US2724759A - Precision wire wound resistors - Google Patents

Description

v- 22, 1955 J. DANIELS 2,724,759

PRECISION WIRE WOUND RESISTORS Filed March 4, 1954 2 Sheets-Sheet 1 FuJ/b/e Mefa/Sfr/p ,4 (0077/79 3 INVENTOR.

1'' BY a) M 57 L ATTORNEYS Nov. 22, 1955 J. L. DANIELS 2,724,759

PRECISION WIRE WOUND RESISTORS Filed March 4, 1954 2 Sheets-Sheet 2 Joseph L.D n' s BY MSW it ATTORNEYS United States Patent PRECISION WIRE WOUND RESISTORS Joseph L. Daniels, Levittown, N. Y., assignor to Vari-Ohm Corp., Amityville, N. Y., a corporation of New York Application March 4, 1954, Serial No. 414,097

17 Claims. (Cl. 201-48) The present invention relates to wire wound precision resistors and to apparatus utilizing such resistors.

Inaccuracies which arise from slight variations in the resistivity and diameter of the resistance wire, variations in the dimensions of the forms upon which the wire is wound, and other factors make it commercially impractical to wind wire resistors to a fixed number of turns on commercially produced forms, if a precision appreciably greater than :l% is desired.

The manufacture of resistors of the wire wound type to achieve a higher degree of precision than :l% has heretofore involved a final adjustment by the careful removal of excess resistance wire, the excess wire being purposely included in the resistor in order to provide a range of adjustability.

An object of the present invention is to provide means for adjusting a wire wound resistor to its correct nominal value with an extremely high degree of precision, such adjustment being made after assembly of the completed resistor and without the necessity for winding or unwinding any resistance wire.

A further object of the invention is' to provide means for the precise adjustment of the resistance value of a relatively inaccurate resistor wound in accordance with conventional production methods.

A further object of the invention is the provision of a variable resistor, potentiometer, or similar apparatus in which the relationship between the angular position of a shaft and the resistance values included between certain terminals of the apparatus is determined with an unusually high degree of precision.

Still another object of the invention is to provide a potentiometer; variable resistor, or similar device in which a resistance value is accurately related to the angular position of a rotatable shaft, the accuracy being obtained by the adjustment of a vernier resistor, such adjustment being made' by increasing the resistance value of the vernier resistor, the vernier resistor being added to the basic adjustable resistor structure after a preliminary approximate adjustment of the basic resistor structure.

"A feature of the invention resides in the fact that adjustability is obtained without the use of movable taps.

Other and further objects and advantages of the invention will become apparent upon reading the following specification together with the accompanyingdrawing forming a part hereof. 7

Referring to the drawing: I

Figure l is an enlarged diagrammatic view showing means for permanently adjusting a wire wound" resistor without the winding or unwinding of any resistance wire.

Figure 2 is a transverse sectional view taken along the line 2-2 of Figure 1, looking in the direction of the arrows.

Figure 3 illustrates means for obtaining a more precise adjustment than is obtainable using the arrangement of Fig. 1.

Figure 4 illustrates an arrangement for the precise adice justment of a relatively inaccurate commercial resistor by means of a small vernier resistor of relatively low resistance.

Figure 5 is a circuit diagram of a precision potentiometer using the arrangements illustrated in Figs. 1 to 3.

Figure 6 is a rear perspective view of the potentiometer shown in Fig. 5, the potentiometer being broken away to illustrate details of construction.

Referring to Figs. 1 and 2, a resistance wire 10 is shown helically wound on a core 11 of suitable insulating material. At the left hand portion of core 11, a strip of electrically conductive fusible metal 12 is disposed in electrical contact with a plurality of adjacent turns of resistance wire 10 under the convolutions 13 of the resistance wire 10, effectively short circuiting these turns and making the total resistance of the entire winding substantially equal to the resistance of the convolutions 14 of the right hand end of the resistor beyond the right hand end 15 of the fusible metal strip 12.

The fusible metal strip 12 is formed of metal having a melting point lower than the melting point of the resistance wire 10. Advantageously, a corrosion resistant metal such as gold or a gold alloy may be used for the metal strip 12, in order that the electrical contact between the metal strip 12 and the convolutions 13 of resistance wire 10 may remain permanent, constant, and unaffected by atmospheric conditions. The width and the thickness of the fusible metal strip 12 will be determined by the maximum current it is required to carry. Where the maximum current is very small, gold leaf or its equivalent may be used.

