US2847640A - Dielectric voltage divider - Google Patents

Description

G. E. PlHL DIELECTRIC VOLTAGE DIVIDER 2 Sheets-Sheet 1 Aug. 12, 1958 Filed March 4, 1954 Q emg e:

L 1 z E United States Patent DIELECTRIC VOLTAGE DIVIDER George E. Pihl, Abington, Mass., assignor, by mesne assignments, to Acton Laboratories, Inc., Acton, Mass.

Application March 4, 1954, Serial No. 414,206

1 Claim. (Cl. 323-79) The present invention relates to a voltage divider, and more particularly to an improved voltage. divider having a constant RC product.

In many applications it would be desirable to have manually and mechanically controlled voltage dividers which have a wide frequency response. Ideally such voltage dividers should have zero frequency distortion, zero phase shift, infinite resolution, and be capable of linear or prescribed non-linear operation.

In accordance with the present invention it is proposed to provide a voltage divider, which to a large degree satisfies the foregoing specifications, by a construction employing a pair of fixed electrodes and. a movable electrode immersed in a liquid dielectric which has a loss characteristic, and hence produces a voltage divider having aconstant RC product.

It, therefore, is an object of the present invention to provide an improved voltage divider having infinite resolution which is simple and economical to manufacture.

A still further object of the invention is to provide an improved dielectric voltage divider which for a given frequency range has zero distortion, zero phase shift and infinite resolution.

Still another object of the invention is to provide an improved dielectric voltage divider of infinite resolution and constant RC product having the desired or prescribed characteristic of operation.

Other and further objects of the invention subsequently will become apparent by reference to the following. description taken in conjunction with the accompanying drawing, wherein:

Figure 1 is a diagrammatic representation of a voltage divider or potentiometer constructed in accordance with the principles of the present invention;

Figures 2 and 3 are further diagrammatic representations of another form of voltage divider or potentiometer contemplated by the present invention;

Figure 4 is a further diagrammatic representation of a type of dielectric voltage divider contemplated by the present invention;

Figures 5 and 6 are still other diagrammatic representations of a further embodiment of the present invention;

Figure 7 is a graph representative of the characteristics of the voltage divider shown in Figure 4;

Figure 8 is a graph representative of the characteristics of the voltage divider illustrated in Figures 2 and 3; and

Figure 9 is a graph representative of the characteristics of the voltage divider shown in Figures 5 and 6.

In accordance with the present invention a dielectric voltage divider has been provided which has a constant RC product. This is accomplished by the simultaneous variation of capacitance and resistance with respect to a movable electrode immersed in a liquid dielectric having a constant loss characteristic and located between another pair of electrodes immersed in the liquid dielec- "ice versely as the configuration or relation between the elec-.

trodes. is changed. Each elementary capacitive. path between. the electrodesmust be compensated by an identical resistive leakage path. This. makes it imperative that any solid dielectric used as insulation should. be. at a minimum and should be placed at locations where the electric field is weak and substantially constant. In. many applications it might be desirable to employ a metal housing for the liquid to reduce the effect of stray capacitive coupling.

An elementary voltage divider is schematically illustrated in Figure 1 having a casing 10 which is connected to ground by a conductor 11. The casing 10 preferably is a metal housing so as to suitably contain the liquid and minimize the effects of stray capacitance. The container 10 is filled with a liquid dielectric having a certain loss characteristic. Immersed in the liquid dielectric are two fixed electrodes A and B, between. which is located a movable electrode D. A suitable source of potential E supplied from a source 11 having one terminal connected to ground is applied to the electrode. A. The electrode B is connected to ground and the casing 10. The movable electrode D is connected to one output. terminal 12, the other output terminal, 13 being connected to ground. It now may be assumed that the movable electrode D is the control electrode from which the output is taken, and that it is movable in a vertical direction. Thus there is obtained. a variable resistive-capacitive voltage divider where the ratio: of the output voltage to the input voltage is given by the following expression:

ii 1 +jRgwCz) Where:

Z =impedance between D and ground Z =irnpedance between D and A R =parallel resistance between D and ground R =parallel resistance between A and D C =parallel capacitance between D and ground C =parallel capacitance between A and D Note that R C =R C for all positions of' D at all frequencies.

From the foregoing it is apparent that the output voltage has zero phase shift and zero frequency distortion, and hence the dielectric voltage divider is suitable for broadband frequency applications.

The foregoing example and analysis assumes the ideal condition where the resistances R R and the canacitances C C exist solely because of the dielectric fluid. In a practical embodiment, stray capacitances also exist between the output terminals and between the other connections to the voltage divider. Generally these stray capacitances are fixed quantities, and hence the divider can be compensated by the use of suitable resistors. In Figure 1 the stray capacitances have been indicated by the capacitors 14 and 15 having dotted line connections to the various electrodes A, B and D. A suitable resistor 16 is connected across the capacitance 14 and another resistor 17 is connected across the capacitance 15. The output terminals now would be identified as terminals 18 and 19. The resultant arrangement would still provide a voltage divider having a constant RC product. It will be appreciated that the addition of the resistors 16 and 17 constitutes a resistive loading which changes the ratio of output to input voltage as a function of the control setting. This, however, can be taken into account when designing the electrode configuration for a particular control characteristic. which it is desired to achieve.

In certain other applications of a dielectric voltage divider of this kind it might be impossible to avoid a resistive loading of the output, in which case it may be desirable to deliberately load with capacitors the voltage divider in order to provide the desired RC product.

