US3760322A

US3760322A - Variable resistor

Info

Publication number: US3760322A
Application number: US00267630A
Authority: US
Inventors: M Kogo; H Honda
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1971-06-30
Filing date: 1972-06-29
Publication date: 1973-09-18
Anticipated expiration: 1990-09-18
Also published as: CA949145A; JPS5241463B1

Abstract

A resistive element has a gradient of resistivity extending along its length. It is held in slidable contact with two brushes spaced-apart from each other. The resistance between the brushes can be varied by changing their position relative to the resistive element and the gradient of resistivity. The resistive element may have an arcuate configuration and the relative position may be changed by rotating the element.

Description

iiite States atent 1191 1111 3,760,322

Kogo et a1. [4 Sept. 18, 1973 VARIABLE RESISTOR 2,798,140 7/1957 Kohring 338/137 [75] inventors:MasanoriKogo;HisashiHonda,both

OfTOkYOJaPa" 2,681,967 6/1954 Harrison et a1. 338/89 2,005,922 6/1935 Stoekle 338/140 Assignee: Nippon Electric Companyjdiinited, I

Tokyo,Japan Filed: June 29, 1972 Appl. No.: 267,630

Foreign Application Priority Data June 30, 1971 Japan 46/48316 U.S. C1 338/1137, 338/142, 338/150 Int. Cl 1101c 9/02 Field of Search 338/89, 137, 138-142,

References Cited UNITED STATES PATENTS 10/1929 Greenewalt 338/141 Primary ExaminerBernard A. Gilheany Assistant Examiner-D. A. Tone AttorneyNicho1 M. Sandoe et a1.

[57] ABSTRACT A resistive element has a gradient of resistivity extending along its length. it is held in slidable contact with two brushes spaced-apart from each other. The resistance between the brushes can be varied by changing their position relative to the resistive element and the gradient of resistivity. The resistive element may have an arcuate configuration and the relative position may be changed by rotating the element.

5 Claims, 11 Drawing Figures PATENTEUSEHBIQB 3.760.322

' satmurd v Z (PR/OR ART) R HG. la Q FlGlb FIG. 2a 2b vanraatn nnstsron BACKGROUND OF THE lNVENTiON Tiis invention relates to a variable resistor for use in electrical circuits and, more specifically, to a variable resistor including a film of resistive material having a gradient of resistivity in a predetermined direction.

in a conventional variable resistor, terminals are provided at the ends of the resistive film and a brush is held in slidable contact with the film to provide a variable resistance between one of the terminals and the brush. The resistance then depends on the position of the brush with respect to the film.

One of the shortcomings of this type of variable resistor is that the inductance component appearing in parallel with the variable resistance undergoes a consider able change depending on the resistance selected. A principal object of the present invention is therefore to provide a variable resistor capable of resistance variation without without causing a change in inductance inductance component.

SUMMARY THE INVENTION THE INVENTION The variable resistor of the present invention comprises a resistive element having element having a gradient of resistivity that varys in a predetermined manner along its length and at least two brushes separated by a predetermined interval and held in slidable contact between the resistive desired The desired gradient of resistivity can be given to the resistive element by tapering its width or gradually changing its composition. More than two slidable brushes may be used as required. When more more than two brushes are used, a different variable resistance can be across every two every two brushes. The brushes so interconnected so that the distance between them remains constant.

In the variable resistor of this invention, a desired variable resistance is provided between the slidable brushes, while maintaining the accompanying inductance component at a fixed value.

Another feature of the present invention is that the variable resistance never takes the value zero ohms. This contributes to the simplification of a circuit including a finite minimum resistance.

BRIEF DESCRTPTTON OF THE DRAWTNGS The present invention will now be described with reference to the accompanying drawings, in which:

FiGS. 1(a) and (b) show equivalent circuits ofa conventional variable resistor for the direct-current and extremely low frequency region and for the high frequency region, respectively;-

FiGS. 2(a) and (b) show equivalent circuits of an embodiment of the invention for the direct-current and low frequency region and for the high frequency region, respectively;

FlG. 3 is a cross-sectional view of a first embodiment of the invention;

FIG. 4 shows a circuit diagram of a bridged-T type variable resistance attenuator to which this invention is applicable;

FIG. 5 shows a cross-sectional view of a second embodiment of of this invention adapted to the attenuator of HG. d;

FIG. ti; is an equivalent circuit ofa third embodiment of the invention;

