US2956254A

US2956254A - Variable electrical resistor

Info

Publication number: US2956254A
Application number: US806822A
Authority: US
Inventors: Jerome G Burns; Harry B Casey
Original assignee: International Resistance Co
Current assignee: International Resistance Co
Priority date: 1959-04-16
Filing date: 1959-04-16
Publication date: 1960-10-11
Anticipated expiration: 1977-10-11

Description

Oct. 11, 1960 J. G. BURNS EAL 2,956,254

VARIABLE ELECTRICAL RESISTOR Filed April 16, 1959 Fig. I 28 I lo 2 24 36 3 t 44 4O 2 ///J////// /AP 2 INVENTORS 2 JEROME G. BURNS 26 HARRY B. CASEY 9 f/gaz/ ATTORNEY VARIABLE ELECTRICAL RESISTOR Jerome G. Burns, Upper Darby, and Harry B. Casey,

Roslyn, Pa., assignors to International Resistance Company, Philadelphia, Pa.

Filed Apr. 16, 1959, Ser. No. 806,822

13 Claims. ((11.338-162) The present invention relates to a variable electrical resistor, and more particularly to a variable electrical resistor having a disc contact.

In general, a variable electrical resistor, such as a rheostat or potentiometer, includes a resistance element, and a contact which engages the resistance element and slides therealong to vary the resistance of the variable resistor. The resistance element of such a variable resistor usually comprises a strip of an insulating material having either a resistance wire helically wound therealong, or a film of a resistance material, such as carbon or a metal, coated thereon. In a variable resistor which uses a film type resistance element, a major problem arises from the fact that the sliding movement of the contact over the film of resistance material wears away the film, and thereby varies the resistance value of the resistance element. In an attempt to overcome this problem, contacts for variable resistors have been developed which have rolling contact with the resistance element rather than a sliding contact. However, it has been found that such rolling contacts are unsatisfactory because they do not wipe off minute particles of dust, dirt, or oxides which may accumulate on the surface of the resistance element. Such minute particles of dust, dirt, or oxides interfere with the electrical engagement between the contact and the resistance film, and provide a noisy contact.

It is an object of the present invention to provide a novel variable electrical resistor.

R is another object of the present invention to provide a variable electrical resistor having a novel disc contact.

It is a further object of the present invention to provide a variable electrical resistor having a rolling disc contact which is provided with a small but controlled amount of slippage for cleaning the resistance path.

Other objects will appear hereinafter.

For the purpose of illustrating the invention there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

Figure 1 is a sectional view, partly in elevation, of the variable electrical resistor of the present invention.

Figure 2 is a sectional view taken along line 22 of Figure 1.

Referring initially to Figure 1, the variable electrical resistor of the present invention is generally designated as 10.

Variable resistor comprises a cup-shaped housing 12 which may be of metal or'an electrical insulating material. Housing 12 has a flat, circular base 14, and a cylindrical outer wall 16. A tubular bushing 18 is secured to the center of the outer surface of the base 14 of housing 12. As shown, bushing 18 is integral with the base 14. However, bushing 18 may be separate from the housing 12, and secured to the base 14 of the housing 12. The outer surface of the bushing 18 is threaded to receive a nut for mounting the variable resistor 10 on a panel.

iCQ

An annular resistance element 20 is within the housing 12, and is secured to the inner surface of the base 14 of the housing 12. Resistance element 20 comprises a fiat, annular disc 22 of an electrical insulating material such as a ceramic or plastic, and a layer 24 of a resistance material, such as carbon or metal, coated on the outer face of the disc 22. As shown in Figure 2, the resistance-element 28 is split so that the resistance layer 24 provides an arcuate resistance path having spaced ends. A separate terminal 26 is secured to each end of the resistance element 20, and is electrically connected to an end of the resistance layer 24. Terminals 26 project through the outer wall 16 of the housing 12.

A shaft 28 extends through the bushing 18 and the base 14 of the housing 12, and is rotatably supported in the bushing 18. Shaft 28 has an enlarged head 30 on its end within the housing 12. The head 30 of shaft 28 is disposed centrally in the space within the inner periphery of the resistance element 20. The end surface 32 of the head 30 is flat, and is inclined at an acute angle with respect to the longitudinal axis of the shaft 28. Head 36 has a recess 34- in its end surface 32. The longitudinal axis of the recess 34 is perpendicular to the end surface 32, and is off-set from the center of the end surface 32. A stub shaft 36 of an electrical insulating material, such as a plastic is rotatably supported in the recess 34 by a ballbearing 38. A thin, circular contact disc 40 of an electrically conductive metal is secured at its center to the outer end of the stub shaft 36. Contact disc 40 is substantially parallel to the end surface 32 of the head 30 so that the contact disc 40 is inclined with respect to the resistance element 20. Contact disc 40 is of a diameter so that a peripheral portion of the contact disc 40 engages the resistance layer 24 of the resistance element 20. As shown in Figure 2, contact disc 40 has a notch 42 extending radially from the periphery of the contact disc 40. Notch 42 is positioned at the split in the resistance element 20.

