HIGH-SPEED RELAY 



The assembly of relays is straightforward, but certain points should be 

 noted: (a) it is essential for all screws and nuts to be really tight; (b) the 

 correct buffer block must be used ; (c) a two-hole washer must be used under 

 the buffer block retaining screws; (d) the correct length mounting screws 

 must be used for contact sets and buffer blocks: short screws give inadequate 

 strength, while long screws may penetrate the coil windings — the standard 

 lengths of 6BA screws used in 3000 type relays are h, |, |i and 1| in.; 

 (e) contact loads on the two sides of the buffer block should be approximately 

 equal; (f) the relay must be adjusted after assembly. 



It is strongly recommended that complete contact sets should never be 

 assembled from individual springs and spacers. It is possible in this way 

 inadvertently to construct contact sets v/hich cannot be operated with a 

 normal armature travel, which do not mate up to standard buffer blocks, 

 which have inter-contact shorts through operating pins, and which cannot 

 ever be adjusted for reliable operation. 



HIGH-SPEED RELAY 



When operate and release lags of less than 5 ms are required it is necessary 

 to use a specially designed relay. Such a relay is shown in Figure 34.9; the 

 operate and release lags have been reduced to about 1 ms. 



Single or twin coil relays are available with resistances up to about 2,000 

 ohms. A single change-over contact is provided, using platinum as the 

 contact material. Up to 0-5 amp may be carried by the contacts, but because 

 of the small gap, operation on d.c. voltages over 50 is inadvisable unless 

 particularly efficient spark-quench circuits are used. 



A high-speed relay is adjusted as follows. Both contact screws are slackened 

 back and the 'make' screv/ is adjusted so that it just touches the lever contact 

 when the armature is held down on to the pole pieces. The make screw is 

 advanced l-\ turn to set the residual gap. The break contact screw is now 

 adjusted to give a contact gap of 0-004 in. The spring is tensioned by turning 

 the knurled knob until 20 g wt is needed at the armature tip to open the 

 break contacts. 



If a high-speed relay is to be operated in a circuit (such as a chopper 

 amplifier) where contact bounce must at all costs be avoided, it is desirable to 

 carry out the adjustment while the relay is being repetitively operated, and 

 with a voltage interrupted by the contacts displayed on an oscillograph. To 

 minimize contact bounce it is sometimes necessary to operate with higher 

 armature tension than 20 g wt. High-speed relays can be operated as choppers 

 satisfactorily up to about 100 c/s, although for a reasonable duty cycle a 

 square wave driving current is advisable. 



The power required by the Siemens high-speed relay for various conditions 

 is shov/n below. 



Operate 200 mW Hold 80 mW 



Non-operate 110 mW Release 60 mW 



For reliable operation, the usual factors of safety (page 517) should be 

 applied. 



Since twin coil relays have two separate coils on different pole-pieces, these 



519 



