U.S. patent number 3,602,861 [Application Number 04/804,591] was granted by the patent office on 1971-08-31 for hybrid element variable resistor.
This patent grant is currently assigned to Bourns, Inc.. Invention is credited to Richard L. Bomar, Donald G. Tweed.
United States Patent |
3,602,861 |
Tweed , et al. |
August 31, 1971 |
HYBRID ELEMENT VARIABLE RESISTOR
Abstract
A resistance element comprising an insulative base bearing first
and second spaced-apart conductive terminals between which
terminals extends a cermet component adherent to the base and a
superposed conductive plastic component adherent to the cermet
component and intimately in electrical contact therewith, both
components having electrical connection at their first and second
ends to respective ones of the terminals.
Inventors: |
Tweed; Donald G. (Riverside,
CA), Bomar; Richard L. (Pomona, CA) |
Assignee: |
Bourns, Inc. (N/A)
|
Family
ID: |
25189354 |
Appl.
No.: |
04/804,591 |
Filed: |
March 5, 1969 |
Current U.S.
Class: |
338/9; 338/162;
338/319; 338/322; 338/308 |
Current CPC
Class: |
H01C
7/18 (20130101) |
Current International
Class: |
H01C
7/18 (20060101); H01c 007/00 (); H01c 009/00 () |
Field of
Search: |
;338/9,308,322,162,319
;174/68.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Bennett, Jr.; Rodney D.
Assistant Examiner: Kinberg; R.
Claims
We claim:
1. A resistance element, said element comprising:
an insulative base;
first and second spaced-apart conductors on said base providing
first and second terminals;
a cermet resistive component on said base, extending between said
first and second terminals and electrically contacting at least a
portion of each of said terminals; and
a conductive plastic resistive component overlying and in intimate
physical and electrical contact with said cermet resistive
component between said first and second terminals and in electrical
communication with said terminals;
the terminal-to-terminal resistance of said cermet resistive
component being greater than that of the said conductive plastic
resistive component
whereby said element possesses desirable characteristics of said
cermet resistive component and desirable characteristics of said
conductive plastic resistive component and is characterized by
marked reduction of undesirable characteristics of said resistive
components.
Description
BACKGROUND OF THE INVENTION
Resistance elements of other than wirewound types and employed in
variable resistors such as potentiometers that are used to adjust
potentials or to trim electronic circuits are classified in two
general principal classes, namely carbonaceous or "conductive
plastic," and metallic such as metal film and cermet. Each of those
classes of elements is characterized by its respective desirable
features and undesirable features. For example, the conductive
plastic elements are characterized by excellent resistance to
wearing and low contact resistance variation (CRV), at least in
lower and intermediate total resistance (TR) ranges. But, as
offsetting undesirable features, such elements are characterized by
relatively high negative temperature coefficient of resistivity
(-Tc), poor temperature stability, and low-power ratings. On the
other hand, cermet resistance elements having acceptable contact
wear or low abrasiveness have generally bad CRV characteristics,
and are limited to low and intermediate TR ranges, since in the
high TR range they are very abrasive. Cermets are, further,
generally much more abrasive than conductive plastic elements of
any TR range, and so cause relatively severe wearing problems in
potentiometers in which extensive movements of the wiper or contact
occurs. In general, cermet resistance elements are characterized by
low to medium positive Tc, in contrast to the high negative Tc of
conductive plastic elements; and they have good temperature
stability and power ratings.
In the prior art, selection of one or another of the noted types of
elements was made, depending upon the TR range into which the
element was desired to fall, and compromises made in respect of
characteristics and specifications desired to be met. Very high
values of TR were substantially impossible of attainment in
elements of either type in elements of very small physical
dimension and which had satisfactory uniformity, CRV, durability
and load-carrying ability. Thus undesirable minimum physical
dimensions were imposed on elements in the high TR range. On the
other hand, attainment of very low values of TR was difficult and
sporadic in the manufacture of elements of either type, having
otherwise acceptable characteristics. Thus, in general, compromises
between + and -Tc, durability, high and low CRV, etc. were made;
and neither cermet nor conductive plastic elements could be made to
cover all of low, intermediate and high total resistance ranges
without involving one or more rather grossly undesirable
characteristics at one or the other extreme of the total resistance
ranges.