The finished resistor is initially manufactured and arranged to have a smaller resistance than the value desired. In order to increase the resistance to the correct value, current is caused to flow through the left hand portion 13 of the resistor in which the fusible metal strip 12 is located. By means of suitable contacts (not shown) the current may be confined to any'desired portion of the resistor. By the use of a very low voltage to produce the current flow, the current will drop as soon as a single turn or single additional turn has been unshunted by the melting of the fusible metal strip 12 between two adjacent convolutions of the resistance wire 10. Experience will indicate with certainty how many additional turns of resistance wire are required to bring the resistance to the correct nominal value, and the metal strip 12 may then be fused accordingly so that the shunting action of strip 12 is eliminated for the correct number of turns.

From the foregoing, it will be seen that the adjustment may be made to bring the resistance value of the resistor incrementally to the nearest resistance value attainable by the inclusion or omission of a full turn of resistance wire 10. Where the turn resistance is small compared with the limits of tolerance, satisfactory results may be achieved using a single resistor as shown in Fig. 1.

Where a greater precision is required, the arrangement shown in Fig. 3 is utilized. In Fig. 3, a fixed resistor 20 is connected in multiple with an adjustable or vernier resistor .21 the vernier resistor 21 having a fusible metal strip 22 similar to the strip 12 of Figs. 1 and 2 which may be used for progressively increasing its resistance. Let it be assumed that the adjustable resistor 21 has. aresistance value of x and that the fixed resistor 20 has a resistance value of y, the resistors 20 and 21 being connected in parallel to terminals 23 to produce a combined resistance value of R. According to the conventional formula for two resistors connected in parallel:

Let it be assumed that the resistance x of the adjustable resistor 21 is made several times greater than the resistance value y of the fixed resistor 20, such that:

a; F'y, or y 5 Then substituting into the expression the following expression is obtained in which y has been From this last expression, it will be seen that if F is made fairly large, say 99, so that the resistance value x of resistor 21 is 99 times the resistance value y of resistor 20, then dR/a'x becomes A This means that a 1% change in the value of x will produce a of 1% change in the total resistance value R measured at terminals 23.

If a final precision of of 1% is desired for the value of R, then a value of 19 for F is required if the value of x is to be adjusted only to the nearest 1%. These figures are merely illustrative and serve to show that by proper proportioning of the resistance values and the precisions of the resistance values y and x of the resistors 20 and 21, respectively, that by adjustment of the resistance value x of the vernier resistor 21 to the nearest turn, a much higher degree of precision is obtainable in the combined resistance value R than in the precision of adjustment of the vernier resistance value x.

It should be further noted, that both resistors 20 and 21 may both be made adjustable by means of a fusible metal strip 22 in each resistor, the lower resistance value y of resistor 20 being first adjusted to the nearest turn, 0 whereafter the resistance value x of resistor 21 is adjusted to the nearest turn. Where a greater degree of precision is desired, three or more resistors of progressively increasing resistance values may be connected in multiple, and the first adjustment may then be made by adjusting the resistor of lowest resistance to the nearest turn so that the combined resistance R is just under the desired final value. The resistor of next higher resistance value is then adjusted so that the value of R is still just under the desired value but with a greater degree of precision than in the original adjustment. This may then be followed by an adjustment of the third resistor, etc. each successive adjustment increasing the combined resistance value R by successively smaller increments until the desired resistance value for R has been attained a with the desired degree of precision.

The resistance wire 20 may be formed of any suitable material having due regard for its resistivity and temperature eoefiicient of resistance.

The fusible metal strip 12 in Fig. l or the fusible metal strip 22 in Fig. 2, is preferably formed of a suitable metal or other electrically conductive material having a melting point lower than that of the resistance wire 10. In this manner, resistance wire 10 may carry a current sufiicient to fuse the metal strip 12 or 22 before the resistance wire might become fused. By the use of suitable contact members (not shown), lateral contact may be made with adjacent turns of resistance wire 20 to fuse the metal strip therebetween without passing current longitudinally along the resistance wire, in which case the heat dissipation factors may be so arranged that the relative melting points of the resistance wire and of the fusible metal strip are of comparatively minor importance.

