RELAY CIRCUITS 



Even with 'capacitance slugging', delays of much more than ten seconds 

 are not possible. By using thermally sensitive resistors delays of up to a 

 minute or so may be obtained. The duration of the delay will not, however, 

 be accurate, but will depend on ambient temperature. In addition, an ade- 

 quate time must be left between operations for the thermal element to revert 



r 



T 



r 



(a) 



IT 



(b) 



Figure 34.21 



Capacitance delay circuits: (a) single coil relay; (6) twin 

 coil relay 



to room temperature. Figure 34.22 shows a group of circuits using the large 

 and cheap thermistor elements of the type used as current limiters (e.g. 

 Brimistors). 



L 



(a) 



(b) 



(c) 



Figure 34.22 Thermistor delay circuits: (a) slow operate, fast release; 

 (b) fast operate, then after delay, release; {c)fast operate, slow release 



Counting circuits 



Relay circuits can be used to 'count down' impulses so that an output is 

 provided only at every second (or third, etc.) input impulse. In this section 

 some of these circuits are described, both for their inherent usefulness and 

 also as examples of relay circuit design. For other examples of scales-of-two, 

 a comprehensive review of scaling circuits by Barnes^ should be consulted. 



Figure 34.23a shows a typical scale-of-two circuit. When the input key L 

 is closed, relay A operates through the make-before-break contact A\; it 



527 