SUMMARY OF THE INVENTION
The present invention eliminates or greatly reduces the magnitude
of all of the aforenoted undesirable characteristics of variable
resistor elements and permits notable meritorious improvements in
the field of nonwirewound resistance elements, by disposing in
intimate contact a cermet resistance element component possessing
the noted characteristics of that class of elements, and a
conductive plastic component. In any case, selection of the cermet
and of the conductive plastic components may be such that
optimization of one or another of the noted characteristics is
attained as well as improvement in others. Thus, for example, very
low or zero Tc may be attained, or very high TR attained with great
wear resistance and high-power rating. As thus combined into a
unitary resistance element, the two components are in a sense
disposed in parallel electrical relationship between end terminals
but are also intimately connected each to the other throughout
their effective lengths by what may be considered to be a multitude
of conductive connectors each of very low value of resistance but
collectively of resistance dependent upon the resistivity of the
components. Preferably, and as herein described with respect to an
exemplary preferred physical embodiment, an elongate cermet
component is formed upon an appropriate surface of an insulative
base or substrate such as a chip or wafer of alumina, and an
elongate component of conductive plastic is formed along and upon
the exposed surface of the cermet component. While the electrical
lengths of the thus superposed components are the same or
substantially so, the widths and/or cross-sectional dimensions as
well as the compositions are susceptible of variations to permit
attainment of optimum or desired characteristics such as TR, Tc,
CRV, durability, etc. By "tailoring" or judicious proportioning of
the compositions and dimensions, etc., of the two components of the
thus-produced unitary hybrid resistance element, substantially zero
value of Tc may be attained while desirably low CRV and great
contact wear resistance are concurrently secured. Also, the
previously impractical highly abrasive very high TR cermet
component with its high-power rating is rendered usable and given
low Tc and great wear resistance by combination with the
superimposed conductive plastic component. Other meritorious
improvements will hereinafter be more full explained. An exemplary
variable resistor hybrid element in two physical shapes is
illustrated in the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pictorial view of an elongate rectangular hybrid
resistance element according to the invention, to no particular
dimensional scale and with sections of superposed cermet and
conductive plastic elements broken away to illustrate details, film
thickness and film interrelationships being somewhat exaggerated
for purposes of illustration;
FIG. 2 is a plan view of the element depicted in FIG. 1, to a
reduced scale;
FIG. 3 is a sectional view, taken as indicated by directors 3--3 in
FIG. 2;
FIG. 4 is a plan view of an arcuate hybrid resistance element
formed on a circular substrate and adapted for use in a single-turn
rotary potentiometer;
FIGS. 5 and 6 are sectional views of structure depicted in FIG. 4,
the sections being as indicated by directors 5--5 and 6--6,
respectively, in FIG. 4; and
FIG. 7 is a schematic diagram illustrating schematically the
electrical relationships of the laterally cross-connected resistive
components of the hybrid element of either of FIGS. 2 and 4.
DESCRIPTION OF THE ILLUSTRATED STRUCTURES
In FIGS. 1, 2 and 3, 10 denotes an insulative base or substrate of
ceramic material such as alumina or steatite. At each end portion
of the substrate there are initially formed respective first and
second terminals 12, 14, which may be, for example, of silver
reduced from applied silver-bearing ink or paint, the reduction
being by firing, as in a reducing or inert atmosphere in a kiln.
Other modes and means of producing such adherent conductive
terminal means, well known in the art, may be employed. As will be
evident to those skilled in the resistor art, the terminals may be
very thin, for example of the order of 1 mil, or other thickness;
and for purposes of illustration are shown of exaggerated thickness
in the drawings.
Applied by conventional cermet film-producing techniques is an
elongate cermet resistive layer or film 16 (FIGS. 1, 2 and 3) which
overlies at its ends respective areas or portions of the terminals
12 and 14, whereby the cermet component is electrically connected
at its electrical ends with the terminals. Overlying all (or a
portion only as depicted) of the cermet film 16, and overlying also
portions of the terminals 12 and 14 is a second resistive component
of the type commonly referred to as "conductive plastic." The
latter layer or film is thus directly connected at its electrical
ends to the terminals 12 and 14, and is intimately cross connected,
throughout its effective electrical length, to the cermet component
16. The conductive plastic resistive component may be of any stable
synthetic resin material with finely divided conductive particles
thoroughly mixed therein, the film or layer being applied while the
mixture is plastic or inklike, and cured in situ on the previously
described structure to form the composite or hybrid resistance
element. Typical conductive particles are finely divided carbon,
finely divided metals such as silver, gold, rutheniun, platinum and
the like. Resins such as phenol formaldehyde, Bakelite, urea resins
and heat-curable synthetic resins, with volatile solvents if
necessary to control viscosity, may be used.
