Abstract:
A metal strip resistor includes a resistor body having a resistive element formed from a strip of an electrically resistive metal material and a first termination electrically connected to the resistive element to form a first junction and a second termination electrically connected to the resistive element to form a second junction, the first termination and the second termination formed from strips of electrically conductive metal material. The resistive element, the first termination, and the second termination being arranged mitigate thermally induced voltages between the first junction and the second junction.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/161,636 filed on Mar. 19, 2009 and U.S. Provisional Application Ser. No. 61/169,377 filed on Apr. 15, 2009, both of which are incorporated by reference as if fully set forth. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to resistors. More specifically, the present invention relates to metal strip resistors configured to assist in mitigating the effects of thermal EMF. 
     BACKGROUND OF THE INVENTION 
     Thermal electromotive force (EMF) is a voltage that is generated when two dissimilar metals are joined together. When there are two of these junctions that are of opposite polarity and the temperature of the junctions are equal, there is no net voltage. When one of the junctions is at a different temperature than the other, a net voltage difference can be detected. A resistor may have a metal resistive element connected between copper terminals, thereby providing two junctions and making the resistor susceptible to adverse effects of thermal EMF. 
     Resistors of this construction are often used to sense current by measuring the voltage drop across the resistor. In cases where the current is low, the signal voltage generated across the resistor is also very small and any voltage caused by thermal EMF can cause a significant measurement error. 
     One prior art approach to addressing this problem has been to change the metal alloy used for the resistive element to one with a lower thermal EMF. In some cases this presents other challenges such as increased cost, an increase in bulk resistivity that creates a resistor geometry that is costly to manufacture, or sacrifices other electrical characteristics such as TCR (temperature coefficient of resistance). 
     Another prior art approach has been to add an ASIC (application specific integrated circuit) that is programmed to compensate for the offset voltage created by the thermally induced EMF. Such an approach adds material cost, complexity to the assembly, and manufacturing cost in terms of assembly steps and equipment. 
     What is needed is to provide a resistor that mitigates the effects of thermal EMF while not imposing constraints on the type of metal resistance alloy used. 
     SUMMARY OF THE INVENTION 
     According to one embodiment a metal strip resistor is provided. The metal strip resistor includes a resistor body having at least one resistive element formed from a strip of a resistive metal material, (such as Evanohm, Manganin, or others), and a first termination electrically connected to the resistive element to form a first junction and a second termination electrically connected to the resistive element to form a second junction; the first termination and the second termination being formed from strips of highly electrically conductive metal material, such as copper or others, with high electrical conductivity. Prior art metal strip resistors are described in U.S. Pat. No. 5,604,477 (Rainer et al.). The resistive element, the first termination, and the second termination are arranged to assist in mitigating effects of thermally induced voltages between the first junction and the second junction. The resistor body may include a fold between a first portion of the resistor body and a second portion of the resistor body. A thermoconductive and electrically non-conductive material may be used to thermally connect the first portion of the resistor body to the second portion of the resistor body and assist in reducing the temperature differential between the first junction and the second junction to thereby mitigate the effects of the thermally induced voltages between the first junction and the second junction. 
     According to another embodiment, a metal strip resistor is provided. The metal strip resistor includes a resistor body having a resistive element formed from a strip of a resistive metal material and a first termination joined to the resistive element to form a first junction and a second termination joined to the resistive element to form a second junction; the first termination and the second termination being formed from strips of highly electrically conductive metal material. The resistor body is folded onto itself and mating surfaces are bonded with a thermally conductive and electrically non-conductive adhesive to thereby equalize the temperature between the two sides of the resistor body thus mitigating effects of thermally induced voltages between the first junction and the second junction. 
     According to another embodiment, a metal strip resistor includes a resistor body having a resistive element formed from a strip of a resistive metal material and a first termination joined to the resistive element to form a first junction and a second termination joined to the resistive element to form a second junction; the first termination and the second termination being formed from strips of highly electrically conductive metal material. The resistive element, the first termination, and the second termination are arranged to provide a first temperature gradient along a length of the first junction and a second temperature gradient along a length of the second junction such that the temperatures at any two adjacent points on opposite junctions are substantially equal. 
