US2238915A - Electric filter - Google Patents

Description

April 22, 1941. M F, PETERS [gl-AL 2,238,915

ELECTRIC FILTER Filed OCT.. 13, 1937 Flc-3.1 H f 2 4 'lIlIlllIlllllllllllllllllllllllllllll1 V ll/ll/ l//I//l/l /Ill//l/Il/lllll FIC-3.8-

IIIIIIIIIIIIIIIIIJ ATTORN EY.

Patented Apr. 22, 1941 ELECTRIC FILTER Melville F. Peters, Beltsville, Md., and R. Harry Stone, Bound Brook, N. J., assignors to Titeflex Metal Hose Co., a corporation of New Jersey Application October 13, 1937, Serial No. 168,810

\ 8 Claims.

This invention relates to electric filters whereby unwanted high frequency current oscillations contained in circuits with lower frequency currents or direct current may be partially or wholly removed or filtered out.

According to a Well known phenomenon, known as skin effect, high frequency oscillations of current are forced outwardly to the surface of the conductor and this is taken advantage'of to damp or separate out the undesired, high frequency oscillations whether sustained or transsient.

The invention is of great use in connection with the ignition circuits of internal combustion engines wherein undesired transient high frequency oscillations are set up by the rupturing of the circuit incident to such use. It is also of value in connection with the circuits of radio apparatus wherein direct current is required and oscillations are not wanted. It is also useful for removal or reduction of transient oscillations occasioned in electric circuits operating elevators, refrigerators and other devices. In fact the invention may be employed Wherever undesired high frequency oscillations are to be reduced or removed from a circuit.

'I'he skin effect may be joined with the use of capacitance or inductance or ohmic resistance, any or all of them, as may be thought advantageous to produce the desired results.

The invention provides a means for filtering out undesired oscillations from a transmission line without increasing the resistance or inductance in the transmission line and it also has the advantage of the filter being small in weight which is of importance in aviation.

It also provides the advantages that the filter may'be made as a unit which may be inserted in and removed from a transmission line and such a unit may be of small size.

The invention also has the advantage that it may be adapted to the elimination of oscillations of different frequencies by suitably constructing or adjusting the filter.

One of the main objects of the invention is to provide a means for eliminating or reducing unwanted current oscillations from an electric transmission line.

A further object of the invention is to provide an electric current filter having the characteristics and with some or all of the advantages as above indicated.

A further object of the invention is to provide an improved method for eliminating or reducing unwanted electrical oscillations from a transmission line.

A further object of the invention is to provide a method of electric filtering which shall have some or all of the advantages as above indicated.

Other and ancillary objects of the invention will appear hereinafter.

In the accompanying drawing which illustrates the invention- Fig.- 1 is a diagrammatic view of an ignition circuit for an internal combustion engine in which is included a current filter embodying the invention;

Fig. 2 is a longitudinal sectional View of the filter of Fig. 1, on an enlarged scale;

Figs. 3 and 4 are similar views, on the same scale as Fig. 2, of modified forms of filter;

Fig. 5 is a fragmentary view in longitudinal section, on an enlarged scale, of a filter similar to that shown in Fig. 1, but having a unitary structure readily attachable to and detachable from a circuit;

Fig. 6 is a fragmentary view, on a still larger scale, of a longitudinal section of another modified form of filter;

Figs. 7 and 8 are longitudina1 sections, showing the parts in exaggerated relationship, of a further modified form of filter; and

Fig. 9 is a diagrammatic view of an ignition circuit including the filter of the present invention which is formed by adding parts to a previous Well known form of current filter.

Referring to the drawing, and rst .to Figs. 1 and 2 showing the application of the invention t0 the ignition circuit of an internal combustion engine, direct current is supplied from the battery I through the transmission conductors 2 and 3 to the -terminals of the primary 4 of the step up transformer 5, the secondary 6 of which has its terminals connected with the terminals of the spark plug l in the cylinder of the engine. In the primary circuit is the usual switch 8 for opening and closing the ignition circuit to place it in and out of operation. The usual automatic interrupter contacts 9 are provided in the primary circuit of the spark coil 5 to cause the interruptions of the direct current necessary to effect transmission through the spark coil or transformer 5 and cause sparking at the spark plug gap 'l in the well known manner. Across the contacts 9 the condenser l0 is connected to prevent sparking at those contacts.

