CA1219927A - Electrical resistor and method and apparatus for making same - Google Patents
Electrical resistor and method and apparatus for making sameInfo
- Publication number
- CA1219927A CA1219927A CA000458388A CA458388A CA1219927A CA 1219927 A CA1219927 A CA 1219927A CA 000458388 A CA000458388 A CA 000458388A CA 458388 A CA458388 A CA 458388A CA 1219927 A CA1219927 A CA 1219927A
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- CA
- Canada
- Prior art keywords
- plate
- notches
- metal plate
- electrical
- resistor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01C—RESISTORS
- H01C3/00—Non-adjustable metal resistors made of wire or ribbon, e.g. coiled, woven or formed as grids
- H01C3/10—Non-adjustable metal resistors made of wire or ribbon, e.g. coiled, woven or formed as grids the resistive element having zig-zag or sinusoidal configuration
- H01C3/12—Non-adjustable metal resistors made of wire or ribbon, e.g. coiled, woven or formed as grids the resistive element having zig-zag or sinusoidal configuration lying in one plane
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- Engineering & Computer Science (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Apparatuses And Processes For Manufacturing Resistors (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The electrical resistor of the present invention 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 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.
The electrical resistor of the present invention 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 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.
Description
,ll. . ~ ( ~ '7 BACKGROUND OF THE INVENTION P
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 cylinderO
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 labor intensive.
The number of parts in such resistors i5 usually six (two end caps, two leads, one cera~ic core, and one resistance wire). In addition to this, a coating of some sort must be provided.
Therefore, a primary 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 coefficient.
A further object o the present invention is the provision of a resistor which can be manufactured with a ~ reduction in the amount of labor required.
A further object of the present invention is the provision of a resistor which has a reduced number of parts therein.
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 cylinderO
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 labor intensive.
The number of parts in such resistors i5 usually six (two end caps, two leads, one cera~ic core, and one resistance wire). In addition to this, a coating of some sort must be provided.
Therefore, a primary 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 coefficient.
A further object o the present invention is the provision of a resistor which can be manufactured with a ~ reduction in the amount of labor required.
A further object of the present invention is the provision of a resistor which has a reduced number of parts therein.
-2-. ~ "~
, c) ~ 7 A further object ~f the present invention is the provision of a resistor which ena~les the production e~uipment to be fully automated and ~ompletely computer controlled. ~, A further object of the present invention is the provision of a process for making a resistor, which can utilize an in-line continuous process for manufacture-A further object of the present invention is the provision of a resistor which can be manufactured at a greatly reduced cost.
A further object of the present invention is the provision of a resistor which can be automatically trimmed to very close tolerances with no additional labor costs.
A further object of the present invention is the provision of a resistor which is manufactured by a versatile production process so that a single type of material and production set-up can be used to make resistors with widely varying resista~ce value ranges.
A further object of the present invention is the provision of a process which automatically and naturally ~anneals and stabilizes-the-electrical characteristics o~
the resistors.~ - ~
A further object of the present invention is the provision of a device which is economical to ~anufacture, durable and stable in use, and efficient in operation.
. Il ~' ~
i ~ 7 The present invention utilizes a ~ectangularly 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 staQgered 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 pro-cess 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 -~ater 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.
~2~9~7 ¦ L er beams have been previoasly ured to cut and trim what are referred to in the art as thick film resistors. These resistor6 include a substrate having a metal ilm imprinted l thereon. The thickness of the metal ~lm 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 proces5.
Heretofore, the high powered resistors have conventionally l 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 s~ooth 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.
BRIEF DESCRIPTTON OF THE FIGURES OF THE DRAWINGS
Figure l 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.
-5~
1~Z~9~27 Figure 4 $s a top plan view o~ a m~dified form of the present invention.
Figure 4A is a sectional view taken along line 4A-4A of Figure 4.
5Figure 5 is a plan view of a modified form of the present invention.
Figure 5A is a sectional view taken along line 5A-5A
of Figure 5.
Figure 6 is a plan view of a modified form of the 10present invention.
Figures 6A and 6B are sectional views taken along lines 6A-6A and 6B-6B of Figure 6.
Figure 7 is a plan view of a modified form of the present invention.
15Figures 7A and 7B are sectional views taken along lines 7A-7A and 7B-7B of Figure 7~
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.
Referring to the drawings, particularly Figure 3, 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 1~ by spot welding or by any other convenient process for electrically connecting the leads to the plate.
c, (~
~ 9~2~ i . I
Plate 12 may be comprised of any conductive material, and the preferred material i~ a nickel chromium alloy. Th~
thickness of the metal alloy ca be in the range of 1 mil to 50 mils~ and the preferred range i~ from a 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 traveling 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 farther apart these grooves are from one another, the less the resistance.