As an example, the metal strip 12 may be formed of a gold-copper alloy having a melting point in the range from 900950 C. such as an alloy containing Au 92%, the balance being Cu. The resistance wire 20 may be a nickel-chromium alloy containing Ni Cr. 20%, sold under the commercial designation Nichrome, and having a melting point in the range l300-l400 C.

Figure 4 illustrates the adjustment of a commercial resistor of low precision to a precise value by means of a small vernier resistor of relatively low resistance. The fixed resistor 20 is produced according to any desired conventional method, its precision being of relatively little importance. The fixed resistor 20 is provided with terminals 24 and 25 and is tapped at 26 near terminal 24. A relatively small resistance y, is included between terminal 24 and tap point 26. A relatively large resistance Y, is included between tap point 26 and terminal 25.

The vernier resistor 21 is wire wound and is provided with terminals 27 and 28. Initially, the fusible strip 22 may extend throughout the entire length of the vernier resistor 21. The adjustable resistance value x of the vernier resistor is connected in multiple with the small fixed resistance value y of the fixed resistor 20. The fixed resistor 20 will always have a total resistance value y-j-Y greater than the desired final precise resistance value R measured between terminals 23. The resistance value y is shunted by the adjustable vernier resistance value x and will initially bring the total resistance value R to a value less than the desired precise value. By progressively fusing the fusible metal strip 22, the final total resistance value R may be adjusted with a high degree of precision. Since only a small portion of the fixed resistor is included between terminal 24 and tap point 26 and this small portion is being adjusted by the vernier resistor 21, a greater degree of precision is obtained in the total resistance R than in the precision of the parallel combination of x and y.

For example, if the value of resistance R is to be 1000 ohmsi of 1% and the value of y between terminal 24 and tap point 26 is 50 ohms, the total resistance Y+y between terminals 24 and 25 may be wound to 1025 ohms:2% bringing the minimum value of total resistance Y+y to about 1005 ohms and the maximum value of total resistance Y+y to 1045 ohms with y unshunted.

With a zero value for x, these limits will be 955 to 995 ohms. By progressively increasing the value of x, a total value of 1000 ohms may be obtained under all circumstances. Since a final precision of of 1% is desired, the total resistance value must be adjusted to the nearest A2 ohm. For the 50 ohm section between terminal 24 and tap point 26, a precision of /2 ohm is a precision of only 1%. The vernier resistor 21 thus requires fewer than turns, since a shunt adjustment is involved, as discussed above in connection with Fig. 3.

Figures 4 and 5 show a precision potentiometer which embodies the invention to obtain precise resistance values which are accurately adjusted and linearly interpolated for predetermined angular positions of the potentiometer control and adjustment shaft. This permits the use of accurately pre-calibrated dials with different potentiometers, thereby avoiding the necessity for individually calibrating each dial for each individual potentiometer. In many instances, where the potentiometer might be controlled by complex mechanism, the potentiometer must necessarily be fitted to follow the angular adjustments imparted thereto by the complex mechanism, it

being virtually impossible to adjust such a complex mechanism to the individual angle-resistance characteristics of the potentiometer.

The potentiometer shown in Figs. and 6 comprises a body 30 provided with a rotatable shaft 31 which is shown journaled in the body 30 by means of front and rear ball bearings 32 and 33. A disc of insulating material 34 is fixed to the rear end of shaft 31 for rotation therewith. A rotary contact arm 35 is fixed to disc 34 by screws 36. Connection with rotary contact arm 35 is established by any desired conventional means, such as a flexible pigtail (not shown) the details of such means being omitted for simplicity of illustration.

A collar 37 is fixed to shaft 31 intermediate the rear ball bearing 33 and the insulating disc 34, the collar 37 having a circumferential groove 38 formed therein. A resilient friction washer 39 is provided with parallel transverse ribs 40 which engage diametrically opposed portions of collar 37, passing through the circumferem tial groove 38. A bent wire 41 engages one side of friction washer 39 and comprises a straight portion 42 which is longitudinally slidable in body 30, extending therethrough in a direction parallel to shaft 31 and terminating in a free end portion 43. By pressing against the free end portion 43 of bent wire 41, the pressure of engagement between transverse ribs 40 of friction washer 39 and collar 37 may be varied to obtain an adjustable frictional drag for holding rotatable shaft 31 in any desired position of angular adjustment, together with the contact arm 35 which rotates therewith.