A helical coil spring 44 of an electrically conductive metal surrounds the head 30 of the shaft 23, and is spaced from the head 30. One end of the spring 44 is secured to the contact disc 40, and the other end of the spring 44 is secured to a terminal 46 which extends through the base 14 of housing 12. A grommet 48 of an electrical insulating material fits between the terminal 46 and the base 14 of housing 12 to electrically insulate the term nal 46 from the housing 12. A pin 50 is secured to the base 14- of the housing 12, and projects into the housing 12. Pin 50 extends through a hole 52in the contact disc 40. A cover 54 extends across the housing 12, and is secured to the outer wall 16 of the housing 12 by screws 56.

The operation of the variable resistor 10 of the present invention is as follows:

Upon rotation of the shaft 28, the stub shaft 36 rotates with respect to the shaft 28. Since the stub shaft 36 and the contact disc 40 are inclined with respect to the longitudinal axis of the shaft 28, rotation of the shaft 28 imparts a wobble motion to the contact disc 40. The wobble motion of the contact disc 40 moves the point of engagement between the contact disc 40 and the resistance layer 24 around the periphery of the contact disc 40, and along the length of the resistance layer 24 to vary the electrical resistance between the terminal 46 and the terminals 26. In essence, the wobble motion of the contact disc 40 rolls the periphery of the contact disc 40 along the length of the resistance layer 24. The rolling of the periphery of the contact disc 40 along the resistance layer 24 minimizes any wear of the resistance layer 24 by the engagement of the contact disc 40 with the resistance layer 24. The notch 42 in the contact disc 40 provides a sharp engagement between the contact disc 40 and the ends of the resistance layer 24, and prevents the ends of the resistance layer from being shorted out across the slit in the resistance element 20 by the contact disc 40.

Since the contact disc 40 is inclined with respect to the resistance element 20, the path of contact on the resistance layer 24 is of a smaller diameter than the diameter of the contact disc 40. Assuming the fictional forces in the bearing 38 to be negligible, and that the pin 50 does not exist, the following situation would exist. Upon rotation of the shaft 28 through 360 degrees, the contact disc 40 would roll along the resistance layer 24 in a contacting path of a diameter equal to the projected diameter of the contacting disc 40. Because of the difference in diameter of the contacting path and the diameter of the contact disc 40, a given point on the contact disc 40 would not return to its original point of engagement on the resistance layer 24, but would be displaced from its original point of engagement with the resistance layer 24 a distance equal to the difference between the circumferential length of the contact disc 40 and the circumferential length of the contacting path along the resistance layer 24. Therefore, in this case, the contact disc 40 would move relative to the resistance element 20 upon rotation of the shaft 28. However, in the variable resistor of the present invention, the pin 59 prevents the contact disc 40 from rotating with respect to the resistance element 20. Thus, upon rotating the shaft 28 through 360 degrees, a given point on the contact disc 4% will return to its original point of engagement with the resistance layer 24. In order for the fixed point on the contact disc 40 to return to its original point of contact with the resistance layer 24, the contact disc 40 must slide along the resistance layer 24 a distance equal to the difference between the circumferential length of the contact disc 40 and the circumferential length of the contacting path on the resistance layer 24. In addition, since the intersection of the longitudinal axis of the stub shaft 36 and the plane of the contact disc 40 is off-set from the longitudinal axis of the shaft 28, an additional amount of sliding is introduced between the contact disc 40 and the resistance layer 24 upon rotation of the shaft 23. This additional amount of sliding is proportional to the perpendicular distance between the point of intersection between the longitudinal axis of the stub shaft 36 and the plane of the contact disc 4t? and the longitudinal axis of the shaft 23. Thus, although the contact disc 40 is rolled along the resistance layer 24 upon rotation of the shaft 28, the variable resistor 10 of the present invention also provides a small amount of sliding between the contact disc and the resistant layer 24. The small amount of sliding between the contact disc 40 and the resistance layer 24 is sufiicient to remove any particles of dust, dirt, or oxides which may accumulate on the resistance layer 24, and thereby prevent a noisy contact. The amount of sliding between the contact disc 40 and the resistance layer 24 can be controlled by the angle of inclination between the contact disc 40 and the resistance element 20, and by the position of the stub shaft 36 with respect to the longitudinal axis of the shaft 28 so as to provide an optimum Sliding action which will cleanse the surface of the resistance layer 24, but will not cause undue wear of the resistance layer 24. Since the various materials which can be used for the resistance layer 24 are of different hardnesses, the maximum amount of sliding between the contact disc 40 and the resistance layer 24 which will be permissible will vary with the particular materials being used for the resistance layer 24, and can be determined experimentally.

