Single-turn Potentiometer

Abstract

A potentiometer comprised of an electrically insulative housing of thermoplastic material having a deep, substantially cylindrical cavity therein, a plurality of termination pins partially molded into the bottom of the housing, a substantially annular resistance element secured at the bottom of the housing and having the ends thereof attached to the heads of two of the termination pins, a disk-shaped cover member adapted to fit into the open end of the cavity, a resilient contact member attached to the underside of the cover and adapted to contact the resistance element and a third one of the termination pins, and an annular sealing member positioned between the cover member and a ledge on the housing. The housing member includes a plurality of nubs extending a short distance into the cavity adjacent the open end thereof to help retain cover member within the cavity. In the preferred embodiment, the resistance element is a wirewound element which is heat inserted into raised portions of the housing on the floor of the cavity; the heads of two of the termination pins include small integral projections for facilitating their electrical connection to the ends of the resistance element; and the tip of the contact member which contacts the resistance element has a knurled surface and is rhodium plated.

Description

BACKGROUND OF THE INVENTION

The subject invention relates generally to the field of variable resistance devices, and, in particular, to single-turn potentiometers. Many different designs of single-turn potentiometers are readily available on the open market whose sizes, specifications and prices vary substantially. In order to provide high reliability, minimum cost and good performance, it is desirable to minimize the number of parts necessary and promote ease and simplicity in manufacture.

Some of the problems therein have been providing reliable connection between termination pins and the ends of resistance elements; securing resistance elements, particularly the wirewound type, within the device; minimizing the contact resistance variation and simplifying final assembly of the device. In many cases leads are attached to the ends of a wirewound resistance element and resistance welded to the heads of termination pins. However, due to the relatively large surface area of the heads of the pins, it is sometimes difficult to make a satisfactory weld. Generally, a wirewound element is attached to the housing by means of a glue. However, the use of glue presents a possible problem of contamination and requires that an accurate amount of glue be carefully applied, generally uniformly over the surfaces to be joined. Such an operation requires a significant amount of time and skill. Prior art efforts to reduce the contact resistance variation have included increasing the pressure of the contact device on the resistance element, which unfortunately promotes wear of the resistance element, and providing multiple contact fingers, with its attendant increase in cost and difficulty in manufacture. Also, it is usually necessary, but often quite cumbersome to hold the pieces of the device together before they are permanently retained.

SUMMARY OF THE INVENTION

Thus it is an object of the subject invention to provide a simple, easy to manufacture single-turn potentiometer having a minimum number of parts;

Another object of the subject invention is to provide a single-turn wirewound potentiometer which permits reliable connections to be made simply and quickly between termination leads and the ends of the wirewound resistance element;

A further object of the subject invention is to provide a single-turn wirewound potentiometer wherein the wirewound resistance element is attached securely to the housing without need for a separate attachment medium;

Yet another object of the subject invention is to provide a single-turn potentiometer which exhibits low contact resistance variation even at high temperatures;

And still another object of the subject invention is to provide a single-turn potentiometer wherein the final assembly can be quickly and easily accomplished.

The subject matter which is regarded as the present invention is particularly pointed out and distinctly claimed in the concluding portion of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIG. 1 is an isometric view of a potentiometer embodied in the subject invention;

FIG. 2 is an exploded isometric view of the potentiometer shown in FIG. 1 with a portion of the housing cut away;

FIG. 3 is a partial sectional view of the apparatus shown in FIG. 1 taken along the plane denoted III--III;

FIG. 4 is a greatly enlarged sectional view showing a portion of a terminal pin, potentiometer housing and molding apparatus in accordance with the subject invention; and

FIG. 5 is a bottom view of the cover and contact member of the potentiometer shown in FIG. 1.

FIG. 6 is a partial sectional view of the cover and contact member of the potentiometer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A potentiometer 10 including the features of the subject invention is best shown in FIGS. 1, 2 and 3. FIGS. 1 and 3 are isometric and sectional views, respectively, of potentiometer 10 completely assembled. FIG. 2 is an exploded view of potentiometer 10 showing the pieces thereof prior to assembly.

