US2541749A - Variable condenser with fixed electrodes - Google Patents

Description

Feb. 13, 1951 c, DE LANGE 2,541,749

VARIABLE CONDENSER WITH FIXED ELECTRODES Filed April 20, 1946 INVENTOR.

combwfilumz ATTMX Patented Feb. 13, 1951 UNITED STATES PATENT OFFICE VARIABLE CONDENSER WITH FIXED ELECTRODES Conn., as trustee Application April 20, 1946, Serial No. 663,681 In the Netherlands May 13, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires May 13, 1962 2 Claims.

This invention relates to a variable condenser, more particularly a trimmer, in which a plate of insulating material may be displaced between the two electrodes. The invention particularly provides means which enable to arrive at an inexpensive series-production of small variable condenser, as trimmers, having excellent mechanical and electrical properties whilst avoiding the disadvantages inherent in the known constructions.

The British patent specification 454,845 describes variable condensers in which a plateshaped insulating piece is movable relatively to at least one of the two electrodes. In several of the forms of construction given in this patent specification one electrode is provided on the movable plate-shaped insulating piece and the other on a stationary carrier. The movable plate or the carrier, or both of them may serve as a dielectric. A ground holder provides for solely the desired relative movements of carrier and insulating piece being possible. In other forms of construction mentioned in the said patent specification each of the two electrodes is provided on a stationary carrier, these carriers being united to form one whole by means of auxiliary parts. A plate of insulating material may be turned between the two electrodes so as to vary the capacity. All these forms of construction have the drawback that as a result of the required small tolerances very accurate and consequently expensive grinding is required for the manufacture of the components. In addition, the components are thin walled. When they are made from ceramic material, which is generally i used for this purpose in view of its favourable dielectric properties, they are consequently fraglie. The use of ceramic material involves still other difficulties. Thin components frequently chip in the manufacture and, this together with the irregular contraction which occurs, makes it necessary to be satisfied with comparatively large tolerances so that long and expensive aftertreatments are involved due to the small tolerances required in the above-described constructions.

Th mechanic stability of variable condensers built up from a number of different components larger than strictly necessary is too weak for the majority of applications, in spite of the uality of the grinding operation. In fact, it is a drawback that the two electrodes are provided on different carriers which ar mechanically connected by means of auxiliary parts. Sudden, though in some cases small variations in capacity are thus involved. It is true that the above-stated patent specification gives one form of construction in which the two carriers and the connecting part are made in one piece, but this is thin walled and can be worked only with dimculty and is fragile due to its particular shape. In addition, the movable insulating piece is rotatably constructed, which imposes high requirements upon the fit of the shaft in view of the desired precision. The use of the present invention overcomes the said drawbacks.

According to the invention, the two electrodes are provided on a common carrier of homogeneous material on each side of a slit provided in it, the smalles v thickness of the material of the carrier being greater than the width of the slit. The auxiliary parts for the mechanical connection of the carriers may thus be dispensed with. Since only the slit of the carrier is used, no additional after-treatment is required after grinding the slit, even when use is made of ceramic material. The electrodes are applied by one of the known methods, for example by burning.

The carrier preferably has a cylindrical shape and comprises an axial bore, the surface of the slit comprising the axis of the cylinder. The cylindrical shape is the easiest shape to be manufactured due to the high symmetry, which also avoids the occurrence of unequal tensions in the material. The axial bore may be provided during the first shaping. The tolerances are very wide both for this bore and for the diameter of the cylinder, so that the after-treatments are dispensed with.

For the purpose of obtaining a low zero capacity, the electrodes are not provided on the whole surface of the sides of the slit. When the movable plate by means of which the capacity is controlled is displaced in such manner that it is located between the uncovered parts of the walls of the slit, the capacity is minimum. Now, electrical lines of force still run, however, from the back of the one electrode to the back of the other electrode wholly or in part through the carrier. The bore reduces the part which passes wholly through the carrier.