As can be seen in Figure 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.
..
~ 27 By placing the slo~ 14 and 16 closer toge~her a~jacent the ends of the resistor, it 15 possible to permit them to dissipate a greater amount o heat adjacent the ends of the resistor while at ~he SaTne time keeping the temperature of the resistor relatively constant across the entire length thereof. Thus, the tendency to form a hot spot adjacent ~he center of the resistor is minimized by the relative spacing of the grooves as shown in Figure 3.
Referring to Figures 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 unaer the trade designation SS-501-3. This machine is a laser welaing system capable of 400 watts average power at up to 200 pulses per second. The maximum energy per pulse is 25 joules. Thè~pulse width variable is from?l~8 m/s to-~ ~ `
, c) ~ 7 A further object ~f the present invention is the provision of a resistor which ena~les the production e~uipment to be fully automated and ~ompletely computer controlled. ~, A further object of the present invention is the provision of a process for making a resistor, which can utilize an in-line continuous process for manufacture-A further object of the present invention is the provision of a resistor which can be manufactured at a greatly reduced cost.
A further object of the present invention is the provision of a resistor which can be automatically trimmed to very close tolerances with no additional labor costs.
A further object of the present invention is the provision of a resistor which is manufactured by a versatile production process so that a single type of material and production set-up can be used to make resistors with widely varying resista~ce value ranges.
A further object of the present invention is the provision of a process which automatically and naturally ~anneals and stabilizes-the-electrical characteristics o~
the resistors.~ - ~
A further object of the present invention is the provision of a device which is economical to ~anufacture, durable and stable in use, and efficient in operation.
. Il ~' ~
i ~ 7 The present invention utilizes a ~ectangularly 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 staQgered 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 pro-cess 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 -~ater 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.
~2~9~7 ¦ L er beams have been previoasly ured to cut and trim what are referred to in the art as thick film resistors. These resistor6 include a substrate having a metal ilm imprinted l thereon. The thickness of the metal ~lm 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 proces5.
Heretofore, the high powered resistors have conventionally l 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 s~ooth 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.
BRIEF DESCRIPTTON OF THE FIGURES OF THE DRAWINGS
Figure l 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.
-5~
1~Z~9~27 Figure 4 $s a top plan view o~ a m~dified form of the present invention.
Figure 4A is a sectional view taken along line 4A-4A of Figure 4.
5Figure 5 is a plan view of a modified form of the present invention.
Figure 5A is a sectional view taken along line 5A-5A
of Figure 5.
Figure 6 is a plan view of a modified form of the 10present invention.
Figures 6A and 6B are sectional views taken along lines 6A-6A and 6B-6B of Figure 6.
Figure 7 is a plan view of a modified form of the present invention.
15Figures 7A and 7B are sectional views taken along lines 7A-7A and 7B-7B of Figure 7~
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.
Referring to the drawings, particularly Figure 3, 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 1~ by spot welding or by any other convenient process for electrically connecting the leads to the plate.
c, (~
~ 9~2~ i . I
Plate 12 may be comprised of any conductive material, and the preferred material i~ a nickel chromium alloy. Th~
thickness of the metal alloy ca be in the range of 1 mil to 50 mils~ and the preferred range i~ from a 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 traveling 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 farther apart these grooves are from one another, the less the resistance.
As can be seen in Figure 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.
..
~ 27 By placing the slo~ 14 and 16 closer toge~her a~jacent the ends of the resistor, it 15 possible to permit them to dissipate a greater amount o heat adjacent the ends of the resistor while at ~he SaTne time keeping the temperature of the resistor relatively constant across the entire length thereof. Thus, the tendency to form a hot spot adjacent ~he center of the resistor is minimized by the relative spacing of the grooves as shown in Figure 3.
Referring to Figures 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 unaer the trade designation SS-501-3. This machine is a laser welaing system capable of 400 watts average power at up to 200 pulses per second. The maximum energy per pulse is 25 joules. Thè~pulse width variable is from?l~8 m/s 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 l laser head 24~ Support table 26 is capable of moving in a f rst direct!ion by means of longitudinal sliding of slides I
L9~7 28. Movement of the work piece in a direction transverse to this first di~ection is caused by virtue of slide blocks 30 ~hich 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 manu~acturing 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 Figure 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 Figure 1. Such computer control systems are known in the art as is exempli-fied by ~nited States patent 4,345,235 dated August 17, 1982.