At its free end, contact arm 35 carries a resilient contact member 44 which is fixedly secured thereto at 45 as by welding, brazing or the like. The contact pressure of contact member 44 may be adjusted by means of an adjusting screw 46 threaded into a bent end portion 47 of contact arm 35.

Contact member 44 is in sliding engagement with a circular wire wound contact resistor 48 wound on a core 49. The resistance of contact resistor 48 is substantially uniform for each unit length throughout its entire length. The contact resistor 48 is laterally confined between two adjacent annular members 50 having peripherally spaced aligned radial V-shaped notches 51 formed therein. A plurality of axially spaced annular members 50 are carried by body 30, the notches 51 on all of the annular members 50 being shown in axial alignment with respect to shaft 31.

Extending transversely across annular members 50 through the aligned V-shaped notches 51 are a plurality of bridging wires 52 which engage the internal periphery of contact resistor 48 at spaced points along its length.

Disposed between further annular members 50 forwardly of and adjacent to contac resistor 48 is a function resistor 54. More forwardly disposed between further annular members 50 and adjacent to function resistor 54 is a Vernier resistor 55. Each of the bridging wires 52 engages spaced points along the internal peripheries of all three resistors 48, 54 and 55, connecting these points in multiple. Resilient ring members 56 formed of stable plastic resilient material such as synthetic rubber, yieldingly press the bridging wires 52 into constant contact with inner peripheries of the three resistors 48, 54 and 55. Bridging wires 52 need not necessarily be straight, but may be bent to skip one or more notches, if desired, so that a greater portion of one resistor is included between adjacent bridging wires than of another resistor. This is indicated diagrammatically at 52a in Fig. 5.

In order to provide mechanical stability over a wide range of ambient temperature variations, the core 49 of contact resistor 48 and of the other resistors 54 and 55 is formed of metal having the same thermal coefficients of expansion as the resistance wire which is wound thereon, the core material being coated with a suitable insulating material. In order to obtain this condition with accuracy, the core is preferably formed of resistance wire material of the same composition as the current carrying resistance wire. The core may also be formed of hollow tubing, if desired.

The function resistor 54 and the Vernier resistor 55 are each provided with suitable fusible metal strips 12 as described for the single resistor shown in Figs. 1 and 2, a fusible metal strip 12 being situated between each two points of contact of each resistor with the bridging wires 52, or a fusible metal strip 12 may initially extend throughout the entire length of each resistor and may thereafter be subsequently fused as desired.

The resistance of the function resistor 54 is adjusted after partial assembly of the potentiometer without the Vernier resistor to provide approximately predfetermined resistance values between each two adjacent bridging wires 52 to conform to incremental values of some desired function of resistance with respect to the angular position of shaft 31. Between adjacent bridging wires, the resistance variation with respect to angular shaft position will necessarily be linear. However, by changing the resistance between adjacent bridging Wires to correspond approximately to the slope or first derivative of the curve of the desired function between these adjacent points a first approximation may be obtained. After finalaccurate adjustment, the resistance value intermediate these positions is the equivalent of interpolation in the case of mathematical tables. After adjustmerit of the function resistor 54 has been completed to obtain slightly more than the desired resistance value between any two bridging wires, the Vernier resistor is applied to the assembly and the resistance between the same two points is then precisely adjusted to bring the combined resistance to the desired value with a high degree of precision between adjacent bridging wires 52 as previously described. In this connection, the resistance per unit length of the Vernier resistor Will preferably be considerably greater than that of the function resistor.

A fixed resistor 57 may conveniently be mounted as shown. The fixed resistor 57, however, is ordinarily not engaged by the bridging wires 52. The fixed resistor 57 may also comprise an adjustable portion containing a fusible metal strip, if desired. I

It is to be understood that the potentiometer construction illustrated may alternatively embody resistors using a core having an insulating surface coated with resistive metal which is removed from the surface as required to adjust the resistance between adjacent bridging wiresto the required value.

Many changes in details of construction will be apparent to those skilled in the art, and such changes may be made without departing from the scope of the invention as defined in the following claims.

What is claimed is:

1. The method of incrementally increasing the resistance of a fixed resistor comprising a plurality of convolutions of a resistance wire and a fusible electrically conductive member in electrical contact with a plurality of adjacent turns, said method comprising the step of progressively fusing said fusible member.