FIG. '7 is an equivalent circuit of a fourth embodiment of the invention;

FHG. b shows diagramatically, the impedance variation characteristics of the invention as compared to those of a conventional variable resistor; and

FIG. 9 shows diagramatically, the frequency characteristics of the conventional bridged-T type variable attenuator and those of the attenuator of FIG. 4 in comparison with the second embodiment of the invention shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. ]l(a) shows an equivalent circuit for a conventional variable resistor comprising a resistive film it) having terminals l and 3 at its ends, and a slidable brush 2 operating in the low frequency range. The desired variable resistance is obtained between the brush 2 and the terminal 3. The resistive film lit) has a uniform specific specific resistance, and its inductance component is proportional to the length, but negligibly small when a direct current or extremely low frequency signal is applied. Likewise, the capacitive component is also negligible. Generally, the impedance Z, between the terminals 1 and the brush 2 is given by:

where R stands for the resistance per unit length of the resistive film 11411; L, for the inductance per unit length of the film x, for the length of the section of the film iltl lying between the terminal ll andthe brush 2; and w w, for the angular frequency. Differentiating Eq. (2) with respect to x and w,

From Eqs. (3) and (4), it is apparent that I Z,| increases linearly with each successive increment of the length x, and non-linearly with each successive increment of the angular frequency w.

The impedance Z, of an ideal variable resistor having neither a capacitance component nor an inductance component may be expressed as follows:

where resistive film is assumed to have a uniform specific resistance R. The ratio A, of |Z l to |Z,| is given More specifically, the absolute value of impedance is always larger in the conventional variable resistor than in the ideal variable resistor. Also, the larger the angular frequency w, or the smaller the specific resistance R of the resistive film, the greater the ratio A, of Eq. (6) becomes. in other words, with the increase of the frequency, the impedance has a greater effect on the variable resistance, making the deviation from the ideal variable resistor greater. An equivalent circuit for a conventional variable resistor at high frequency is shown in MG. i(b).

in the equivalent circuits of the variable resistor of the the invention shown in FlGS. 2(a) and slidable slidable brushes 5 and 6 are in contact with aresistive film 30. The film 30 (resistive element) has a nonlinear gradient of resistivity respect respect to its length. Two brushes 5 and 6, spaced by a predetermined a predetermined interval, are held contact slidable contact with the resistive film 30. between interval between the brushes and the nonlinear gradient in the resistivity of the film 30 are that the so that the resistance and the brushes 5 and 6 exhibits an proportion in linear proportion movement the lengthwise movement of will be brushes. It will be apparent, is not that the invention is not limited to this resistive film arrangement. In general, the gradient may change continuously, linearly or nonlinearly. For simplicity in this explanation, it is assumed here gradient, in other in the gradient, in other words, the rate of the change of the resistivity of linear film 30, is in linear proportion its length. the position selected along its length.

Assuming that the distance from the lefthand end of the resistive film 30 to the brush 6 is x, the resistance therebetween is xR, and the spatial interval between the brushes 5 and 6 is unity, then the impedance Z earse. hebwsh 5 and 6 an be aasam.

where x a l and the variables are the same as those in Eqs. (1) and (2). The ratio A of the impedance lZ i of the present variable resistor to the impedance iZ l of the ideal variable resistor is given by:

9) where x Derived from Eqs. (2) and (3), the ratio A of the air-- solute value of the impedance of the present variable resistor to that of of the conventional one is given b In other words, in the range of x 2 1.0, the variable impedance Z of the present variable resistor is smaller than that Z of the conventional resistor and closer to that of the ideal resistor. FIG. 8 shows the value of A given in Eq. (10) with respect to the length x.

In such a variable resistor, the resistance obtained across the brushes 5 and 6 cannot take the value zero ohms even under the state of unity thus maintaining a certain minimum resistance value. The variable resistor of this invention is therefore highly desirable for those circuits which require a minimum constant resistance.

Referring now to H6. 3, the first embodiment of the invention, an equivalent circuit of which is shown in FIG. 2, two

brushes

106 and 107 are attached to a cylindrical housing 105 and held in contact with an arcuate resistive film 102 attached to a disc-shaped base I03. The film 102 may form a complete circle. The resistive film 1102 has a nonlinear gradient of resistivity in the circumferential direction. This gradient is attained by tapering the thickness or the width of the film 102. Alternatively, it may be attained by varying the composition of the material of which the film is made. By the expression gradient of resistivity it is meant that the resistance per unit length of the film varies in a predetermined manner as as it is measured at different points along its length. The base 103 is pressed downwardly by a spring 104% thus permitting rotation under the control of a rotatable disc 1101. The desired variable resistance value is thus obtained between the brushes I06 and 107, by changing the relative position of the brushes and the film 102.