The interruption of the circuit at the contacts 8 and also the sparking at fthe plug gap 1 produces transient high frequency oscillations of electro-motive force in the primary circuit which it is desirable to eliminate and prevent them from reaching or acting upon the battery,

To eliminate the high frequency oscillations, a filter F is placed in the transmission conductor 2 in the primary circuit which includes the battery. This comprises the transmission conductor II which is connected in series in the conductor 2, the conductor II being an integral part of the conductor 2. Surrounding the conductor II and spaced therefrom is a cylinder I2 of good electric conducting material such as copper. The space between the conductor II and the cylinder I2 is filled with a. poorly conducting material I3, so that it contacts with both 4the cylinder I2 and the conductor I I. The ends of the cylinder are closed by heads I4 of insulation such as Bakelite or hard rubber. Such heads may serve to retain the material I3 within the cylinder when such material is granular or liquid or such as would be liable to run out of the cylinder. The heads may also engage the cylinder and the transmission conductor I I to hold them in proper relative positions.

The cylinder I2 is connected with ground at I5. Also the primary circuit on the other side of the battery from the filter is connected to ground at I6.

When the switch 8 is closed, the well known interrupter contacts 9 will interrupt the primary) circuit and cause the electro-motive force -to be transmitted through Ithe spark coil 5 to the spark plug gap "I where the usual sparking will take place in the cylinder of the engine, The transient high frequency electric oscillations occasioned by such operation, however, are eliminated by the nlter F. The reason for this filtration is that the oscillations of very high frequency are forced outwardly from the conductor II and across the poorly conducting material I3, by the operation of skin effect, to the cylinder I2 and thence dissipated to the ground. Also the oscillations are dissipated by being transformed into heat in the poorly conducting material. The grounding at I6 also provides a shunt about the battery so that none of these high frequency ostion circuit illustrated will be provided with the necessary and usual appurtenances of an ignition circuit such as a timer etc., certain of such features, however, not being necessary to an illus- 'tration of the present invention, have been omitted.

The poorly conducting material I3 may be anything suitable, either solid, liquid or granular. One suitable material has been found to be a mixture of graphite and clay mixed in proportions to give the desired conductivity or, reciprocally, the desired resistance. This graphite and clay may be powdered, mixed together and drypacked into the filter, or it may be bound together with sodium silicate or other suitable binder. Instead of the graphite a powdered metal, such as copper, aluminum, etc., may be mixed with the clay. Also instead of the clay, the graphite or powdered metal might be mixed with powdered or flaked mica, A satisfactory poorly conducting material I3 has also been formed by mixing 100 parts litharge glycerine with 30 parts of lamp black, graphite or manganese dioxide.

The poorly conducting material I3 may also consist of metallic oxides, sulphates and sulphides, which are of well known high resistances, either alone or in combination with clay, graphite or other suitable materials, or the poorly conducting material I3 might be formed of powdered metals mixed with metallic oxides, sulphates or sulphides either alone or with a suitable base to give the proper resistance.

cillations can -pass through the battery. 'I'heelectro-motive forces whereby the operation of skin effect forces the high frequency oscillations outwardly across the poorly conducting material to the cylinder I2, are of such value, der pending upon the frequency of the oscillations, that they force the high frequency currents across the poorly conducting material, whereas low frequency or direct currents do not develop such high skin effect voltages and hence such currents are not forced across the poorly conducting material or at least to only a negligible extent, battery voltage or any voltage developed in relation to the direct or low frequency current being confined within the conductor, except for a possible negligible leakage across the poorly conducting material I3. 'I'he conducting path between the conductor II and the cylinder I2 should therefore be made of such high resistance that only a negligible amount of the low frequency or direct currents which it is desired to retain in the transmission conductor shall leak to the cylinder I2, and on the other hand the resistance of such path should be made such that the skin effect voltages due to the high frequency oscillations, which it is desired to eliminate from the transmission circuit, shall be transmitted across the poorly conducting material to the cylinder and thus eliminated.