Following completion of the hybrid resistance element comprising
the base or substrate 10, terminals 12 and 14, cermet component 16
and conductive plastic component 18, the element may be subjected
to desirable tests and inspection, and incorporated into one or
another of conventional variable resistors, or in the case wherein
the element is used as an adjustable fixed-value resistor having
zero or negligible Tc, the element may be potted or provided with
other adjuncts such as a housing or other mechanical protection.
Since incorporation of the element in a housing and/or combining it
with adjunct means is not per se of the present invention, and
since such housing and adjunct means are known in the resistor art,
they are not further herein described nor illustrated.
In FIGS. 4, 5 and 6 there is illustrated the application of the
principles of the invention to an element for a single-turn rotary
potentiometer. Therein, 20 denotes a ceramic disc or substrate, 22
and 24 denote respective conductive filmlike terminals of the same
character as terminals 12 and 14, and 26 and 28 denote,
respectively, cermet and conductive plastic arcuate filmlike strips
each of which at its ends contacts the terminals, as indicated in
FIG. 5. Other than in respect of physical shape and dimensions, the
unit and its conductive and resistive components may be
substantially identical to the comparable components of the unit
illustrated in FIGS. 1, 2 and 3.
In FIG. 7, there is a purely schematic representation of the
electrical arrangement of the conductive and resistive components
of either of the previously described units, the reference numerals
referring to the unit depicted in FIG. 1 by way of example only. In
the electrical schematic diagram, an elongate conductive plastic
component 18 of relatively high resistivity or TR and an elongate
cermet component 16 of relatively low resistivity or TR are
interconnected along their active extents by numerous conductors,
and each of the cermet and conductive plastic components is in
electrical contact with a terminal (12, 14) at either end. As
employed, for example, in a potentiometer, a movable wiper or
contact, C, is adjusted along the hybrid element in contact with
the conductive plastic component of the hybrid element.
As herein employed, the term hybrid element is intended to mean a
two-component resistance element not of the wirewound type which
element comprises a cermet component overlaid by a conductive
plastic component which is in intimate contact with the cermet
component along an elongate zone between two conductive terminals
which define electrical ends of the zone and of the components.
Typical variable resistor constructions in which hybrid resistance
elements as hereinabove described are usable are illustrated in the
following numbered U.S. Pat. Nos. 2,926,324; 2,976,507; 3,178,664
(adaptable for the construction depicted in FIGS. 1, 2 and 3);
2,917,721; 2,958,839; and 3,111,640 (adapted for use with
constructions shown in FIGS. 4, 5 and 6).
In the case of prior art cermet elements, attainment of higher and
still higher TR is accompanied by commensurate increases in
abrasiveness of the element surface and consequent decrease in
effective lifespan (number of cycles of wiper contact operation)
due to increasing rapidity of wearing away of the wiper contact.
This undesirable feature intensifies or is accentuated as physical
size of the element is decreased, as is necessary in the continuing
effort to reduce the volume and weight of electronic components.
Thus when attempts have been made to produce cermet elements of
very high values of TR and of commercially acceptable dimensions,
the effective lifespan was reduced to very low values and so low as
to render the product commercially unacceptable. For example, in
rotary potentiometer cermet elements it is desirable to have
effective lifespan of the order of several millions of rotational
cycles of operations; and in attempts to obtain cermet elements
having a TR in the range from 1 to 10 megohms the lifespan average
was reduced to values in the range from a few thousand cycles to a
few hundred thousand cycles, far below the desirable minimum. As a
consequence of that situation, resort to other types of elements
was had in all cases wherein long lifespan was a requisite, with
resultant lower load capacity, less desirable CRV and lower maximum
operating temperature. Conductive plastic elements, as is well
known, are characterized by low load capacity and low maximum
operating temperature, as well as change of TR with passage of
time. By application of the present invention, using cermet
components that are otherwise entirely too abrasive for use and
with lifespan capability of very low order and with undesirable
+Tc, elements are made not only capable of enduring many millions
of operational wiper cycles, but with additional meritorious and
unobvious improvement in Tc and CRV. Thus, hybrid elements
according to the invention not only retain the previous
advantageous and desirable characteristics of high-power rating and
high operating temperature rating of cermet elements, while
concurrently extending the practical total resistance (TR) range
and reducing the Tc to near zero or negligible values, but further
are characterized by extremely high lifespan figures. These
features are illustrated by typical hybrid rotary potentiometer
elements which, for example, operated through millions of
operational cycles before element or wiper contact failure.
* * * * *