. For a low value of resistance, there would be relatively few cuts of very short length, whereas for a high value of resistance, there would be many cuts of greater length.

(e) The XY table 26 then begins to move the part under the laser beam so that the laser beam can cut the slots as desired. This would be done at as fast a possible rate until the very last slot is being cut. At that time, the speed of the cutting operation would be reduced so that the resistance of the resistor could be determined to a fine tolerance. The resistance tolerances of less than onehalf percent are easily obtainable with this method.

(f) The resistance value of the resistor is monitored constantly during the cutting process, and at the end when the resistor reaches its proper value in tolerance, the resistor is placed in additional automatic handling equipment (not shown) and moved along in the production line.

The mounting of leads 18, 20 is shown to be done by a spot welding process, but other processes such as butt welding could be utilized.

Furthermore, after the resistor is complete, it is potted, coated or molded into a protective coating in conventional fashion.

One particular advantage which has been found to result from the user of laser beams to cut the grooves 14, 16 is that the laser beam at the same time that it is cutting the groove, also anneals the metal, thereby causing the metal to maintain a sta ble electrical characteristic. In the absence of this annealing process, the resistor tends to be unstable over extended use, and the resistance value of the resistor will change throughout use. This is particularly true if the grooves 14, 16 are provided by some mechanical means such as cutting, sawing or other means. Such mechanical means for producing the resistor have been found to produce highly unstable resistors which do not maintain a constant resistant value throughout use.The laser beam, presumably because of its smooth cutting action, and because of its simultaneous annealing of the metal, has been found to provide a surprisingly improved stability over that obtained by other means.

Referring to Fig. 4, a modified form of the invention is shown and is designated by the numeral 40.

Resistor 40 includes leads 18,20 similar to the leads 18, 20 shown in the device 10 of Fig. 3. Similarly, a metal plate 12 is utilized and is provided with a plurality of grooves 14, 16 extending inwardly from the opposite edges thereof. However, plate 12 is bent into a C-shaped configuration as is demonstrated by the cross-sectional view of Fig.

4A. The side edges 42, 44 are bent inwardly to form the C-shape cross-sectional configuration, thereby reducing the geometric size of the resistor so that it will fit into a confined area within the cir cu itry.

Another modified form of the invention is shown in Flg. 5 and is designated by the numeral 46. Resistor 46 includes the same components described for the previous modifications including leads 18, 20, plate 12, and slots or grooves 14, 16. However, after plate 12 has been provided with grooves 14, 16, it is twisted approximately 180 so as to give the plate 12 a helical or screw- like configuration.

Another form of the invention is shown in Fig. 6, and is designated by the numeral 48. Resistor 48 includes a cylindrical ceramic or molded plastic housing 50 which includes a longitudinal bore 52 extending axially therethrough. Bore 52 is shown to be rectangular in cross-section, but it is also possible that bore 52 could have other cross-sectional shapes such as ovular or circular. Resistor 10 (Fig. 3) is placed within bore 52 so that it does not touch housing 50. Then a molding compound such as commonly used for housed resistors, is placed within the bore so that it completely surrounds plate 12 of resistor 10 and protects plate 12 of resistor 10 from outside influence. The molding compound is designated by the numeral 54.The two preferred molding compounds for use in this process are manufactured by Dow Corning Company, 592 Saginaw Road, Midland, Michigan and are designated as Dow Corning Silicon Molding Compounds under the product numbers 1-5201 and 480.

Referring to Fig. 7, another form of the invention is shown for molding resistor 10. The molding compound designated by the numeral 56 is formed around plate 12 of resistor 10 so that it completely surrounds plate 12 but permits leads 18, 20 to protrude outwardly from the ends thereof. This molded resistor is designated by the numeral 58.

The present invention provides a significant improvement over previous wire wound resistors.

The resistor of the present invention contains only three parts, whereas the number of parts in a wire wound resistor is six, without counting the coating material. The production equipment necessary to produce this newly improved resistor can be fully automated and completely computer controlled.

The resistance material to make the part can be purchased in long,continuous rolls, which can be automatically fed into the equipment along with copper wire for the leads. The automatic equipment can then completely make the part without having any requirement of workers touching the resistor again. The cost of manufacturing the resistor is significantly reduced from the manufacturing process presently used for standard wire wound resistors.