To reduce also the part which passes via the carrier through the insulating piece, the carrier has ground in it, in addition, a transverse slit of which the surface is at right angles to that of the slit in which the electrodes are provided, the electrodes being provided on only one side of the transverse slit in such manner that in the zero position the movable plate is located entirely on the other side of the transverse slit.

Those parts of the walls of the slit which are covered with electrodes now do not adjoin in any point the ceramic material due to the presence of the axial bore and the transverse slit. As has already been explained above, this results in a decrease of the capacity between the two electrodes, but still another advantage is thus obtained, since it is not necessary to use mandrels in applying the electrodes. There is no danger that the electrodes will extend over a larger area than is desirable.

In order that the tolerances may not be made unnecessarily small, the thickness of the movable plate is smaller than the width of the slit. It IS nevertheless not necessary to take steps to counteract the displacement of the plate in the transverse direction of the slit (normal to the surface of the slit), since such displacements do not bring about variation in capacity. The plate is preferably made from ceramic material having a high dielectric constant and low losses. In this case the variation in capacity with a displacement of the plate are maximum, th loss angle of the condenser being small.

The carrier may also be made from ceramic material and for this purpose the same material may be used as for the movable plate. It use is made of a different ceramic material, it is possible to give the temperature coefhcicnt of the condenser a definite value by means of a correct choice of the dimensions and the material. To reduce the zero capacity it is possible to make the carrier from ceramic material having a low dielectric constant.

The dielectric of the described condenser is constituted in part by air and in part by insulating material. To reduce the loss angle still further than is obtainable in this manner, the two sides of the movable plate may be made conductive and be connected to each other in a conductive manner. In this case solely the air serves as the dielectric. If, now, the tolerances for the width of the slit and the thickness of the plate are chosen. smaller, the difiference between the slit width and the plate thickness may be made smaller, thus renderin possible a larger variation in capacity. It is evident that in this form of construction constructive steps must be made to cause one or the walls of the slit to be always engaged by the movable plate, since otherwise short-circuit of the electrodes could occur. To avoid such a short-circuit, the electrodes of the plate may be covered with a thin insulating layer.

It is advantageous to cover also the electrodes provided on the slit walls of the carrier with a thin insulating layer. On the one hand the above-stated risk of short-circuit is thus still reduced and on the other hand the wear of the electrodes on the slit walls brought about by the sliding of the movable plate is thus avoided.

In such a condenser the plate and the carrier may advantageously be made from ceramic ma terial having a very low coefficient of expansion such as the titanates. A condenser having a very low temperature-coeflicient is thus obtained. The use of such materials which generally bring about high dielectric losses, is in this case possible since only air and quartz occur as the dielectric so that the loss angle remains small.

In order that the invention may be more clearly understood and readily carried into effect, it will be explained more fully by reference to the accompanying drawing.

Fig. 1 is a perspective view of one embodiment of a variable condenser according to the invention.

Fig. 2 is the front-view thereof (as viewed in the direction a of Fig. 1).

Fig. 3 is the plan view.

Figs. 4 and 5 are sections of the variable condenser taken over the surfaces of which the in tersection with the plans of the drawing is indicated by III and IV in Fig. 2.

Figs. 6 and '7 are a perspective and a sectional View of another form of construction.

Referring first to Fig. 1, th ceramic body I which comprises an axial bore 2 has a slit 3 ground in it. A transverse slit t is ground at right angles to slit 3 (see Figs. 1 and 2).' The sides of slit 3, so far as located below the transverse slit 4, are covered with electrodes 5 and 6 (see Fig. 2). Said electrodes are connected to two supply conductors l and 3 which are in line with each other, thus reducing the mutual inductance and the zero capacity. A plate-shaped insulating piece 5 may move up and down in the slit 3 provided between the electrodes 5 and 6. Th capacity is larger, according as a larger part of this plate is located between the electrodes 5 and 6. The insulating piece 9 is held by a spring l0 located in a groove ll of the carrier The zero capacity, 1. e. the capacity which exists between the electrodes 5 and 6 when the insulating piece 9 is located entirely above the transverse slit 4 in the position indicated in dotted line by l2 in Fig. 2, is reduced due to the bore 2 and the transverse slit 4 being provided and the width d of slit 3 (see Fig. 5) being large.

