RELAYS AND RELATED MECHANISMS 



Copper slugs fitted to the coil of a relay slow down its action. If the slug 

 is fitted at the armature end the relay is both slow to operate and slow to 

 release. If, however, a heel-end slug is used, only the release of the relay is 

 delayed. Slugs of three sizes — |, 1 and 1| in. — are available to give various 

 degrees of delay. With relays in standard adjustment operate lags of about 

 100 ms and release lags of up to 500 ms can be obtained with 1| in. slugs. 



In the section on Relay Circuits it will be shown that the lags of relays can 

 be modified by external circuit arrangements ; some methods of achieving 

 long delays are also given. 



Marginal adjustment of relays 



The current at which a relay will operate is usually rather ill-defined for 

 the following reasons. The spring load increases as the springs deflect, and 

 the tractive force on the armature increases as it approaches the pole piece. 

 If the spring force increases more rapidly than the tractive force, the relay 

 can be stable in a half-operated condition for certain values of the operating 

 current. Whether or not the contacts are closed by this current depends 

 critically upon the contact clearances. However, if the tractive force always 

 increases more rapidly than the spring load, a current sufficient to overcome 

 the resting spring load will always operate the relay fully. 



For this reason, when a relay is required to operate on a well-defined 

 current the spring force must be arranged to increase only slightly with 

 deflection. This can best be done by heavily pre-tensioning the lever springs 

 on to the armature, so that the increase of force due to deflection is a small 

 proportion of the total force. 



The ratio of release current to operate current depends on the armature 

 residual, and if an adjustable residual screw is used some control of the 

 release current can be achieved. If a small differential (i.e. difference between 

 operate and release currents) is required, a large residual is necessary, and 

 the armature travel may have to be increased. 



The adjustment of a relay to definite operate and release currents thus 

 becomes a matter of varying three interdependent parameters, viz. spring 

 tension on to the armature, residual screw adjustment and armature travel. 

 Inevitably a marginally adjusted relay is less sensitive than one in standard 

 adjustment. Any attempt to combine sensitivity with marginal operation by 

 using very light spring tensions and minute contact clearances usually results 

 in unreliabihty. 



Assembling relays 



Even in a laboratory where relays are frequently needed it is not possible 

 to stock complete units to suit ah applications. It is then most convenient to 

 stock the separate components of 3000 type relays and to assemble them as 

 required. 



A suitable selection of coils should first be provided. Resistances from 

 50 to 10,000 ohms will cover most requirements; a number of twin coil 

 units are also desirable. Complete contact sets in useful combinations should 

 be available in both left- and right-hand versions. Buffer blocks of the five 

 different types are needed, together with armatures, yokes, mounting screws 

 and coil nuts. 



518 