Referring now to FIG. 4, a bridged-T type variable attenuator to which the present invention is applied permits a variation in the attentuation depending on the resistance of resistors Rs and Rp which is selected. When conventional variable resistors are used in this attenuator, the voltage standing wave ratio (VSWR) is unavoidably increased with an increase in the frequency. This tends to cause an increase in the impedance Z Referring now to FIG. 5, a second embodiment is shown which is adapted to the variable attenuator of FIG. 4 and has a dual variable resistor which may be viewed as two separate variable resistors. Each of these variable resistors is the equivalent to one resistor of the circuit shown in FIG. 2. The dual variable resistor has two brushes 11M and 117 fixed to a base 119 which also serves as a cover. Two arcuate

resistive films

112 and 112 having nonlinear resistivity gradients are attached to bases I13 and 1M and kept in slidable contact with the brushes 116 and 117. The resistive films 112 and 112' are moved as a disc 101i on which the bases I13 and 1114 are mounted is rotated. These elements are contained in a cylindrical housing 105 and a closure including a base 119. A pair of brushes are provided for each of the resistive films 112 and T12.

With the dual variable resistor of FIG. 5 used as the resistors Rs and Rp of the the bridged-T variable attenuator variable attenuator of FIG. 4, the impedances of the resistors Rs and Rp exhibit virtually no change in the inductive components, as indicated by Eq. (7). Therefore, the voltage standing wave ratio (VSWR) can be kept unchanged as shown in FIG. 9, which illustrates two groups of curves showing the attenuation vs. frequency and the voltage standing wave ratio (VSWR) vs. frequency characteristics of the present variable attenuator as those with those of variable-resistor-based attenuator. variable-resistor-based attenuator.

In the bridged-T type type variable attenuator of FIGA, resistances resistances r and r of the resistors Rs and Rp can be expressed as a function of value attenuation value R0 as follows:

r R0(e l)i Where a is given in neper.

The zero attenuation value is achieved by making the resistance rs and rp zero and infinity, respectively. Conversely, infinite attenuation achieved achieved by making rs and rp infinity and zero, respectively. Practically, however, these requirements cannot be met, infinite impedance impdance is difficult to achieve with a conventional variable resistor. This results in a residual attenuation which causes a high voltage standing wave ratio. Therefore, to achieve a satisfactorily low voltage standing wave ratio with the conventional device, at least one resistor must be in series with either its or Rp or both. In contrast, the variable resistor of the invention never takes the value zero ohms. This makes it it possible to dispense with any additional resistance ele ments and consequently to simplify miniatuarize miniaturarize the attenuator as a whole. This applies to any apparatus or device in which the present invention finds application.

Referring now to FIG. 6, a third embodiment of this invention has four

brushes

202, 203, 204, and 205 spaced at predetermined intervals and held in slidable contact with a resistive film 201. This variable resistor makes it possible to to obtain two desired variable resistance values from one resistive film 201, one from the

brushes

202 and 203 and the other from the brushes 204 and 205. In other words, a dual variable resistor can be constructed using one resistive film 201i.

Brushes

203 and 204 may be connected to each other when these two brushes are to be at an equal potential. It will be apparent that more than four brushes can be used.

Referring now to FIG. 7, the fourth embodiment of the invention includes a resistive film

30I having terminals

304 and 305 at the ends, respectively, and brushes 302 and 303 held in slidable contact with the resistive film 3011. With a voltage applied between the

terminals

304 and 305, the desired voltage is derived from the

brushes

302 and 303. In this variable resistor, the inductive component of the output impedance impedance is small and constant. Therefore, the use of this variable resistor in a high frequency device will contribute substantially to better performance.

In the foregoing description, we have explained the present invention with reference to several exemplary embodiments and their application. However it will be apparent to those skilled in the art that other modifications are possible without departing from the spirit and scope of the invention.

