RELAYS AND RELATED MECHANISMS 



there are many differences in mechanical construction. In this section these 

 mechanical details will be discussed and illustrated by examples of commer- 

 cially available relays. 



Many relays use a clapper-type armature, either hinged or pivoted at one 

 end (e.g. Figure 34.13a). The contact spring tension is then not usually 

 called upon to restore the armature, this function being performed by a 

 separate return spring. For very heavy-duty relays a solenoid type action is 

 often used, with a plunger moving into a hollow coil. 



Instead of a buffer block it is quite common practice to use stiff buffer 

 springs behind the appropriate contact springs {Figures 34.13b and c). In 

 another design {Figure 34.13a) the moving contacts are loosely attached to 

 the armature, the contact pressure being provided by coil springs. Flexible 

 'pigtails' connect the contacts to terminal tags. 



Miniature relays {Figure 34.13b) are available with designs somewhat 

 similar to a telephone relay, although the number of contact sets is usually 

 more limited. 



With all the types of relay illustrated it is essential that the manufacturer's 

 data be consulted for details of operating power, contact rating, etc. 



Relays for a.c. operation 



Relays whose coils are energized with a.c. generally have laminated yokes 

 and armatures. In addition, a copper ring is often embedded in the pole 

 face to prevent the magnetic flux ever dropping to zero. Otherwise the 

 construction of the relay follows conventional practice. An example of an 

 a.c. relay is given in Figure 34.13c. 



Any d.c. relay can be adapted to work from a.c. when combined v/ith a 

 metal rectifier. One of the circuits shown in Figure 34.14 may be used. 



(a) (b) (c) 



Figure 34.14 Operation of ordinary relay on a.c. 

 Mercury relays 



For very high currents mercury switches {Figure 34.15) have many advan- 

 tages over solid contacts. Thus with a tube about 1 in. long, 250 V 15 amps 

 can safely be switched by a tilting movement of about 10 degrees. Various 

 contact combinations— make, break, change-over and make-before-break — 

 are available, and time-lags of up to three minutes can be arranged by using 

 constricted tubes. 



While complete assemblies of relay mechanism and mercury switch are 

 commercially available, it is quite possible to mount a tube on a standard 

 relay frame, such as the 3000 type. The mounting should be slightly resiUent 



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