Magnetic relay
This application is a continuation-in-part application of U.S. Ser. No. 790,967, filed Apr. 26, 1977, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an improved structure for an electromagnetic relay, the energizing coil of which is surrounded by a metal envelope. 
     In a known embodiment as disclosed in U.S. Pat. No. 3,434,080 an electromagnetic relay has metal foils wrapped underneath the energizing coil about a sealed reed switch capsule and above the energizing coil. However, this and the other known types of relays are not satisfactory to provide a high sensitivity enough for small size constructions to be used with a low-power energization for being compatible with MOS transistor technology. In these applications the power involved may not exceed a level in the order of about 10 mW and experiments have shown that the known structures for relays do not permit to obtain a suitable high sensitivity with an energization power lower than about 10 mW. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to improve the sensitivity of electromagnetic relays with a lower power energization for making them compatible with MOS technology. 
     Another object of the invention is a high-sensitivity small-size electromagnetic reed relay which can be easily and reliably assembled by mass production. 
     In accordance with the invention there is provided a magnetic relay having an electromagnetic coil with an open center extending lengthwise therethrough and switch contact means having coacting ends disposed lengthwise in said open center of said coil. An outer tube of magnetic material is positioned to enclose the coil lengthwise. A pair of radially expandable disks of magnetic material are positioned in the outer tube across the ends of the coil, each disk having a central opening aligned with the open center of the coil. A pair of inner tubes of magnetic material are positioned axially aligned with each other through the central openings in said disks and in the open center of the coil about the switch contact means with the adjacent ends of said inner tubes spaced apart to form a gap adjacent the coacting ends of the switch contact means, said inner tubes being of a size in the open center of the coil to pass freely through the central openings in the disks and of a larger size at the location of the disks whereby the inner tubes urge the disks to expand radially into tight contact with the inside of the outer tube. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal sectional view of the assembly in accordance with an embodiment of the invention as applied to a reed relay; 
     FIG. 2 is a longitudinal sectional view of an exemplary bobbin for the coil; 
     FIG. 3 is a cross-sectional view along line III--III in FIG. 1, with reed capsule removed; 
     FIG. 4 is a schematic view showing the method of positioning the inner tubes into the coil; 
     FIG. 5 is a graph illustrating the optimization of the sensitivity of the switch contact means; 
     FIG. 6 depicts a variation of the assembly of FIG. 1, with reed capsule removed. 
    
