OTHER DESIGNS OF RELAY 



Polarized relays are particularly suitable for use with 'chopper' amplifier 

 circuits (Chapter 39). For this application special types are available with 

 platinum contacts for low noise, and with screening between the coil and the 

 contacts to minimize spurious pick-up. Chopper relays should be of the 

 each-side stable type, giving good contact pressure with high speed of 

 operation; chopping frequencies of up to 300 c/s can be used. A chopper 

 relay should be adjusted when running in its final circuit by observing the 

 contact waveform on an oscillograph. 



A particularly sensitive form of polarized relay is based on a moving-coil 

 meter movement (Chapters 32 and 33) with contacts instead of a pointer; 

 such a relay may operate on less than 1 ^aW. 



If an appreciable load is to be switched, an intermediate relay is essential 

 as the contacts of moving-coil relays have a very low power rating. Moving- 

 coil relays are both delicate and expensive, and in most applications can be 

 replaced by a transistor and an ordinary relay (see Chapter 29). There are, 

 however, a few purposes for w hich the use of a moving-coil relay is essential. 



Ordinary non-polarized relays can be made to operate only on current of 

 one polarity with the aid of a rectifier {Figure 34.11). Naturally none of the 



f ' 



, j f f f 



' I I ^' I I I I ^^ I 



(a) 



(b) 



(0 



(d) 



Figure 34.11 



Rectifier-polarized relay: (a) and (6) constant-current feed; 

 (c) and {d) constant-voltage feed 



other advantages of the polarized relay is thereby obtained. Conversely, a 

 polarized relay can be operated by a current of either polarity if it is con- 

 nected in the circuit of Figure 34.12. 



Figure 34.12 Operation of polarized relay on current of either polarity 



OTHER DESIGNS OF RELAY 



Many different designs of relay exist besides those already discussed. In all 

 cases the fundamentals described for the P.O. 3000 type apply, although 



521 