When the condenser is not adjusted to its minimum capacity the insulating piece 9 serves as a dielectric. Said insulating piece is preferably made from ceramic material having a high dielectric constant and low losses.

In this form or construction the width of the slit is 1 mm., the thickness of the movable plate 0.980 mm., the surface of each of the electrodes 0.75 cm? and the dielectric constant of the material of the movable plate 12.5. In this case the minimum capacity is 0.6 cm. and the maximum capacity 6 cm. When in this case use is made for the movable plate of a material of which the dielectric constant is 100, the maximum capacity becomes 20 cm. The ceramic body I has a cylin drical shape havin a diameter of 15 mm., the axial bore having a diameter of 3 mm.

The insulating piece 3 may alternatively be provided on each side with conductive electrodes which are connected in a conductive manner. Subsequently, these electrodes, like the electrodes 5 and 6 provided on the sides of th slit, may be covered with a thin insulating layer, for example quartz applied by vaporization. A condenser is thus obtained in which only air and a thin layer of quartz serve as the dielectric. Such a con struction has the advantage of a small loss angle and a large variation in capacity.

In Fig. 6 the upper half of the cylindrical carrier 20, which comprises an axial bore 22, has provided in it a slit 2| of which the surface cornprises the axis of the cylinder. Fig. '7 is a section of which the plane is indicated by broken line 1-4 in Fig. 6. The two adjacent walls of the slit, as far as located above the line F-G in Fig. 7, are covered with a conductive electrode. A plate 23 may slide in a slit 2|, said plate being secured to a guiding rod 24 which may move in the axial bore 22. The plate 23 is displaced due to the guiding rod 24 being driven, for example, by a screw mechanism, which is not shown in the drawing. In this case the electrodes also extend over the wall of the axial bore for the purpose of electrically connecting the parts located at the left and at the right of the bore. The supply conductors are provided as in the previous form of construction and are indicated by 25 and 26 in Fig. 6. In the zero position the plate 13 is located entirely below the line FG. The capacity which exists between the two electrodes is maximum when the plate 23 is in the uppermost position.

Condensers according to the invention are characterised by great rigidity, stability and low losses. An inexpensive series-production is possible owing to the small number of operations and the avoidance of constructions in which compon nts have to fit exactly into each other. The temperature coefficient, the capacity variation and the minimum capacity may be regulated within wide limits by the choice of the material and the dimensions. The invention is particularly advantageous when applied, to trimmers of small size, such as are used in radio technics at very high frequencies.

I claim:

1. A variable condenser comprising a body of homogeneous material having a longitudinal dimension with a slit in the body parallel to said dimension, and with a slot in the body transverse to the longitudinal dimension and to the parallel slit, the side surfaces of the slit on one side of the slot being covered with metallic coatings to constitute the electrodes of the condenser, a plate of insulating material floatingly movable in the slit to vary the condenser capacity, an external groove 3 in the body and a resilient member anchored in the groove and engaging the plate to hold the latter from casual displacement.

2. A variable condenser comprising a unitary carrier body of homogeneous insulating material provided with a rectangular slit having planar walls, a plate of insulating material movable in the slit, and a transverse slit in the carrier body normal to the rectangular slit, the walls of the rectangular slit on one side of the transverse slit being provided with directly opposed metal coatings which constitute the electrodes of the con denser and between which the plate of insulating material is movable to alter the capacitance of the condenser from a maximum value when fully inserted therein to a minimum value when the plate of insulating material is disposed in that portion of the rectangular slit on the opposite side of the transverse slit.

CORNELIS DE LANGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,212,231 Gossel Aug. 20, 1940 FOREIGN PATENTS Number Country Date 454,845 Great Britain Oct. 5, 1936 523,869 Great Britain July 24, 1940

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