RELAYS AND RELATED MECHANISMS 



powers apply to operation by either coil alone. If, however, power is applied 

 equally to the two coils, the total power requirement of the relay is only half 

 the value tabulated above. 



It is advisable to keep the mean dissipation in the coil below 1-5 W to 

 prevent overheating. 



POLARIZED RELAY 



By incorporating a permanent magnet into the magnetic circuit a relay of 

 high sensitivity and speed can be constructed. In addition, the direction of 

 movement of the armature becomes sensitive to the direction of the operating 

 current; such relays are therefore known as 'polarized relays'. 



The design of a typical polarized relay is illustrated in Figure 34.10. The 



Contacts 



[]^<^<^ Pivoted 



armature 



Laminated 

 yoke 



Figure 34.10 Polarized relay 



exact arrangement of the armature control spring depends on the required 

 resting position of the armature when the relay is un-energized. In a 'one-side 

 stable' relay the armature normally rests against one fixed contact, changing 

 over to the other only if an operating current of the correct polarity is 

 supplied. The armature of an 'each-side stable' relay will rest stably on 

 either fixed contact; when operated by a current of the appropriate polarity 

 the armature snaps over to the other fixed contact. 'Centre stable' relays 

 have no bias on the armature, which may rest anywhere between the fixed 

 contacts. Some form of electrical bias is then necessary to centralize the 

 armature, and in extreme cases to prevent it resting hghtly on one or other 

 fixed contact. In another type of centre-stable polarized relay the armature 

 is positively constrained in the centre of the contact gap ; this is achieved 

 only at the expense of some loss of sensitivity. 



The operating power for a polarized relay is between 0-2 and 2 mW, 

 according to the design. Coils usually have a maximum dissipation of 1-2 W. 

 Multiple coil windings can be provided. The contacts, which are normally 

 a palladium-copper alloy, can carry up to 10 W with overall maxima of 

 1 amp and 200 V. Since small contact clearances (a few thousandths of an 

 inch) are used it is essential that efficient spark-suppression circuits should be 

 provided. The time of transit of the armature between the fixed contacts is 

 about 1 ms. 



520 



