Reed Relay And Crossbar Exchange
 
    A reed relay is a device based on the fact that an electric current passing through a coil of wire
    produces an electro-magnet, with the ends of the coil having opposite magnetic polarities,
    as in Figure. 18

    If now two thin strips of material that can be magnetized are placed inside the coil, the strips
    will become magnetized when the current is flowing in the coil. If the two strips are placed so
    that one end of each overlaps , they will have opposite magnetic polarities and so will attract
   each other, as shown in Figure. 19

    These two strips can be used to form a switch in another electrical circuit.
 
 
Fig.18                                                                    Fig 19
Coil of wire as a simple                                           Principle of operation of
electro-magnet                                                        a reed relay
 
 

    Selectors in crossbar exchanges have horizontal and vertical bars operated by electromagnetic
    relay coils, so that, with a crossbar switch also, the contacts at a particular point in a matrix
    may be operated under the control of these relays.

    Crossbar switches and reed relays are both used in telephone exchanges. The basic concept
    is however quite different from that of step-by-step exchanges.

    Both crossbar and reed relay switching depend on the operation of a switching matrix, the
    principle of which can be explained by considering the circuits which are to be connected
    together as being arranged at right angles to each other in horizontal and vertical lines. These
    lines represent inlets and outlets of the switch. This idea is illustrated in Figure. 20a

    The intersections between horizontal and vertical lines are called cross points. At each cross-
    point some form of switch contact is needed to complete the connection between horizontal
    and vertical lines, as shown in Figure 20b . Any of the 4 inlets can be connected to any of the 4
    outlets by closing the appropriate switch contacts. For example ;

        a) Inlet  1  can  be  connected  to  outlet  2  by  closing  contact  B.
        b) Inlet  4  can  be  connected  to  outlet  3  by  closing  contact  R.

    Considering Figure 20a and 20b again, it can be seen that with  4  inlets and  4  outlets there are
    16  cross points.

 
 Fig. 20

    Obviously, the number of cross points in any matrix switch can be calculated by multiplying
    the number of inlets by the number of outlets. This is further illustrated in Figure. 20c
    If there are  n  inlets and  m  outlets, then the number of cross point is  (n x m).
 
        a) If  n  is larger than  m  , that is if there are more inlets than outlets, then not all the inlets
            can be connected to a different outlet. When all the outlets have been taken, there will
            be some inlets still not in use.
        b) If  m  is larger than  n  , that is there are more outlets than inlets, then, when all inlets are
            each connected to an outlet, there will be some outlets still not in use.

    So, the maximum number of simultaneous connections that can be carried by a matrix switch
    is given by which ever of the number of inlets or outlets is smaller.
 
 
 
 
 
 

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