We claim:

I. A variable resistor comprising an electrically resistive element the resistance of which changes along its length in a predetermined manner to form a gradient of resistivity, and at least two spaced-apart movable brushes connected to each other so that the distance between them is fixed and in slidable contact with the resistive element, whereby the resistance obtained between the brushes can be selected bypositioning the brushes relative to the resistive element and the gradient of resistivity but the inductance between the brushes remains substantially constant regardless of the position selected.

2. A variable resistor set forth in claim I, wherein said brushes and said resistive element are supported by a housing whereby said brushes are held against said resistive element.

3. The variable resistor set forth in claim ll, further comprising a disc-shaped member to which said resistive element is attached.

0. The variable resistor set forth in claim 3, further comprising a housing by which said disc-shaped member is rotatably supported.

5. The variable resistor of claim I, further comprising a cylindrical housing and a disc-shaped member rotatably supported within the housing, the resistive element having an arcuate configuration and being attached to the disc-shaped member for rotation therewith.

Patent No.

UNITED STATES PATENT ()FFICE Dated September Line Line

Line

Page 1 of A Inventor(s)KOqO, M. et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 4, Iiis" should be --This--;

delete "without" (1st occurrence) delete "inductance";

delete "element having" (lst occurrence) delete "every two";

Column 2 delete "specific" (lst occurrence);

"andthe" should be --and the--;

n II

delete w delete "the" (lst occurrence) after insert -2(b)--; delete "slidable";

"so" should be -are-;

II II o w po-m lo MCI-(-29) Page 2 01" UNEEED STA'lES PATENT ()FFICE (IER'H FEE/VIE (W CQRR M11310 N Patent No. ,7 1 Dntmh QQBEQQQQP Inventor(s) Kogo, M. et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3 Line 2, change "respect"(lst occurrence) to with-; Line 3, after "spaced" insert ---apart--; delete "a predeterminedflst occurrence); change "contact" to ---in--;

Line 5, after "30," change "between" to --The-;

Line 7, change "that the" to -selected-;

change "and" to -between--;

Line 8, change "proportion" to --increase;

change "movement" to --to--;

Line 9, change "will be" to the;

Line 10, change "is not" to -however-;

change "gradient in other" to -that the change-;

Line 15, change "linear" to -the--;

Line 16, change "its length" to -to-;

Line 32, Z 2,, should be 52 g 52 E lage 5 of UNE'EED STATES PA'EESNT OFFICE GERTENQATE UP CGRREUHN Patent No. 3,760 ,322 Dated September 18, 1973 Inventor(s) K090, M. et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4 Line 17, "attentnation" should be attenuation;

Line 42, delete "variable attenuator";

Line 50, after "as" change "those" to compared--;

after "of" insert a conventional--; Line 51, after "attenuator"(lst occurrence) delete "variable resistor based attenuator.-;

Line 52, before "variable" delete "type";

Line 53, change "resistances" (1st occurrence) (:0 --the--;

Line 54, after "of" change "value" to the- Line 61, after "attenuation" change "achieved" to ---is--;

Line 63, after "met" insert -because-'-;

Line 64, after "impedance" delete "impdance";

-' OHM 0-1050 (1O 19) Page LP of UNHED STATES PATENT OFFICE CERTlFlQATE UP CURRECTIQN Patent No. 3,760 ,322 Dated September 18, 1973 Inventor(s)KOg'O, M. et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5 Line 4, after "it" delete "it"; Line 6, change "miniaturi'ze"t0 and;

Line 14, after "to" delete -to-;

Line 30, after "output" delete "impedance f' Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM F'O-lOfll) (HLFIH

Claims

1. A variable resistor comprising an electrically resistive element the resistance of which changes along its length in a predetermined manner to form a gradient of resistivity, and at least two spaced-apart movable brushes connected to each other so that the distance between them is fixed and in slidable contact with the resistive element, whereby the resistance obtained between the brushes can be selected by positioning the brushes relative to the resistive element and the gradient of resistivity but the inductance between the brushes remains substantially constant regardless of the position selected.

2. A variable resistor set forth in claim 1, wherein said brushes and said resistive element are supported by a housing whereby said brushes are held against said resistive element.

3. The variable resistor set forth in claim 1, further comprising a disc-shaped member to which said resistive element is attached.

4. The variable resistor set forth in claim 3, further comprising a housing by which said disc-shaped member is rotatably supported.

5. The variable resistor of claim 1, further comprising a cylindrical housing and a disc-shaped member rotatably supported within the housing, the resistive element having an arcuate configuration and being attached to the disc-shaped member for rotation therewith.