It will be understood of course that the igni- 75 The poorly conducting material I3 may be a suitable liquid as for instance water, either pure or having sal-t dissolved in it until the desired conductivity is attained. l

In the ignition system of Fig. 1, an alternating current source may be substituted for the battery, such alternating current source being of a relatively low frequency compared to the high frequency oscillations to be eliminated by the filter, the system and filter then operating as above described.

I n order lto shorten the filter, the wire II of Fig. 1 may be spiraled or placed in the form of a helix as shown by the transmission conductor II' of Fig. 3. This ceiling of the Wire also aids the operation of the filter in that it provides an inductance which impedes progress of the high frequency oscillations along the transmission conductor.

In the modification as shown in Fig. 4, the outer cylinder I2 of Fig. 1 may be formed with circumferential ridges I'I on the tube I2' to increase the average diameter of the tube and hence increase the resistance of the path from the transmission conductor II" to the tube I 2 whereby the poorly conducting material I3' may be made of greater conductivity and still provide the necessary resistance to retain the direct current or low frequency oscillations within the transmission conductor, except for the negligible leakage as above referred to.

It may often prove desirable to form the filter in a unit which may be readily inserted in or removed from any circuit, either to effect repairs or replacements or for other purposes. A construction whereby this may be effected is illustrated in Fig. 5 in which at each end of the metal cylinder I2" corresponding to the cylinder I2 of Fig. 1, there is inserted a socket piece I8 of high grade insulation in which is mounted the metal socket piece I9 Ito which is secured in conducting relation the transmission wire II" corresponding to the transmission conductor Il of Fig. 1, and between which and the cylinder I2" is placed the poorly conducting material I3" corresponding to the poorly conducting material I3 of Fig. 1.

The metal part I9 is provided with the spring contacts 20 adapted to receive and retain the plug 2I forming part of the metal ferrule 22 fitted over the end of the insulated transmission line conductor 2' corresponding to the conductor 2 of Fig. l, the metal conductor 24 of which is secured in electrical connection with the ferrule 22 by solder' g or any other suitable manner. The end, herefore, of the filter may be connected with the transmission conductor 24 by inserting the ferrule 22 within the insulating socket piece I3 and the conducting clip 2| within the clips or spring contacts 20. This connection to the end of the filter with the transmission line may be obviously effected by withdrawing the ferrule 22 from the filter. At the other end of the filter similar provision is made for connecting and disconnecting the transmission conductor with the filter end.

Another form of filter which may be inserted ln the transmission line as above indicated in connection with Fig. 1, is the form shown in Fig. 6. This construction has the advantage that a considerable amount of impedance to the flow of high frequency oscillations in the transmission conductor is provided by providing a yconsiderable amount of inductance in the transmission conductor. In the construction of filter as shown in Fig. 6, the outer metal cylinder I2 of the filter which corresponds to the outer cylinder I2 of the filter in Fig. 1, has within it the poorly conducting material I3' corresponding with the poorly conducting material I3 of Fig. 1, and in such material is the helically coiled transmission conductor IIIV corresponding to the conductor II of Fig. l, but differing therefrom in that it is coiled whereby inductance is provided in the transmission conductor which impedes the passage of the oscillations to be eliminated along such conductor and aids the skin effect in forcing such oscillations outwardly to the exterior cylinder I2"'. A support for the material I3" is provided by the cylinder 25 formed of suitable insulation and the outer cylinder I2' is grounded at I5' corresponding to the grounding of the cylinder I2 at I5 in Fig. 1.

