RELAYS AND RELATED MECHANISMS 



needed when, for example, two relays are operated in paralled by L, but only 

 one is required to hold. 



The inverse circuit is shown in Figure 34.19c. Here the relay is released if 

 L is opened and remains released even if L re-closes. The circuit may be 

 re-set by the make contact shown dotted. 



External control of operate and release lags 



In a previous section it has been shown that the operate and release lags 

 of a relay depend to a great extent on the external circuit configuration. It 



1 



r 



Series 

 resistance 

 to give fastx 

 build-up S 

 of current 



High resistance 

 so that only 

 just enough 

 current flows 

 to hold relay 



A/ 



A/ 





(a) 



(b) 



(c) 



Figure 34.20 



External control of relay timing: (a) fast operate, fast release; 

 (b) slow operate, fast release; (c) lag of relay A depends on operation of 



relay B 



will be seen from Table 1 that some of the conditions are mutually incom- 

 patible ; thus a relay for fast operation must be provided with an excess of 

 operating power, while for fast release just sufficient power should be used. 

 Fortunately the operation of the relay can be used to change the external 

 conditions with suitable contacts, so that it is possible to meet both operating 

 and release requirements. 



If the second coil on a twin coil relay is short-circuited, an effect similar to 

 that of a slug is produced, and the relay action is slowed down. The coil 

 can be short-circuited by one of the relay's own contacts. 



Some examples of the use of relay contacts to give specified operate and 

 release conditions are shown in Figure 34.20: the contacts need not, of 

 course, belong to the relay whose timing is being controlled. Thus the speed 

 of operation of a relay can depend on whether a second relay is operated or 

 released (Figure 34.20c). 



It is not possible by any circuits to this type to exceed the delay available 

 from a slugged relay (operate, 100 ms; release, 500 ms). For greater delays 

 a resistance-capacitance circuit can be used, as shown in Figure 34.21a. Still 

 longer delays can be obtained with a twin coil relay, as in Figure 34.21b. 

 Again the capacitor can be switched by one of the relay's own contacts so 

 that operate and release lags can be controlled independently. 



526 