    
     DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
     In Fig. 1 there is shown the structure of the invention as applied to an electromagnetic reed relay, that is an electromagnetic relay with reed switch contacts. An energizing coil 1 for producing the excitation magnetic field has an axial open center in which a reed switch capsule 2, known per se, is located lengthwise. An outer tube 3 of a magnetic material extends lengthwise about the coil. A pair of disks of magnetic material 4,5 are positioned in the outer tube 3 across the ends of coil 1. Each disk has a central opening which is aligned with the open center of coil 1. The disks 4 and 5 are made to be radially expandable. 
     In the exemplary embodiment illustrated in FIG. 1 the coil 1 comprises a bobbin 11 for supporting the excitation winding 12, the bobbin 11 having a pair of flanges 13, 14. As more readily apparent from FIG. 2, the bobbin 11 is provided with a second pair of flanges 15,16 each defining a pair of coplanar circular segmental slots 17A,17B and 18A, 18B with the corresponding one of flanges 13 and 14. Each of said slots has a depth intercepting the open center of bobbin 11. 
     Each disk is comprised of a pair of coplanar plates 10 each generally in the form of a circular segment as shown in FIG. 3. The pair of coplanar plates 10 are disposed symmetrically with respect to each other and with the chordal portions thereof confronting. Each chordal portion has a recess or opening 20 in the center thereof such that when a pair of coplanar plates 10 are symmetrically positioned, the two recesses 20 are confronting thereby to form a central opening 21. 
     A pair of inner tubes 6, 7 of magnetic material are positioned axially aligned with each other through the central openings 21 and in the open center of coil 1 about the switch contact capsule 2. The adjacent ends of the inner tubes 6, 7 are spaced apart to form a gap 8 adjacent the coacting ends of the switch contacts in capsule 2. The tubes 6 and 7 have advantageously a size in the open center of coil 1 to pass freely through the central openings 21 and a larger size at the location of the disks 4 and 5. This larger size at the location of the disks is advantageously in the form of an outward flare 22. When in position these flared ends of the inner tubes 6 and 7 urge the disks 4 and 5 radially into tight contact with the inside of the outer tube 3, thereby assuring a magnetic path having minimum reluctance. 
     The inner tubes 6 and 7 are advantageously inserted into position by the following method in accordance with a further aspect of the invention. Reference will be made to FIG. 4 which shows one flared tube 7 prior to being positioned in the assembly. As apparent from this drawing, the inner tubes have an outward flare 22 the outer diameter of which is greater than the diameter of the central openings 21 in disks 4 and 5. For example, with inner tubes having an external diameter of 3.6 mm, the outer diameter at the flared ends is typically about 4 mm. To be positioned in the open center of coil 1 the flared tube 7 is forced axially through the central opening 21 into the open center of the coil 1 until its outer end is received in the central opening 21 into cooperation with the chordal portions of the pair of plates 10, whereby said plates are urged to expand radially into tight contact with the inside of outer tube 3. 
     Experiments have shown that the structures according to the invention permits to achieve a relay having a higher sensitivity than prior art relays. 
     Comparative tests have been made on several small-size samples A built with straight inner tubes in accordance with the prior art and on several samples B built with outwardly flared tubes in accordance with the teaching of the invention. 
     The constructional particulars of the samples are the following: 
     Switch Capsule Clare Picoreed Switch 102287 
     diameter: 2.54 mm 
     length: 26.4 mm 
     Inner tubes 
     internal diameter: 2.54 mm 
     external diameter: 3.6 mm 
     length: 9.6 mm 
     Outer tube 
     internal diameter: 9.6 mm 
     external diameter: 10.4 mm 
     length: 20.3 mm 
     Flanges 
     opening diameter: 3.65 mm 
     For the samples B, the inner tubes have been flared at one end such that the outer diameter at the flared end is about 4 mm. 
     The test results obtained with the two groups of samples are given in the table below indicating the ratio ρ of the operate ampere-turns of the used capsules to the operate ampere-turns in the applicant&#39;s testcoil: 
     
         ______________________________________                                    
Samples          A           B                                            
______________________________________                                    
1                0.65        0.56                                         
2                0.73        0.56                                         
3                0.72        0.57                                         
4                0.70        0.54                                         
5                0.70        0.57                                         
______________________________________                                    
 
    
     The values in the table above clearly show that the construction in accordance with the invention (column B) requires substantially less magnetizing energization for operating the reed contacts than the prior art construction (column A). 
     It has further been found that an optimization of the sensitivity can be obtained in the structure according to the invention by choosing for the annular gap between the inner tubes 6 and 7 a length l such that the ratio of this length to the diameter d of the tubes is comprised in the range extending from 0.45 to 0.85. The graph of FIG. 5 is a curve showing the influence of the gap length l on the ratio ρ for tubes having a diameter d of about 2.54 mm. It can be seen that the optimum gap is about 2 mm in the structure in accordance with the invention. 
     It is also possible to implement a relay in accordance with the invention by using one flared inner tube only. With such an embodiment the improvement in sensitivity is of course lower than with the embodiment of FIG. 1 but an improved sensitivity is nevertheless obtained as against the prior art. 
     The embodiment of FIG. 6 is similar to that of FIG. 1 with respect to the left hand portion thereof. The only difference is that the right hand disk in FIG. 1 is replaced by a permanent magnet 18 located between flanges 14 and 16, said permanent magnet being magnetized such as to produce a radial magnetic flux shorting the outer tube 3 and the inner tube 17. The latter need not to have an outwardly flared end, though this is quite feasible. This embodiment permits the switch contact means to be kept closed during the time that the coil is not energized. A variation is providing the permanent magnets embedded in a non-magnetic material, e.g. a plastic material. Also the disk may be replaced by several permanent magnets placed symmetrically about the open center of coil 1.