The foregoing analysis has assumed that the dielectric constant of the liquid and resistance characteristic remain fixed under all conditions. It, of course, will be appreciated that some dielectric fluids might have characteristics which change as a function of temperature or frequency. in general it has been felt, however, that where a single pure dielectric is employed, the characteristics are substantially constant. For the greater part the alcohols have shown the greatest promise, although acetone and ethyleneglycol also have been employed.

For a voltage divider where the movable electrode is actuated substantially in the manner shown in Figure 1, the output characteristic would be linear assuming a constant input voltage for the signal source 11.

Another embodiment for the invention is schematically illustrated in Figures 2 and 3 showing a casing 21 containing fixed electrodes 22 and 23 connected to suitable terminals 24 and 25 cooperating with a grounded terminal'26. The terminals 24, 25, and 2.6 are for connection to a center-tapped voltage source. A movable electrode 27 arranged to rotate in a circular path perpendicular to the electrodes 22 and 23 is connected to a terminal 28. The movable member 27 is insulated from the liquid except for its tip end indicated by a circle in the drawing. The cooperating output terminal 29 is connected to ground. This embodiment produces a nonlinear relation between the input and output and is generally referred to as a sine-cosine potentiometer. The relationship between the constant input voltage and the output voltage expressed in terms of angular rotation of the member 27, is shown in Figure 8, from which it will be seen that the curve is a true sinusoidal curve. In one embodiment the dielectric materials employed were of the alcohol family which might be isopropyl, ethyl or methyl. Such voltage divider had infinite resolution and operated over a frequency range from zero to 400 cycles per second without distortion or phase degradation. The input resistance was about 50,000 ohms, and the output resistance was 1,000,000 ohms.

In another embodiment it is desired to provide an approximately logarithmic response function such as that illustrated in the graph of Figure 7. Therein it is shown that the input voltage was constant and that the output voltage is shown as the logarithm plotted against a dial setting or indication of the linear movement of the movable electrode. Such structure is diagrammatically shown in Figure 4 wherein there is a container 31 which might be of metal and which is connected to ground. The container has a fixed electrode 32 which is connected to an input terminal 33, the other input terminal 34 being connected to ground. Mounted on the end of a movable rod 35 is a movable cup-shaped electrode 36 which is connected to an output terminal 37 the other output terminal 38 being connected to ground.

For convenience in manufacture the fixed electrode 32 is actually a solid element having a circular top surface, while the movable electrode 36 is a hollow cylinder with the remote end closed by a plate. For accurate control of the linear movement of the movable electrode 36, the shaft 35 is driven through a suitable gearing arrangement, not illustrated, which is coupled to an indicator and dial.

In a still further application of the dielectric voltage divider, it was desired to have an output characteristic similar to that shown in the graph of Figure 9 which shows the output voltage as compared to the percent of current in one of two actuating coils which rotate the movable electrode. This arrangement is shown in Figures 5 and 6, wherein there is a container 41 which is connected to ground and to one of the input terminals 42. The other terminal 43 is connected to an electrode A fixed within the container 41. Another similar electrode B is connected to ground. One output terminal 44 likewise is connected to ground, and the other terminal 45 is connected to a sector-shaped fixed electrode D. A movable electrode F is suitably supported on a shaft 46 and actuated by an electromagnetic device. The movable electrode Fhas two vanes, one of which is always associated with the fixed electrode D. The other vane is positioned so as to be closer or farther away from the two fixed electrodes A and B.

Supporting shaft 46 for the vane electrode F is actuated by a rotor 47 having a plurality of permanent magnets arranged in opposite directions so that one north pole is at the bottom and the other north pole is at the top. Two actuating coils 48 and 49 are provided which are supplied with direct current to position the movable electrode F to produce the desired output voltage.

One application of the voltage divider shown in Figures 5 and 6 was in a compressor amplifier where the divider was preceded by a cathode follower and succeeded by a cathode follower. A constant potential source of direct current was used as reference and supplied to a direct current amplifier. A portion of the alternating current from the last cathode follower was passed through a rectifier to control the operation of the direct current amplifier which supplied current to the coils 48 and 49 to control the position of the movable electrode F. Such an arrangement constitutes a substantially distortionless regulator having suificient loop gain to minimize the changes in output level. The compression range of such system is approximately 35 db, and the useful frequency extends beyond one megacycle.

While for the purpose of illustrating and describing the present invention certain preferred embodiments have been shown, it is to be understood that the invention is not to be limited thereby since such other embodiments are contemplated as may be commensurate with the spirit and scope of the invention set forth in the accompanying claim.

I claim:

A voltage divider having capacitance and resistance comprising a casing containing a liquid dielectric having a finite resistance characteristic, at least two of parallel fiat fixed electrodes immersed in said dielectric liquid, and a third immersed electrode movable in a circular path between said fixed electrodes, said path being perpendicular to the planes of said fixed electrode.

References Cited in the file of this patent UNITED STATES PATENTS 1,755,739 Kositzky Apr. 22, 1930 1,838,987 Cooper Dec. 29, 1931 2,036,084 Roder Mar. 31, 1936 2,087,003 Miller July 13, 1937 2,147,728 Wintringham Feb. 21, 1939 2,176,451 Berndt Oct. 17, 1939 2,419,111 Bostwick Apr. 15, 1947 2,659,039 Bourgonnier Nov. 10, 1953 2,720,626 Wing Oct. 11, 1955

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