Potentiometer 10 includes a molded plastic housing 12 having a substantially cylindrical cavity 14 therein; three termination pins 16, 18, 20 having the heads thereof 16h, 18h and 20h molded into the bottom of housing 12; an annular resistance element 22 which is secured to housing 12 and is located within cavity 14; a cover member 24 shaped to fit the open end of housing cavity 14; a resilient contact member 26 having contact arms 28, 30 adapted to contact resistance element 22 and termination pinhead 18h, respectively; and a sealing member 32 adapted to be disposed between the cover member and a ledge 34 which extends within cavity 14. Housing 12 is preferably of a one-piece thermoplastic molded construction and may conveniently be cylindrical in shape. Adjacent to the open end of cavity 14 a plurality of retaining nubs 12n integral with housing 12 extend into the cavity. Nubs 12n are used to provide temporary retention of cover 24 in the cavity during final assembly of the device. Ledge 34, which is preferably integral with housing 12, is located part way down the cavity. Preferably, ledge 34 has a groove 34g therein for accommodating sealing member 32. Also located within the cavity is a stop member 12s which extends up from the bottom of the housing and is preferably integral with the wall of the cavity. Stop member 12s is located so as to engage a portion of cover member 24 when the desired limit of rotation thereof is reached. Annular raised portions 12r are located on the bottom of the housing within the cavity about the circumference of cavity 14. Raised portions 12r are used to engage a wire-wound resistance element when it is heat-inserted therein.

A central protrusion 12p extends into the cavity from the bottom of housing 12 and is preferably integral therewith. Termination pin 18 extends through protrusion 12p. Termination pinhead 18h, which acts as a collector for the potentiometer, is located on the top surface of protrusion 12p in a position so as to be contacted by contact arm 30.

Housing 12, while preferably of one-piece construction could be comprised of two or more pieces which are joined together. For example, a substantially cylindrical member could be joined to a disk-shaped member to provide a suitable housing.

All of the termination pins 16, 18, 20 are molded into housing 12 while the housing itself is being molded. One feature of the subject invention is that each of the two termination pinheads 16h, 20h has a small integral projection 16p, 20p formed on the exposed surface thereof which greatly facilitates the welding of leads from resistance element 22 to the pinheads. Projections 16p and 20p tend to help concentrate the energy from the welding apparatus into a small area thereby making the welding operation easier to perform and the resultant joint more reliable. Preferably the projections are formed on the pinheads during the molding operation by the use of molding apparatus such as that which is partially shown in FIG. 4. The mold has an upper piece 36u and a lower piece 36l. The upper portion 36u has a depression 38 therein on the surface which is adapted to contact one of the termination pinheads 16' and 20'.

When molding is to take place, termination pins 16, 18, 20 are inserted through apertures in lower mold portion 36l. Upper mold portion 36u is positioned in place and pressure is applied while the liquid plastic is inserted in the space between the molds. The application of pressure causes metal from termination pinheads 16h, 20h to flow into the depressions 38 so as to form projections 16p and 20p thereon. After the plastic has hardened, the molds are removed.

Resistance element 22 preferably includes an annular mandrel 22m which has an insulating coating thereon 22c. The mandrel may be of any desired material such as metal, plastic or ceramic. If a non-conductive material is used, the coating 22c need not be used. A resistance wire 22w is helically wound over coating 22c. Portions of the resistance wire may be coated with a material such as varnish. However, the top surface of resistance wire 22w must be clean so that suitable contact may be made with contact arm 28. Connecting leads 221 are attached and electrically connected to the ends of resistance wire 22w. These leads may be readily attached by any conventional means, preferably resistance welding, to the projections 16p, 20p on the respective termination pinheads.

In accordance with the subject invention, wirewound element 22 may be attached to housing 12 by heat insertion. Resistance wire 22w is heated by passing a suitable electric current therethrough. Then the heated element is inserted into cavity 14 and pressed against the raised portions 12r. The hot resistance element 22 melts some of the plastic of portions 12r allowing element 22 to embed itself therein and be firmly attached thereto without need for any additional attachment means. Due to the use of this method, a substantial savings in material and labor is effected. Also, a reliable attachment is effected between resistance element 22 and housing 12 which eliminates any potential problems due to contamination by the use of glue or cement. After resistance element 22 is heat-inserted into housing 12, connection leads 221 may be electrically connected to termination pins 16 and 18, as described above.

As can be best seen in FIGS. 2, 3 and 5, cover member 24 is preferably of a one-piece thermoplastic construction which preferably includes a beveled portion 24b about the circumference of the upper surface thereof and an elongated slot 24s disposed along a diameter on the upper surface. Slot 24s is used to allow rotation of the cover member by use of a screwdriver or similar implement and also provides an indication of the approximate position of the contact member on resistance element 22. Of course, any other suitable means by which the cover member may be rotated, such as a knob, may be provided. The diameter of the disk-shaped cover member 24 is slightly smaller than the diameter at the open end of cavity 14 to permit insertion of cover member 24 therein but yet allow retaining nubs 12n to firmly frictionally engage cover member 24 during assembly. On the underside of cover member 24, a step 24a is provided along the circumferential edge thereof to permit the retention of sealing member 32 therein after assembly. Also on the underside of cover member 24 are three posts 24p, which are used for retaining contact member 26 thereon, as well as an integral cover stop member 24m which extends downwardly and is adapted to engage housing stop member 12s when the limits of desired rotation of cover member 24 are reached.

