U.S. patent number 3,760,324 [Application Number 05/292,353] was granted by the patent office on 1973-09-18 for single-turn potentiometer.
This patent grant is currently assigned to Bowins, Inc.. Invention is credited to Kenneth B. Baldwin, Ronald D. Sullivan.
United States Patent |
3,760,324 |
Baldwin , et al. |
September 18, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
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.
Inventors: |
Baldwin; Kenneth B. (Corona,
CA), Sullivan; Ronald D. (Riverside, CA) |
Assignee: |
Bowins, Inc. (Riverside,
CA)
|
Family
ID: |
23124297 |
Appl.
No.: |
05/292,353 |
Filed: |
September 26, 1972 |
Current U.S.
Class: |
338/174;
338/184 |
Current CPC
Class: |
H01C
10/301 (20130101) |
Current International
Class: |
H01C
10/00 (20060101); H01C 10/30 (20060101); H01c
005/02 () |
Field of
Search: |
;338/174,175,162,164,184,199 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Tone; D. A.
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.
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.
* * * * *