UNISELECTORS 



RELAY-TYPE MECHANISMS 



The mechanical construction of certain mechanisms is based on that of 

 relays, and since thsy are commonly used in combination with relays they 

 are briefly described in this section. 



Electromagnetic counter 



The standard Post Office electromagnetic counter or message register is 

 shown in Figure 34.28. It counts up to 9999 but has no method of re-setting. 

 An operating power of about 300 mW is required, and impulses can be regis- 

 tered at speeds up to five per sec. 



Other designs of counter available include types with manual or electrical 

 reset, and types which print their readings on a paper strip each time a second 

 coil is energized. 



Adaptations of the 3000 type relay 



The mechanical movement of most relays can be adapted to purposes 

 other than the operation of contacts. Two mechanisms based on the 3000 

 type relay are shown in Figure 34.29. The operating power required for 

 devices of this type must be determined experimentally. It should be 

 remembered that because of magnetic saturation it is not possible to achieve 

 armature tractive forces of more than about 800 g wt ; this will require an 

 operating power of about 2 W. 



Modifications to relays do not usually prevent their use in their original 

 form: thus both the devices of Figure 34.29 carry auxiliary contacts con- 

 trolling other circuits. 



UNISELECTORS 



The uniselector is an electrically driven rotary selector switch which has 

 many applications for counting impulses and carrying out complicated 

 sequential switching operations. 



The construction of a uniselector is shown in Figure 34.30. Electrically it 

 provides a 25-way switch of up to eight poles, the switches being either of 

 the break-before-make ('non-bridging wiper') type, or make-before-break 

 ('bridging wiper'). Non-bridging wipers have narrow contact tips which can 

 only touch one bank contact at a time, while bridging wipers have broader 

 tips which short-circuit two adjacent contacts while moving. By cutting off 

 opposite ends of alternate wipers 50-way switches can be produced; thus 

 contacts 1-25 will be on level I while contacts 26-50 will be on level 2. 

 Wipers 1 and 2 are, of course, commoned. 



Uniselectors require about 30 W to operate ; they can be driven at up to 

 about 25 steps per second. Interrupter contacts are provided so that the 

 mechanism can be arranged to 'self-step' and rotate continuously. Spark 

 suppression is essential for reliable operation. 



The adjustment of a uniselector is somewhat complicated but essential for 

 reliable operation. The steps are as follows: 



(1) Check that the pressure of each wiper tip on the bank contacts is 

 about 30 g wt. Adjust if necessary with the two-pin relay adjusting tool. 



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