US2680177A - Logarithmic potentiometer - Google Patents

Description

June 1954 A. ROSENTHAL 2,680,177

LOGARITHMIC POTENTIOMETER Filed Nov. 15, 1951 3 Sheets-Sheet l INVENTOR. L EILIIS Fl. REISENTHFIL.

BY QWWA w. [BENT June 1, 1954 L. A. ROSENTHAL 2,680,177

LOGARITHMIC POTENTIOMETER Filed Nov. 15, 1951 3 Sheets-Sheet 2 INVENTOR. LCILIIS H. HDSENTHFJL SENT June 1, 1954 L. A. ROSENTHAL 2,680,177

LOGARITHMIC POTENTIOMETER Filed Nov. 15, 1951 3 Sheets-Sheet 3 4Q 4 INVENTOR.

1.. GU15 H. R0 SENTHHL FJEENT Patented June 1, 1954 2,680,177 LOGARITHMIC POTENTIOMETER Louis A. Rosenthal, N

signor to Myron A.

ew Brunswick, N. J., as- Coler, New York, N.

Application November 15, 1951, Serial No. 256,504

12 Claims.

This invention relates to logarithmic potentiometers and particularly the type utilizing composition resistance elements.

A need has long existed for a rugged logarithmic potentiometer which is low in cost, sufficiently accurate for precision applications and suitable for high frequency use. Commercially available resistance wire Wound on a tapered card or the step potentiometer (step ladder network) in which a switching mechanism varies the output in a stepwise fashion by changing the location of rithmic voltage divider.

It is an additional object of this invention to provide a wide range logarithmic potentiometer.

A further object of this invention is to provide a logarithmic potentiometer capable of carrying relatively large currents.

It is still another object of this invention to provide a logarithmic voltage divider suitable for high frequency applications.

These and other objects, features and advantages will be made more apparent by reference to the following description and the accompanying drawings in which the same symbols refer to the same things.

Figure 1 presents in a combined isometric and schematic drawing a simple version of a logarithmic potentiometer utilizing a slab of conductive material.

Figure 2 shows, in elevation, a preferred embodiment of the potentiometer element of this invention.

Figure 3 shows isometrically, an embodiment of the potentiometer of this invention in a form suitable for an angular type potentiometer.

Figure 4 shows, in elevation, an assembled potentiometer incorporating the element shown in Figure 3.

Figure 5 is a sectional view of the potentiometer shown in Figure 4.

Figure 6 shows, in cross-section, a plan view of the potentiometer of Figure 4.

In Figure 1 there is shown a slab 2 of resistive material having a highly conductive layer 3 over face 12 and a strip of similar highly conductive material 5 along the intersection of face 6 and face 7. A source of potential 8 is connected between strip 5 and layer 3. Movement of the probe 9, a distance :1: along face 1 would result in a logarithmic variation of output voltage E as indicated by meter l0 connected between probe 9 and layer 3.

If this slab has the dimensions h, w, and Z the voltage E will vary with .r, the position along probe surface 1, according to the equation:

The two dimensional solution of La Places equation shows that for an infinitely long slab the potential distribution over face 4 of the slab is of the form E=Esesin ay where y is the distance in centimeters above face An infinitely long slab would not allow any reflections since all voltages propagated down the slab are attenuated. slab the factorsin ay Along the top edge of the becomes sin 1r/2 or 1. The

thickness .0 of the slab will determine the power dissipation qualities (of the potentiometer) and. the impedance level of a potentiometer made from a given conductive plastic.

I have found that in practice the theoretical values indicated above are not attained and only about the first forty percent of the simple slabshaped element is usable with any degree of accuracy.

It is believed that since the slab is not infinitely long, the voltage Es applied between contacts 3 and 5 propagates a wave which is not zero when it reaches the right hand edge, a reflected wave being introduced. The reflected wave produces a voltage which decreases in logarithmic manner from right to left. Accordingly, the output voltage is the sum of at least two exponential voltages.

I have found that by providing a tapered portion it, the face of which is coated with highly conducting material (shown in Figure 2), the propagated wave is dissipated much as an ocean wave on a beach. Since practically all reflected waves are eliminated only the propagated wave is significant and the desired logarithmic variation of voltage with linear change of the contact position is obtained. A greater useful attenuation is attainable since the right hand side is at true zero potential.

The particular taper angle is not critical and an angle of 45 has been found satisfactory.

The element shown in Figure 3 is intended for use in an angular potentiometer. A materialof appropriate specific resistivity is 'rnolded'in the form of a cylinder. Two cuts, along faces 18 and 2e, are made to remove an angular section. It is apparent that a more complex imold-could be utilized which will directly mold the taper portion and thus eliminateithacuttingoperation. The equipotential surface may be created by such means as spraying the element with metal, electroplating, and coating with conductive lacquer. Terminals may be attached by the use-of metal inserts or by soldering to sprayed metal areas.