For very high frequency oscillations the interior of the cylinder 25 lmay be left vacant, that is, filled with air, but where the oscillations are of sufficiently low frequency to permit of it the inductance in the coil II may be increased by placing upon the interior of the tube or sleeve 25 the iron filings 26 which are grounded at 28, the ground connection being with a rod 21 extending for a considerable distance through the iron fillings so as to establish good contact therewith. These iron filings constitute a highly permeable core for the magnetic lines of force generated by the magneto-motive force of the coiled conductor I IW and hence serve to increase the inductance of that coil for the purposes as indicated.

Another form of filter is shown in Fig. '7 which may be placed in a transmission conductor for the purpose of filtering out high frequency oscillations as above described. To accomplish this the filter may be inserted in any circuit subject to have developed therein the high frequency oscillations to be eliminated or filtered out. The filter of Fig. 7 comprises the transmission conductor 29 upon they surface of which and in good electrical contact therewith is a relatively high resistance coating or sleeve 30.V The filter of Fig. 7 operates by the high frequency electric oscillations, tending to be transmitted along the conductor 29, being forced outwardly by skin effect into the relatively high resistance conducting medium, coating or sleeve 30 where they are dissipated or damped out by the resistance, the low frequency or direct current remaining in the conductor 29 because of the lack of their development of skin effect voltages to force them intothe outer high resistance medium 30. A suitablematerial for the transmission conductor 29 is the usual copper, while a suitable material for the coating 3l)l has been found to be silver oxide.

In Fig. 8 is shown a filter of the character described in Fig. 7 having a transmission conductor 29 of copper surrounded by a medium, coating or sleeve 30', of relatively high resistance. Such coating or sleeve 30 is grounded at 3l so that the dissipation of the high frequency oscillations to be filtered out, and which are forced into the outer coating or sleeve by skin effect," is not only effected by the damping effect of the resistance of the coating but it is also aided by the connection of the coating torground at 3I through which such oscillations may be dissi- Dated.

Some circuits may be already equipped with inductance coils designed to damp out the unwanted high frequency oscillations by the inductance of those coils. The dissipation or ridding of the circuit of such oscillations may be greatly enhanced by the application of the filter of the present invention thereto.

In Fig. 9 of the drawing is shown an application of the invention which is accomplished by adding parts to the usual arrangement of filter consisting in a condenser or condensers connected across the two sides of the circuit, and an inductance or inductances connected in series in the circuit. Thus the spark plug contacts 35 in the engine cylinder are respectively connected with the terminals of the secondary of the transformer 36. The primary of the transformer is connected through the interrupter contacts with one terminal of the battery 38, a spark-suppressing condenser 39 being connected about the interrupter contacts 3'I, while the other terminal of the primary is connected through the induction coil 40 and the switch 4I for turning on and off the spark, with the other terminal of the battery 38. An old form of current filter is provided in this circuit by connecting the condensers 42 and 43 across the circuit, connections being established with the conductor 44 by connecting the condensers 42 and 43 with ground at 45 and 46 respectively, while the conductor 44 is connected with ground at 41, the inductance 40 being connected in series in the ignition circuit between the points of connection of the condensers 42 and 43. This old form of filter may have features of the present invention added to it whereby its efficiency is greatly increased by enclosing the inductance 40 Within. a metal cylinder 48 having the end heads 49 of Bakelite or other suitable insulating material which serve to space the inductance coil from the metal cylinder. The metal cylinder is lled with a suitable poorly conducting material 50 as referred to above, such poorly conducting material contacting with the metal of the coil 40 and with the metal cylinder 48. The cylinder 48 is grounded at 5I. 'Ihis structure of encased coil within a metal cylinder filled with poorly conducting material is substantially the same as the structure shown in Fig. 3, and the operation of eliminating the unwanted oscillations is as hereinbefore set forth, the operation of the old form of filter, as referred to, being greatly enhanced. In modifying the old form of filter to include the features of the present invention, it is simply necessary i1' the inductance coil 40 is bare, to place the cylinder 48 about it and ll in the space within the casing with the poorly conducting material, the end heads, of course, being applied. li, however, the coil III, as installed in the old f'llter, is formed of insulated wire, the improved form tof filter may be obtained by substituting a struc'- ture as Fig. 3 for the insulated inductance coil of the old lter arrangement. While with the inductance coil having but a single layer satisfactory results may be produced, the desired effect is increased if the coil has a number of layers,

of conductors.