The computer control system 40 senses the electrical resist-ance 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 cuttin~ 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 _ g _ lZ~ 7 extends inwardly in overlapping and staggered 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, thereb~ increasing the effective resist-ance of the dsvice.
The procedure for cutting the resistance to the desired value includes the following steps:
(a) The resistor is placed in po~ition 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 i5 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 resist-ance, 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 tha 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 resistox could be determined to a fine tolerance.
The resistance tolerances of less than one-half percent are easily obtainable with this method.
~9~27 (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 equlpment (not shown) and moved along in the production llne.
The mounting of leads 18, 2Q is shown to be done by a spot welding process, but other processeæ such as butt weld-ing could be utilized.
Furthermore, after the resistor is complete, it is potted, coated or molded into a protective coating in conven-tional fashion.
One particular advantage which has been found to result from the use 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 stable electrical characteristic. In the absence of this annealing process, the resistor tends to be unstable over ext~nded use, and the resistance value of the resistor will change throughout use. This is parciularly 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 surpris-ingly improved stability over that ob~ained by other means.
~2~2~
¦ Referring to Figure 4, a modified form of the invention is shown and is designated by the numeral 4~. Resisto~ 40 includes leads 1~, 20 similar to the leads 1~/ ~0 5hown in the device lO of Figure 3. Si~ilarly,~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 demon-strated by the cross-sectional view of Figure 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 circui$ry.
Another modified form of the invention is shown in Figure 5 and is designated by the numeral 46. Resistor 46 includes the same components described for the previous modifications including leaas 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-1ike configuration.
Another form of the invention is shown in Figure 6, and is designated by the numeral 48. Resistor 48 incluaes a cylindrical ceramic or molded plastic housing 50 which includes a longitudinal bore 52 extending axially there-through. 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 lO (Figure 3) is placed within bore 52 so that it does not touch housing 50. Then a molding compound such as ommonly useù for hoased resistors, is plsced within ~2~9~,7 the bore s~ that it completely surrounds plate 12 of resistor 10 and protects plate 1~ of resistor 10 from outside influence.
The molding compound is designated by the numeral 54.
The two preferred molding cornpounds ~ use ir~ this process ar~
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 Figure 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 controlledu 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. I
When the part is automatically cut with the laser, as in the present invention, the resistance measuring device is ... .,~... ,. ..... ~ ..... , ..... . I
~LZ~ 7 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 r0sist~r values by using only one type and size o reslstance material, and one production set-up. The co~puter can automatlcally compute the required number o slots and depths o slots so as to make the resistor to the tolerance and resistance value requixed.
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 resist-ance material which later cause the resistance and thestability o the resistor to change with time. The present process automatically assures that the resistor is stable throughout extended periods of use.
Thus, it can be seen that the device accomplishes at least all of its stated objectives.
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 l laser head 24~ Support table 26 is capable of moving in a f rst direct!ion by means of longitudinal sliding of slides I
L9~7 28. Movement of the work piece in a direction transverse to this first di~ection is caused by virtue of slide blocks 30 ~hich 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 manu~acturing 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 Figure 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 Figure 1. Such computer control systems are known in the art as is exempli-fied by ~nited States patent 4,345,235 dated August 17, 1982.
The computer control system 40 senses the electrical resist-ance 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 cuttin~ 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 _ g _ lZ~ 7 extends inwardly in overlapping and staggered 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, thereb~ increasing the effective resist-ance of the dsvice.
The procedure for cutting the resistance to the desired value includes the following steps:
(a) The resistor is placed in po~ition 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 i5 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 resist-ance, 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 tha 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 resistox could be determined to a fine tolerance.
The resistance tolerances of less than one-half percent are easily obtainable with this method.
~9~27 (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 equlpment (not shown) and moved along in the production llne.
The mounting of leads 18, 2Q is shown to be done by a spot welding process, but other processeæ such as butt weld-ing could be utilized.
Furthermore, after the resistor is complete, it is potted, coated or molded into a protective coating in conven-tional fashion.
One particular advantage which has been found to result from the use 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 stable electrical characteristic. In the absence of this annealing process, the resistor tends to be unstable over ext~nded use, and the resistance value of the resistor will change throughout use. This is parciularly 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 surpris-ingly improved stability over that ob~ained by other means.
~2~2~
¦ Referring to Figure 4, a modified form of the invention is shown and is designated by the numeral 4~. Resisto~ 40 includes leads 1~, 20 similar to the leads 1~/ ~0 5hown in the device lO of Figure 3. Si~ilarly,~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 demon-strated by the cross-sectional view of Figure 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 circui$ry.