When the part is automatically cut with the laser, as in the present invention, the resistance measuring device is constantly measuring the resistor and this allows the resistor to be trimmed to very close tolerances. The new resistor is versatile in that it can be made to many different resistor values by using only one type and size of resistance material, and one production set-up. The computer can automatically compute the required number of slots and depths of slots so as to make the resistor to the tolerance and resistance value required.

The process of cutting the resistance material with a laser beam automatically and naturally anneals the resistance material so that the electrical characteristics of the resistor are substantially stable after the part is completely made. Other methods of cutting the resistance materials such as sawing, abrading, etc. introduce stresses into the resistance material which later cause the resistance and the stability of the resistor to change with time. The present process automatically assures that the resistor is stable throughout extended periods of use.

Claims

1. A method of creating an electrical resistor comprising attaching a pair of electrical leads to the opposite ends of a flat metal plate having a thickness of from 1 mil to 50 mils and having opposite lateral edges extending between the opposite ends, using a laser beam to cut completely through the thickness of the plate so as to form staggered and inwardly extending notches in the side edges of the plate to increase the effective electrical resistance thereof, the laser beam at the same time annealing the metal of the metal plate so that the electrical characteristics of the metal plate are stable after the notches have been cut.

2. A method as claimed in claim 1, in which the electrical resistance of the plate is measured prior to cutting the notches and electronically calculating the length of the notches and the distance between the notches which will result in a predetermined resistance value for the plate after the notches are cut.

3. A method as claimed in claim 2, in which the notches are spaced closer to one another adjacent the opposite ends of the metal plate and further apart from one another adjacent the longitudinal center of the metal plate so as to cause the greatest amount of energy dissipated by the metal plate to occur adjacent the opposite ends thereof.

4. An electrical resistor comprising a flat metal plate having a thickness from 1 mil to 50 mils and having opposite lateral edges and opposite ends, a pair of electrical leads operatively attached to the flat metal plate at the opposite ends thereof, the metal plate having a plurality of notches extending inwardly from the opposite lateral edges thereof in spaced apart relationship to one another with the slots of one of the opposite sides being staggered with respect to the slots of the other of the opposite sides, each of the slots extending completely through the thickness of the plate and being formed by a laser beam cut so as to anneal the metal of the plate and impart stability to the electrical characteristics of the metal plate.

5. An electrical resistor as claimed in claim 4, in which the notches adjacent the opposite ends of the plate are spaced apart a distance which is less than the distance between the spaced apart notches adjacent the longitudinal midpoint of the plate.

6. An electrical resistor as claimed in claim 4 or 5, in which the lateral edges of the plate are curved back towards the longitudinal centerline of the plate so as to reduce the outer geometric dimensions of the resistor.

7. An electrical resistor as claimed in claim 4, 5 or 6, in which the flat metal plate is twisted so as to cause the upper and lower surfaces thereof to define a helical surface.

8. An electrical resistor as claimed in any of claims 4 to 7, in which a dielectric sleeve has a cylindrical outer surface and a longitudinal bore extends axially therethrough, the plate being positioned within the bore with the leads extending outwardly beyond the opposite ends of the bore, a hardened dielectric molding material surrounding the plate and filling the bore so as to hold the plate therewith.

9. An electrical resistor as claimed in any of claims 4 to 8, in which a hardened molding compound surrounds the plate, the molding compound being shaped into a cylindrical shape and completely covering it with the leads protruding axially outwardly from the opposite ends of the cylindrically shaped molding compound.

10. An apparatus for creating an electrical resistor, comprising a frame, a support on the frame for holding a plate of electrical resistance material, first circuity connected to the support for measuring the resistance of the plate, computer circuitry connected to the first circuitry and capable of receiving in memory a proposed resistance value for the plate, the computer circuitry being capable of computing the effective electrical length of the plate to create the proposed resistance value, a laser element on the frame adjacent the support for projecting a cutting laser beam at the plate for cutting staggered notches in opposite side edges thereof, a co-ordinate control means on the frame for moving the laser element on x and y co-ordinates to cut notches of various depths and at varied spacing in opposite edges of the plate, the computer circuitry being operatively connected to the laser element and to the co-ordinate control means to cut staggered spaced apart notches in opposite edges of the plate to create an effective electrical length in the plate to give it an electrical resistance equal to the proposed resistance value.

11. A method of creating an electrical resistor substantially as herein described.

12. An electrical resistor constructed and arranged to operate substantially as herein described with reference to and as illustrated in any of the figs. of the accompanying drawings.

13. An apparatus for creating an electrical resistor substantially as herein described.