Electro-magnetic relay with first and second spring biasing means
FIELD OF THE INVENTION 
     The present invention relates to improvements in high sensitivity electro-magnetic relays according to the parent patent. 
     BACKGROUND OF THE INVENTION 
     French Pat. No. 2,331,877) describes an electro-magnetic relay having a moving armature which is mounted on an armature carrier which is subjected to a return spring whose tension is adjustable by means of a screw. 
     The aim of the invention is to improve the system for adjusting the tension of the return spring of the moving armature and thereby to improve the operation of the relay. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high sensitivity electromagnetic relay comprising a moving armature held against two pole pieces of a magnetic circuit by means of a permanent magnet wherein the armature is subjected to the action of two return springs tending to draw the armature away from the pole pieces against the attraction of the permanent magnet, the springs comprising a first, non-adjustable spring and a second, adjustable spring, the springs being independent component parts arranged to act in concert. 
     The first spring is preferably so adjusted as to supply an opposing couple which is slightly below that required to adjust the relay to its least armature holding force, thereby guaranteeing a minimum relay torque. 
     The second spring is preferably a spiral spring placed between two branches of a magnetic circuit on the pivot axes of the armature and supplying the adjustment tension which corresponds to the desired sensitivity. 
     The spiral spring is preferably held fast to the rotation shaft of the armature and is associated with a ratchet wheel system comprising a tooth-wheel fast to the same shaft and a blade for stopping one tooth of the tooth-wheel, which blade may be deformed so as to release the tooth-wheel. 
     An embodiment of a relay in accordance with the invention is described by way of example with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation section, partially broken away, of the relay in its casing; and 
     FIG. 2 is a plan view of the relay of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the embodiment shown, the relay comprises a permanent magnet 1 held between two U-shaped magnetic side pieces 2, having two arms 4 which form the poles that cooperate with the magnetic armature 6 and having two other arms 5 which form a pivot support for the armature carrier 7. 
     A release winding 10 is mounted on one of the pole branches 4 and its flux when it is fed with current, opposes that created by the permanent magnet 1 in the moving armature 6. The armature 6 is mounted in a cage 11 formed at the end of the armature carrier 7, the other end of which includes two separate arms 12 mountd free to rotate on an axle 13. The axle 13 is itself mounted for free rotation in two holes in the arms 5 of the magnetic circuit. The axle 13 includes a flat 14 enabling it to rotate the inner loop of a spiral spring 15, as well as a ratchet wheel constituted by a flat washer 16 having teeth on its outer circumference which engage a hook 18 in a slot of a stop-blade 17. The blade is fastened to a relay housing 20 and is sufficiently flexible to free the tooth-wheel by virtue of its upper end 21 moving away from the housing. The adjustment spring 15 and the toothed wheel 16 are placed between the two arms 5 of the magnetic circuit. The spring 15 is a spiral spring and its other end 22 is fastened to a projection 23 on the armature carrier. One or both of the ends of the axle 13 are shaped by means of a slot or a flat to enable the axle to be rotated and hence to enable the toothed wheel and the adjusting spring to be rotated. 
     A tension spring 24 is fixed between a high point 25 of the armature carrier 7 and a fixing point 26 of the housing. The spring 24 tends to draw the armature 6 away from the pole pieces 4. The relay is placed in the housing 20 which is provided with a cover 27. 
     An engagement and resetting lever 28 is mounted to rotate between lugs 29 and 30 of the housing and its cover, and presses against the engagement button 31. 
     The housing 20 and its cover 27 are arranged to be held together by a single screw 32 passing through the relay via an isolating tube 33 without communicating with the inside of the housing and hence those parts of the relay which are sensitive to dust. The engagement button 31 is freely mounted with clay in a hole 34 of the housing in such a manner as to reduce friction and to avoid jamming, while still retaining sufficient guidance. 
     The hollowed-out form of the base 35 of the engagement button, as it rests on the plate 36 of the armature carrier, tends to place the button perpendicularly to its contact face on the plate. The hole 34 in the housing has a chamfer 37 on its inner side thereby providing guidance over a very small length and enabling the button to lean over appreciably without taking up a position which would be likely to cause jamming. This arrangement in conjunction with the material used: (a low co-efficient of friction thermoplastic material of the acetal copolymer type of resin) reduces energy losses due to friction during release. 
     The shape of the housing and the possibility of adjusting the spiral spring 15 driving axle 13 from either of its ends makes mounting on the housing or the cover side of the relay equally possible. 
     Operation of the device described is as follows: 
     The tension spring 24 provides an opposing couple which is slightly below that required to set the relay to its least total holding force, thereby guarateeing the minimum motor couple for each manufactured relay. 
     When the relay is being adjusted by the adjusting axle 13, rotation of this axial causes the toothed wheel 16 to rotate and the spiral spring is wound in a direction to increase the opposing couple and adjust the release current. The fixing of the teeth of the toothed wheel in the blade 17 fixes the angle of rotation and the tension of the spring. The adjustment can be brought back by disengageing the hook 18 of the blade 17 from the teeth of the wheel 16. The release of the armature 6 of the magnetic circuit cause the armature carrier 7 to rotate and the button 13 leaves the housing. This movement of the button represents the work supplied by the relay. 
     Resetting the relay is performed by the action of the engagement lever 28 on the release button 31 to stick the armature back on the magnetic circuit.