     According to another embodiment, a method of manufacturing a metal strip resistor includes joining a resistive metal material with an electrically conductive material to form a resistor body with a plurality of junctions between the resistive metal material and the electrically conductive material, folding the resistor body, and bonding the resistor body on one side of the fold to the resistor body on an opposite side of the fold with a thermoconductive and electrically non-conductive adhesive to thereby form a metal strip resistor configured for mitigating effects of thermally induced voltages. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates a metal strip resistor prior to folding; 
         FIG. 2  illustrates a metal strip resistor prior to folding with a dual resistive element; 
         FIG. 3  illustrates the metal strip resistor of  FIG. 1  after folding; 
         FIG. 4  illustrates the metal strip resistor of  FIG. 2  after folding; 
         FIG. 5  is a cross sectional view of the metal strip resistor of  FIG. 3 ; 
         FIG. 6  is a cross sectional view of the metal strip resistor of  FIG. 4 ; 
         FIG. 7  illustrates a resistor with a geometry for mitigating effects of thermally induced voltages by maintaining an equal temperature gradient along each junction thus equalizing the temperature differential across the resistive element at any two adjacent points on opposite junctions; 
         FIG. 8  illustrates another resistor with a geometry for mitigating effects of thermally induced voltages by maintaining an equal temperature gradient along each junction thus equalizing the temperature differential across the resistive element at any two adjacent points on opposite junctions; 
         FIG. 9  illustrates another resistor with a geometry for mitigating effects of thermally induced voltages by maintaining an equal temperature gradient along each junction thus equalizing the temperature differential across the resistive element at any two adjacent points on opposite junctions; 
         FIG. 10A-10D  illustrates another metal strip resistor for mitigating effects of thermally induced voltages; and 
         FIG. 11A-11D  illustrates another metal strip resistor for mitigating effects of thermally induced voltages. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments disclosed herein provide a resistor for mitigating effects of thermal electromotive force (EMF). This allows the use of any number of types of metal resistance alloy regardless of thermal EMF and negates any termination to termination temperature differential. The embodiments disclosed herein achieve desirable results by using appropriate resistor geometries, metal forming, and/or heat transfer materials. 
     Note that, rather than change a resistor&#39;s resistive element material and/or termination material, or add compensation circuitry to offset the thermal EMF of a specific set of resistor metal alloys, the embodiments disclosed herein provide for using a geometry that brings both metallic junctions to the same temperature. In overcoming the problem in this way the embodiments disclosed herein function regardless of the metal alloys used and their specific thermal EMF characteristics. Thus, the embodiments disclosed herein are not limited to particular types of materials and materials may be selected to optimize other electrical characteristics such as TCR, resistance, or stability without concern for the thermal EMF. This is a significant advantage. 
       FIG. 1  illustrates a metal strip resistor  10  with a resistor body  11  prior to folding. The resistor body  11  has a first termination  16  and a second termination  20 . The resistor body  11  includes at least one resistive element  13 . The first termination  16  and the second termination  20  comprise metal strips. The resistive element  13  also comprises a metal strip of a different alloy than the termination metal. The strips are joined to provide for electrical and mechanical connections between the first termination  16  the second termination  20  and the resistive element  13 . A first junction  15  is provided where the first termination  16  is joined to the resistive element  13  and a second junction  17  is provided where the second termination  20  is joined to the resistive element  13 . 
     A fold line  12  is shown at the midpoint which is substantially equidistant between each end of the resistor body  11  and which extends through a mid point of the resistive element  13  such that a first resistive element portion  14  and a second resistive element portion  18  of the resistive element  13  are on opposite sides of the fold line  12 , and such that the first termination  16  and the second termination  20  are on opposite sides of the fold line  12  and the first junction  15  and the second junction  17  are on opposite sides of the fold line  12 . The resistor body  11  is subsequently folded on a line  12  which is substantially equidistant from each end of the resistor body  11 . It is understood that the fold line can be located at various locations along the resistor body other than the midpoint. 
