RELAY CIRCUITS 



therefore a relay will have a significantly greater release lag if its controlling 

 contacts are fitted with a spark-quench circuit. 



Conventional spark-quench circuits sometimes fail to suppress arcing at 

 contacts breaking large direct currents. In these rare cases a small horse-shoe 



1 



Inductive 

 load 



Non -linear 

 resistor - — 



■R 



(a) (b) 



Figure 34.18 Spark-quench circuits 



Typical values 



magnet can be mounted so as to give a strong magnetic field at right angles 

 to the arc path. The resulting electromagnetic forces 'blow out' the arc 

 immediately after it forms. 



Self-locking circuits 



If the relay in Figure 34.19a is operated by closing the key L, it will there- 

 after remain operated via A\ even when L is released. The relay is said to be 



A\ 



{ 



Aj, 



'W 



A/ 



(a) 



A\ 



} 



Reset 



A/ 



T 



IT 

 T 



(b) (c) 



Figure 34.19 Self-locking circuits 



'held' via the 'hold contact' Al. The relay can of course be released if a 

 series break contact is provided. Another common arrangement is shown in 

 Figure 34.19b; a separate 'hold winding' is provided. This type of circuit is 



525 