Another advantage of the variable resistor 10 of the present invention is that it does not require a sliding contact between the terminal 46 and the contact disc 40, as is necessary in a variable resistor which uses a rotating contact. Such a sliding contact is subject to being worn out which would break the electrical connection between the terminal and the contact member, and also provides the variable resistor with additional noise. However, in the variable resistor 10 of the present invention since the contact disc 40 does not rotate with respect to the housing 12, the spring 44 does not wind up as a clock spring upon rotation of the shaft 28, but expands and compresses under the wobble action of the contact disc 40. Thus, the rotation of the shaft 28 is not limited by the spring 44, so that the shaft 28 can be rotated as many times as desired in either direction. Although the variable resistor 10 of the present invention is shown with the resistance element 20 being mounted on the inner surface of the base 14 of the housing 12, the resistance element 20 can also be mounted on the inner surface of the cover 54 with the resistance layer 24 facing the contact disc 40.

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

We claim:

1. In a variable resistor, an annular resistance element having a resistance material path on one planar surface thereof, a shaft rotatable about an axis which extends through the center of said resistance element and which is perpendicular to the resistance material path, a flat contact disc of an electrically conducted metal rotatably mounted at its center on said shaft, said contact disc being inclined at an angle with respect to said resistance material path and being in contact with said resistance material path, and means preventing rotation of said contact disc with respect to said resistance element.

2. A variable resistor in accordance with claim 1 in which the center of the contact disc is off-set from the longitudinal axis of the shaft.

3. A variable resistor in accordance with claim 1 in which the shaft has an enlarged head on its end, the end surface of said head being inclined at an acute angle with respect to the longitudinal axis of said shaft, and the contact disc is rotatably mounted on said head and extends substantially parallel to the end surface of said head.

4. A variable resistor in accordance with claim 3 in which the enlarged head on the shaft has a recess in the end surface of the head, the longitudinal axis of said recess being perpendicular to the end surface of said head, a stub shaft rotatably mounted in said recess, and the contact disc being mounted on said stub shaft.

5. A variable resistor in accordance with claim 4 in which the longitudinal axis of the recess in the head is off-set from the center of the end surface of the head.

6. A variable resistor comprising a housing having a cylindrical wall and a substantially flat, circular wall extending across each end of the cylindrical wall, an annular resistance element mounted on the inner surface of one of the circular walls of said housing, said resistance element having a resistance material path on its surface facing the other of said circular Walls, a rotatable shaft extending through one of the circular walls of said housing, the longitudinal axis of said shaft extending through the center of said resistance element and being substantially perpendicular to the resistance material path, a flat contact disc of an electrically conducted metal within said housing and rotatably mounted at its center on the end of said shaft, said contact disc being inclined at an angle with respect to said resistance material path and being in contact with said resistance material path, and means for preventing rotation of said contact disc with respect to said housing.

7. A variable resistor in accordance with claim 6 in which the center of the contact disc is off-set from the longitudinal axis of the shaft.

8. A variable resistor in accordance with claim 6 in which the shaft has an enlarged head on its end within the housing, the end surface of said head being inclined at an acute angle with respect to the longitudinal axis of said shaft, and the contact disc is rotatably mounted on said head and extends substantially parallel to the end surface of said head.

9. A variable resistor in accordance with claim 8 in which the enlarged head on the shaft has a recess in the end surface of the head, the longitudinal axis of said recess being perpendicular to the end surface of said head, a stub shaft rotatably mounted in said recess, and the contact disc being mounted on said stub shaft.

10. A variable resistor in accordance with claim 9 in which the longitudinal axis of the recess in the head is oifset from the center of the end surface of the head.

11. A variable resistor in accordance with claim 10 including a terminal extending through a circular Wall of the housing, and a helical coil spring in said housing between the contact disc and said circular wall of the housing, one end of said spring being connected to said contact disc and the other end of said spring being connected to said terminal.

12. A variable resistor in accordance With claim 11 in which the resistance element has a slit therethrough to provide a discontinuous resistance material path, and the contact disc has a notch therein extending radially from the periphery of the contact disc, said notch being positioned along the slit in said resistance element.

13. A variable resistor in accordance with claim 12 in which the means preventing rotation of the contact disc relative to the housing comprises a pin extending from a circular wall of the housing through a hole in said contact disc.

References Cited in the file of this patent UNITED STATES PATENTS 1,704,154 Stoekle Mar. 5, 1929