Contact member 26 is preferably of a one-piece metal construction having resistance element contact arm 28 collector contact arm 30 adapted for engagement with resistance wire 22w and collector terminal pinhead 18h respectively. Both contact arms are preferably bent along designated fold lines so as to provide a desired spring force acting between the contact arm and the surface to be contacted. In order to minimize contact resistance variation, contact arm 28 preferably has a roughened surface formed on the resistance wire contacting tip 28t, as best seen in FIG. 5. The roughening may be accomplished by knurling or dimpling the tip surface. Contact member 26 may be rhodium plated or have the surface contacting portions thereof rhodium plated to also help minimize contact resistance variation, particularly at high temperatures. Contact member 26 also includes cut out portions 26c including a circular aperture and two semicircular cutaway portions for permitting attachment to cover member 24, as best shown in FIG. 2.

Contact member 26 is preferably attached to cover member 24 by heat staking. This is accomplished by positioning contact member 26 so that posts 24p extend through cut out portions 26c. Heat is then applied to posts 24p which are of thermoplastic material, so that the posts melt to cover a portion of contact member 26 and firmly secure contact member 26 to the underside of cover member 24, as best seen in FIGS. 3 and 5.

Sealing member 32 is annular in shape and preferably of a resilient material such as rubber. Typically, this would be a commercially available o-ring.

Final assembly of device 10 takes place after resistance element 22 has been secured to housing 12 and electrically connected to termination pins 16, 20 and contact member 26 has been secured to cover member 24. First, sealing member 32 is inserted into groove 34g and cover member is inserted into the cavity 14 and snapped in place. Retaining nubs 12n frictionally engage the outer edge of cover member 24 so as to firmly retain it therein against the spring force exerted by contact arms 28, 30. Then the upper edge of housing 12 is heat formed over beveled portion 24b of cover member in at least a plurality of sectors so as to form a retaining lip 121, as best seen in FIGS. 1 and 3, which permanently retains cover member 24 within cavity 14 while allowing cover member 24 to be freely rotatable within desired limits. When cover member 24 is retained, sealing member 32 is compressed between housing 12 and cover member 24 so as to effect an air tight seal. The use of retaining nubs to effect temporary retention during final assembly reduces the amount of labor and production hardware necessary for the final assembly process and simplifies the heat forming operation. It should be noted that in the preferred embodiment the heat formed lip 121 does not extend completely around the circumference of the housing so that orientation portions 12o are formed to give an approximate indication of the position of contact arm on resistance element 22.

Thus the subject invention provides a variable resistance device structure in a single-turn configuration which has a minimum of parts, is simple and reliable, can be quickly and inexpensively manufactured and assembled, and has desirable electrical characteristcs.

It is obvious that many modifications to the embodiment disclosed and shown may be made within the true scope and spirit of the subject invention. Thus it is intended that the scope of the subject invention be limited only in accordance with the following claims.

Claims

What is desired to be secured by Letters Patent of the United States is:

1. A single-turn variable resistance device comprised of:

a. a housing of thermoplastic material having a substantially cylindrical cavity therein;

b. an annular resistance element located approximately at the bottom of said cavity and attached to said housing;

c. a plurality of termination pins, each having a portion thereof molded into said housing, including two resistance element termination pins electrically connected to the ends of said resistance element and a collector termination pin;

d. a substantially disk-shaped cover member adapted to fit into an open end of said cavity;

e. a resilient contact member attached to the underside of said cover for adjustably contacting a portion of said resistance element and simultaneously contacting a portion of said collector termination pin; and

f. a sealing member disposed between said cover member and a portion of said housing for providing contamination protection for said resistance element;

wherein said housing includes retaining nub means extending within said cavity adjacent the open end thereof for retaining said cover member in said cavity.

2. A device as in claim 1 wherein a heat formed lip of said housing is located adjacent the open end of said cavity so as to permanently retain said cover within said cavity.

3. A device as in claim 1 wherein said resistance element is of the wirewound type.

4. A device as in claim 3 wherein said resistance element has been heat-inserted into said housing and a portion of said housing fixedly secures said resistance element.

5. A device as in claim 4 wherein said housing includes raised portions at the bottom of said cavity which have been heat-bonded to said resistance element.

6. A device as in claim 1 wherein said contact member includes a resistance element contacting tip which is roughened to minimize contact resistance variation.

7. A device as in claim 1 wherein said contact member includes a resistance element contacting tip which is rhodium plated.

8. A device as in claim 1 wherein said contact member has been attached to said cover member by heat staking.

9. A device as in claim 1 wherein said two resistance element termination pins each include a small, integral projection on the heads thereof which extend into the cavity to which the ends of said resistance element have been connected.

10. A device as in claim 1 further including a housing stop member extending into said cavity and a mating stop member on said cover member which is adapted to engage said housing stop member when the desired limit of rotation of said cover has been reached.