RELAY CIRCUITS 



A/\ 



Bl 



A\ 



h 



\v 



B/1 



^ 



, ^ei 



X 



Figure 34.24 Alternative scale-of-two circuit 



4/1 



-v-/- 



^'/ 



*\ o- 



3 



e/1 



B\ 



< 



~T 



r~^-n 



•— r-- 





o- 



A i. 



r.M 



Figure 34.25 Fast scale-of-two circuit 



With three relays it is possible to design very simple and fast scalers 

 Figure 34.26 shows a design in which only one contact per relay is needed; 

 it is therefore suitable for use with high-speed relays. For the highest speed 

 of operation each relay should be fed from a high-impedance source ; for a 

 145 ohm relay, a 50 V supply with a 600 ohm series resistor is suitable. 

 Pulse rates of up to 100 per second can then be handled. 



When it is required to count down by more than two, a series of n scales- 

 of-two can be cascaded to give a division by 2". Alternatively, ring circuits 

 of the type shown in Figure 34.27 can be used. Although illustrated as a 

 scale of three the circuit can be extended to any greater number of relays. 

 Contacts BA, C4 ensure that when the supply is switched on relay A operates 

 and holds ; if high speed of operation is not essential these contacts may be 

 replaced by a large capacitor. 



529 