     Prior to folding, one half of what will be the inside of the folded resistor is coated with a material that has good thermal conductivity yet is not electrically conductive (thermally conductive material). The thermally conductive material can also include an adhesive that will bond the two halves of the resistor body together.  FIG. 3  and  FIG. 5  illustrate the resistor after folding and bonding. The resistor body is folded in half onto itself. As shown in  FIG. 5 , there is a gap  22  between the halves. The gap  22  may have a size in the range of 0.001 inch (0.0254 mm) to 0.005 inch (0.127 mm), although the gap may be larger or smaller. The gap  22  is filled with a thermally conductive material or adhesive  30  such as a material which includes an elastomer and a thermally conductive filler. Other thermally conductive materials could be used to achieve the desired objectives of bonding and thermal transfer from one half to the other while electrically insulating one half from the other. 
     By thermally connecting each half of the resistor  10  in this manner the temperature of each of the two copper-to-resistive alloy junctions are held at equal temperatures thus negating any net voltages from the thermal EMF of the junctions. Thus, the thermally conductive material  30  allows heat to be transferred between opposite sides of the resistor so that the first junction and the second junction are held at substantially equal temperatures to thereby mitigate effects of thermal EMF. 
     Another embodiment is shown in  FIGS. 2 ,  4  and  6 . The resistor of  FIGS. 2 ,  4  and  6  is the same as the resistor of  FIGS. 1 ,  3  and  5  except that the resistive element  13  is a dual resistive element such that the first portion  14  is separated from the second portion  18  by a highly electrically conductive metal material  24 . Note that in  FIG. 2  there are junctions  15 A,  15 B on opposite sides of the first portion  14  of the resistive element  13  and there are junctions  17 A,  17 B on opposite sides of the second portion  18  of the resistive element  13 . As best shown in  FIG. 6 , the dual resistive element allows for the conductive material  24  to be in the center of the folding line  12  so that mechanical stress is not induced into the resistive element  13 . This configuration assists in preventing possible resistance problems which may occur if the fold line is through the resistive element. Although this configuration has four junctions  15 A,  15 B,  17 A,  17 B, instead of two, there are opposite junctions at each of the two possible temperatures. Thus, this configuration still results in mitigation of thermal EMF. 
       FIGS. 10A-10D  illustrate another embodiment similar to that shown in  FIG. 1 .  FIG. 10D  illustrates the resistor body  11  prior to folding. Note that the geometry of the unfolded resistor body  11  is similar to the shape in  FIG. 1 , except that the second termination has a notch  26  in its outer edge to assist in folding into the configuration best shown in  FIG. 10B . 
       FIGS. 11A-11D  illustrate another embodiment of a resistor shows a resistor element which uses less welded strip by eliminating the terminal protrusions yet uses the same method of forming and bonding the metal junctions to prevent any junction temperature differentials. 
       FIG. 7 ,  FIG. 8  and  FIG. 9  show other examples of resistor geometries that provide for mitigating effects of thermal EMF associated with junctions, but without using folding. Each is of the metal strip resistor construction. Each of the copper (or other conductor)-to-resistive alloy junctions in any of these designs may have a temperature gradient along the length of each junction caused by any possible temperature differential between the two terminals. As shown in  FIGS. 7 and 8 , the resistor body  11  can include electrically conductive portions that are generally tapered or triangular in shape. Since the temperature gradient along the length of each junction is the same regardless of which side of the resistive element, the temperature at any two adjacent points on opposite junctions is substantially equal, and each junction is of an opposite polarity, thus thermally induced voltages are equal and opposite cancelling each other out. Note that various configurations are contemplated for mitigating thermal EMF in this manner. 
     Therefore, a metal strip resistor for mitigating the effects of thermal EMF has been disclosed. The embodiments disclosed herein provide a resistor for mitigating effects of thermal EMF. The embodiments disclosed herein allow the use of any number of types of metal resistance alloy regardless of thermal EMF and negates any terminal to terminal temperature differential. The embodiments disclosed herein achieve desirable results by using appropriate resistor geometries, metal forming, and/or heat transfer materials. The present invention contemplates numerous variations, options, and alternatives including variations in the geometry used, the types of materials used, and others.