RELAYS AND RELATED MECHANISMS 



then holds through the operation of the same contact. A 1 also provides an 

 operate path for B, which however has its other coil connection earthed via 

 Bl and L. When L is released, B operates. The second operation of L 

 short-circuits A y'm B\ and L, and it releases. B is now held via Al and L, 

 and upon the opening of L it releases. The sequence of relay operations is 

 shown in Figure 34.23b. An auxiliary contact on either A ox B thus closes 

 once per two operations of L. 



A/] 



Closed Open 



Operated 



Released 



B 



(b) 



Figure 34.23 Scale-of-two: (a) circuit; (b) contact sequence diagram 



A circuit with similar performance is shown in Figure 34.24. The make- 

 before-break contact is dispensed with, but the operating contact L is no 

 longer earthy. The resistor R serves to protect the supply if contact B\ 

 bridges it due to faulty adjustment or other failure. 



Both the circuits described rely on the releasing of a relay by short-circuit- 

 ing. This inevitably makes their operation slow, due to the large release lags 

 inherent in this method. It is not possible to overcome this defect with a 

 two-relay circuit using a simple make contact for input. 



A circuit in which the release of both relays is achieved by open-circuiting 

 is given in Figure 34.25. The sequence of operations is again shown by 

 Figure 34.23b. Now the input is applied by a make-before-break key, which 

 must however never dwell in the half-operated condition. If this cannot be 

 prevented by its mechanical design, an auxiliary relay C may have to be 

 provided. 



528 