While the invention has been illustrated in what are considered its best applications, it may have other embodiments Without departing from its spirit and is not, therefore, limited to the structures shown in the drawing.

What we claim ls:

1. The combination with a low resistance transmission conduet'or, of a high resistance conducting medium about said conductor into which electrical oscillations are adapted to be driven by skin effect, a relatively low resistance conducting material in electrical connection with said medium and a conductor connected with said low resistance conducting material for conducting said electrical oscillations to a dissipating medium outside the transmission line, said low resistance conductors being continuously in contact with said medium.

2. The combination with a low resistance transmission conductor, of a high resistance conducting medium about said conductor into which electrical oscillations are adapted to be driven by skin effect, a relatively low resistance conducting material in electrical connection with said medium and a connection for conducting the electrical oscillations from said last mentioned conductor of low resistance, said low resistance conductors being continuously in contact with said medium and a low resistance conductor connected with said low resistance conducting material for conducting the said electrical oscillations to a dissipating medium outside the transmission line.

3. The combination with a low resistance transmission conductor, of a high resistance conducting mediumabout said conductor into which electrical oscillations are adapted to be driven by skin effect, a relatively low resistance conducting material in electrical connection with said medium and a ground connection for conducting the electrical oscillations from said last mentioned conductor of low resistance, said low resistance conductors being continuously in contact with said medium.

4. The combination with a transmission line adapted to have generated in it currents of dif- CII ferent frequencies, of a. high resistance conducting mediumabout said transmission line into which electrical oscillations are adapted to be driven by skin effect," a. relatively low resistance conducting material in electrical connection with said medium and a. connection for conducting the electrical oscillations from said last mentioned conductor of low resistance to a dissipating medium outside the transmission line, said resistance being of a value to reduce any leakage of current desired to be retained in the transmission conductor to a negligible amount but to permit the passage of the high frequency currents to be eliminated, said transmission line and said low resistance conducting material being continuously in contact with said medium.

5. The combination with a transmission line adapted to have generated in it a direct current and an oscillatory current, of a high resistance conducting medium about said transmission line into which electrical oscillations are adapted to be driven by skin effect, a relatively low resistance conducting material in electrical connection with said medium and a connection for conducting the electrical oscillations from said last mentioned conductor of low resistance to a dissipating medium, said resistance being of a value to permit the passage of the oscillatory currents but to reduce the direct current flow therethrough to a negligible amount, said transmission line and said low resistance conducting material being continuously in contact with said medium.

6. The combination with a low resistance transmission conductor, of a low resistance conducting tube surrounding said conductor but spaced therefrom, and a high resistance conducting material in the space between said conductor and said tube and in electrical connection with both said conductor and said tube, said transmission conductor being adapted to have impressed upon it oscillatory currents which are forced across the said high resistance material to said tube by skin effect, said low resistance transmission conductor and said tube being continuously in contact with said material, and a connection for conducting the electrical oscillations from said tube to a dissipating medium loutside the transmission line.

7. The combination as specified in claim 6 but with the path across said high resistance material being such that the high frequency oscillations desired to be eliminated will be forced thereacross by skin effect but low frequency currents desired to be retained in the transmission conductor will not be forced across said material more than a negligible amount.

8. The combination as specified in claim 6 but with the path across said high resistance material being such that the high frequency oscillations desired to be eliminated will be forced thereacross by skin effect but direct current desired to be retained in the transmission conductor will pass over said path to not more than a negligible extent.

R. HARRY STONE.

MELVILLE F. PETERS.

Images