The embodiment of-thisinvention inanangular potentiometer is shown-,in-Figures f-l, -and 6. The resistance-elementE-Z, such as ,is shown in Figure 3 is attacliedto:insulatingplastic base A brass bushing -26 base 2 3. The shaft 28 supports an insulating arm 3!! which carries moving contact t2, which is made of carbon, a conncctingwirefi l between contact 32-and a beryllium copper spring contact 35 which rides against-copper pickup'ring 38. A strap 4b forms an integralpart of the ring -ele- :mentand serves as apterminal thus eliminating connections and provides a desirable short path for high frequency applications. An insulating strip-42 is provided to support-the terminals. A

connection :44 :is provided between terminal 135 and sprayed'metal equipotential face 48. Similarly an electrical connection 58 is provided between terminal 52 and contact face 54. .A protective cover=5fi is:provided. This'cover may be transparent'and it has been found desirable to cement it to the insulating base to seal out dirt and moisture. Wire clip 58 looks shaft 28 into bushing 2'6. Nuts 60 are used to mountthe unit ontoapanel board.

The following example clearly shows the ad- L vantage derived fl'0ll'lith6 practice of the preferred embodiment of this invention wherein ,a tapered portion is provided.

vA. potentiometer was madeby molding a ring froma composition material containing about 5 carbon. black and ya phenolic resin which exhibiteda specific resistivity of 1130 ohm-centimeters when molded.

The ring .was 2 inches in outside diameter,

1%; inches in inside diameter and /2 inch high. The ring was cutthrough inplanes parallel to its axis .and radiusattwopoints about inch apart leaving a ringsegment subtending about 320 of are.

.A conductive .equipotential surface was formed on one of the ring shaped flat faces and on, one of the. cut faces by means of an air drying silver lacquer. Terminals were applied to the equipotential surface and to the uncoated fiat surface adjacent to the uncoated cut surface. The unit was constructed substantially as described above.

Moving-the contactthrough 108 of are (which represents 34% of the availabletrack) provided anattenuation of '30 decibels. The variation of this attenuation with angular displacement of the contact followed'the desired logarithmic re- ,lationship with-an accuracy ofabout 1%. It was foundthat if an accuracy of 5% were tolerable theuseful trackcovered 153 ofrotation (equivais integrally molded -.to the iii) ably at least one-eighth inch in thickness.

lent to 48% of the total available track) with a corresponding increase to 38 db of attenuation.

The ring was dismounted from the potentiometer and a segment wa removed from one end to provide a 45 tapered portion similar to the one shown in the element of Figure 3.

The resulting cut surface was painted with silver lacquer so as to form a continuous equipotential surface withthe flat surface and the potentiometer was reassembled.

The variable contact of the modified potentiometer was rotated through an angle of -(56% of theavailable total) with an attenuation of 45 decibels without deviation from a logarithmic relationship of-more than 1% at any point.

Through 250 of rotation (78% of the available track) an. attenuation of 60 decibels was obtained with an adherence to function within 5%.

This experiment clearly shows the importance of the tapered portion.

Itiszapparent'that the shaped resistance elementmaybe made in many diverse ways; for example, it may be machined from a large block,

cast in a mold or molded under pressure. The choice of material would depend on the desired characteristicsythere being many suitable materials commercially available.

invention may be as great as four-tenths of its outside radius without serious departure from the desired function. For the purposes of the invention it is preferred that for small sizes of potentiometers, say 1.5 to 4 inches in diameter, the thickness-of the ring be held to 0.03 to 0.4 times the outside radius of the ring. For larger potentiometers a thicknessof /4 to /2 inch will generally be appropriate.

If a ring-shaped elementis made thicker than about 0.4 times its outside radius modified logarithmic relationships between angular rotation and output potential result. These may be useful incertain specific applications.

While there has been described what are at present considered the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made herein without departing from the invention.

dimensional block of resistive material having a first contact making face, a second face having a first portion essentially parallel to said first face and havin a second portion tapered so as to meetsaid first face, a highly conductive coating on said second face including said tapered portion, a movable contact for establishing electrical connection to said contact making face, means for moving said contact, means for applying' a potential between a fixed location at the end of said contact making face opposite said tapered end and spacedfrom said highly conductive'coating, and means to connect an output circuit between said movable contact and said highly conductive coating.

2. The apparatus of claim 1 wherein the thickness of said block is at least one-sixteenth of an inch.

3. A resistance element for inclusion in a logarithmic potentiometer device having a contact making face and a highly conductive layer spaced from said contact making face, said layer having a first portion substantially parallel to said con and a second portion at an angle to said contact making face.

4. The element of claim 3 in the form of a hollow cylinder wherein said contact making face is perpendicular to the axis of revolution.

5. A logarithmic potentiometer comprising an insulating plastic base, a resistance element in the form of a hollow cylinder having a contact making face and a conductive equipotential surface parallel to said face, means to fixedly conalong said contact making surface, a terminal, means to electrically connect said movable contact with said terminal.

6. The element of claim 5 wherein the thickness of said resistance element is within the range 0.03 to 0.4 of the outside radius of said cylinder.

7. The apparatus of claim 5 wherein the radius 8. A logarithmic potentiometer comprising an insulating plastic base, a resistance element in dius being not greater than two inches.

10. Apparatus for obtaining an electric signal which varies in a logarithmic manner with a linearly varying adjustment comprising a section and second faces, said first and third faces being coated with a highly conductive material, means for electrically contacting a said second face,

wherein the cylindrical element is between 2.5 to 32 of the wall of said hollow References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,114,330 Borden Apr. 19, 1938 FOREIGN PATENTS Number Country Date 560,769 France July 18, 1923

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