34 



RELAYS AND RELATED MECHANISMS 



K. E. MACHIN 



INTRODUCTION 



A relay is a device in which an electrical input produces a mechanical 

 movement, this movement opening or closing electrical connections. It is 

 thus an electro-mechano-electrical transducer with only two positions — 

 unoperated and operated. The mechanical movement is usually produced 

 by an electromagnet which moves a number of contacts to set up the 

 controlled electrical connections. 



The two main uses of relays are: (a) controlling a large amount of 

 electrical power with a small amount, or from a remote position; (b) setting 

 up and controlling complicated circuit interconnections. Numerous 

 examples of (a) are described in Chapter 29. An example of (b) is the use 

 of relays in a large power supply unit to do the following operations: 



(1) 30 seconds after the unit is switched on, switch on the negative HT; 



(2) as soon as the negative HT is on, switch on the positive HT; (3) if at any 

 stage the negative HT fails, switch off positive HT and ring an alarm bell 

 continuously; and (4) if the positive HT fails, ring the alarm bell inter- 

 mittently. 



As a further example, a relay circuit can be used to record on a smoked 

 drum only every fourth impulse received from the closing of a pair of 

 contacts. 



In this Chapter the fundamentals of relay design are not discussed since 

 the reader is never likely to construct his own relays. Instead, practical 

 information is given about the use of relays and about their properties 

 and hmitations. Since many thousands of different designs of relay exist 

 it is quite impracticable to discuss them all. Detailed descriptions are 

 given of those relays most likely to be useful, while a few general charac- 

 teristics of the others are mentioned. 



Attention has been concentrated largely on the Post Office 3000 type 

 relay, since this is readily available, standardized and of proved reliability 

 and usefulness. The general features of relay design will be illustrated with 

 reference to it, and much of the information given will be relevant to relays 

 of other design. 



NOTATION AND SYMBOLS 



Relay circuits were formerly drawn with the coils and contacts of each 

 relay near together. A circuit drawn in this way is shown in Figure 34.1. 

 Even a simple circuit of this type looks confused; more complicated 

 circuits drawn using the 'attached-contact notation' are almost unreadable. 



510 