Another modified form of the invention is shown in Figure 5 and is designated by the numeral 46. Resistor 46 includes the same components described for the previous modifications including leaas 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-1ike configuration.
Another form of the invention is shown in Figure 6, and is designated by the numeral 48. Resistor 48 incluaes a cylindrical ceramic or molded plastic housing 50 which includes a longitudinal bore 52 extending axially there-through. 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 lO (Figure 3) is placed within bore 52 so that it does not touch housing 50. Then a molding compound such as ommonly useù for hoased resistors, is plsced within ~2~9~,7 the bore s~ that it completely surrounds plate 12 of resistor 10 and protects plate 1~ of resistor 10 from outside influence.
The molding compound is designated by the numeral 54.
The two preferred molding cornpounds ~ use ir~ this process ar~
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 Figure 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 controlledu 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. I
When the part is automatically cut with the laser, as in the present invention, the resistance measuring device is ... .,~... ,. ..... ~ ..... , ..... . I
~LZ~ 7 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 r0sist~r values by using only one type and size o reslstance material, and one production set-up. The co~puter can automatlcally compute the required number o slots and depths o slots so as to make the resistor to the tolerance and resistance value requixed.
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 resist-ance material which later cause the resistance and thestability o the resistor to change with time. The present process automatically assures that the resistor is stable throughout extended periods of use.
Thus, it can be seen that the device accomplishes at least all of its stated objectives.
Claims (10)
1. The 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 said opposite ends, the thickness of said plate being sufficient to permit said plate to be self-supporting; using a laser beam to cut completely through the thickness of said plate so as to form staggered and inwardly extending notches in said side edges of said plate to increase the effective electrical resistance of said plate, said laser beam at the same time annealing said metal of said metal plate so that the electrical characteristics of said metal plate are stable after said notches have been cut.
2. A method according to claim 1 and further comprising measuring the electrical resistance of said plate prior to cutting said notches, and electronically calculating the length of said notches and the distance between said notches which will result in a predetermined resistance value for said plate after said notches are cut.
3. A method according to claim 2 and further comprising spacing said notches closer to one another adjacent the opposite ends of said metal plate and further apart from one another adjacent the longitudinal center of said metal plate so as to cause the greatest amount of energy dissipated by said metal plate to occur adjacent said opposite ends thereof.
4. An electrical resistor made by the process of claim 1.
5. An electrical resistor comprising: a flat metal plate having a thickness of from 1 mil to 50 mils and having opposite lateral edges and opposite ends, the thickness of said plate being sufficient to permit said plate to be self-supporting; a pair of electrical leads operatively attached to said flat metal plate at said opposite ends thereof; said metal plate having a plurality of notches extending inwardly from said opposite lateral edges thereof in spaced apart relationship to one another with said slots of one of said opposite sides being staggered with respect to said slots of the other of said opposite sides; each of said slots extending completely through the thickness of said plate and being formed by a laser beam cut so as to anneal the metal of said plate and impart stability to the electrical characteristics of said metal plate.
6. An electrical resistor according to claim 5 wherein said notches adjacent said opposite ends of said plate are spaced apart a distance which is less than the distance between said spaced apart notches adjacent the longitudinal midpoint of said plate.
7. An electrical resistor according to claim 5 wherein said lateral edges of said plates are curved back towards the longitudinal centerline of said plate so as to reduce the outer geometric dimensions of said resistor.
8. An electrical resistor according to claim 5 wherein said flat metal plate is twisted so as to cause said upper and lower surfaces thereof to define a helical surface.
9. An electrical resistor according to claim 5 comprising a dielectric sleeve having a cylindrical outer surface and a longitudinal bore extending axially therethrough, said plate being positioned within said bore with said leads extending outwardly beyond the opposite ends of said bore, a hardened dielectric material surrounding said plate and filling said bore so as to hold said plate within said bore.
10. An electrical resistor according to claim 5 comprising a hardened dielectric material surrounding said plate, said dielectric material being shaped into an elongated body member completely covering said plate with said leads protruding axially outwardly from the opposite ends of said body member.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000458388A CA1219927A (en) | 1984-07-06 | 1984-07-06 | Electrical resistor and method and apparatus for making same |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000458388A CA1219927A (en) | 1984-07-06 | 1984-07-06 | Electrical resistor and method and apparatus for making same |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1219927A true CA1219927A (en) | 1987-03-31 |
Family
ID=4128260
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000458388A Expired CA1219927A (en) | 1984-07-06 | 1984-07-06 | Electrical resistor and method and apparatus for making same |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1219927A (en) |
-
1984
- 1984-07-06 CA CA000458388A patent/CA1219927A/en not_active Expired
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