2. The method of obtaining a resistor. having a desired resistance value to a high degree of precision, said method comprising the steps of: shunting at least a portion of said resistorv with a further resistor, and adjusting the resistance of said further resistor, whereby the precision of the combined resistances of said resistor and said further resistor is greater than the precision of adjustment of said further resistor.

3. The method of incrementally increasing the combined resistance of a plurality of fixed resistors connected in parallel, one of said resistors comprising a plurality of convolutions of resistance wire and a fusible electrically conductive member in electrical contact with a plurality of adjacent turns, said method comprising the step of progressively fusing said fusible member, whereby said combined resistance is progressively incrementally increased by successive increments each of which is less than the resistance of a single convolution the resistance of which has been caused to become eifective by said fusing of said fusible member.

4. A resistor of which the resistance may be incrementally increased, comprising: a plurality of convolutions of resistance wire; and an electrically conductive fusible member in electrical contact with a plurality of adjacent convolutions of said resistance wire, whereby the fusing of said fusible member between two adjacent convolutions will increase the resistance of said resistor by the resistance of one of said convolutions.

5. A resistor according to claim 4, in which said fusible member is formed of a gold alloy.

6. A resistor according to claim 4, in which the melting point of said fusible member is lower than the melting point of said resistance wire, whereby said fusible member may be fused by the passage of an electrical current through said resistance wire.

7. A resistor according to claim 4, further comprising an insulative core upon which said resistance wire is wound, said fusible member being in the form of a thin strip disposed between at least a portion of said turns of resistance wire and said core.

8. A resistor comprising a core formed of resistance wire material; an insulating coating on said core; and a winding of resistance wire of the same composition as said core wound upon said insulating coating, whereby dimensional changes in said core accompanying changes in ambient temperature will conform to the corresponding dimensional changes in said resistance wire.

9. A potentiometer of the class described, comprising: in combination with a body; a shaft rotatably disposed in said body; a contact arm carried by said shaft for rotation therewith; a circular resistor concentric with said shaft and carried by said body; a resilient contact member carried by said arm and engageable with different longitudinal portions of said resistor, the provision of a further resistor electrically connected to said first named resistor at spaced points along both of said resistors, said further resistor comprising means for adjusting the resistance thereof to obtain a combined resistance between said spaced points whose precision is greater than the precision of said adjustment.

10. A potentiometer according to claim 9, in which said adjustable resistor comprises a core having an insulating surface coated with a resistive metal which may be removed as required to increase the resistance between said spaced points.

11. A potentiometer according to claim 9, in which said connection at said spaced points is eifectedby bridging wires extending between said resistors, said potentiometer further comprising resilient means urging said bridging wires into contact with said resistors.

12. A potentiometer according to claim 9, in which both of said resistors are of circular configuration of equal radius and concentric with said shaft, said resistors being axially spaced from each other.

13. A potentiometer according to claim 9, wherein said bridging wires extend substantially parallel to said shaft.

14. A potentiometer of the class described, comprising: in combination with a body; a shaft rotatably disposed in said body; a contact arm carried by said shaft for rotation therewith; a circular resistor concentric with said shaft and carried by said body; a resilient contact member carried by said arm and engageable with different longitudinal portions of said resistor, the provision of a further resistor electrically connected to said first named resistor at spaced points along both of said resistors, said further resistor comprising a plurality of turns of resistance wire and an electrically conductive fusible member in electrical contact with a plurality of adjacent turns of said resistance wire intermediate at least two of said spaced points, whereby the resistance of said first named resistor intermediate said two spaced points may be incrementally adjusted by fusing of a portion of said fusible member intermediate adjacent turns of said resistance wire with which said fusible member is in contact.

15. A potentiometer according to claim 14, in which said connection at said spaced points is effected by bridging wires extending between said resistors, said potentiometer further comprising resilient means urging said bridging wires into contact with said resistors.

16. A potentiometer according to claim 14, in which both of said resistors are of circular configuration of equal radius and concentric with said shaft, said resistors being axially spaced from each other.

17. A potentiometer according to claim 14, wherein said bridging wires extend substantially parallel to said shaft.

References (Iited in the file of this patent UNITED STATES PATENTS

Images