RELAYS AND RELATED MECHANISMS 



strip bends and operates the contacts. A snap action is usually arranged, 

 either by magnetizing the contacts or by using a bowed spring. 



The operating time depends on ambient temperature and upon the 

 immediate past history of operation of the relay. Operate and release lags 

 of the order of one minute are possible with a relay of this type, but the 

 accuracy of the timing is low (of the order of ±30 per cent). The operating 

 power required is several watts. 



Thermal delay relays are most useful for the control of sequential circuit 

 switching (e.g. switch on heaters— wait — switch on HT) where the disadvan- 

 tages of low accuracy and high power consumption are of little significance. 



Another device dependent on thermal expansion is the hot-wire vacuum 

 switch (Figure 34.17). With a relay of this type currents of up to 25 amps 

 can be switched, the operating power needed being only a few watts. Hot- 

 wire switches can only be used when the delays of several seconds inherent 

 in their operation can be tolerated. 



RELAY CIRCUITS 



In this section certain selected topics in the design of relay circuits are 

 discussed. The uses of relays are so diverse that it is impossible to do more 

 than illustrate by these examples some of the techniques available. 



Circuits designed by telephone engineers tend to follow certain stereotyped 

 conventions, based on the conservatism which is essential in the design of 

 rehable telephone systems. However, the combination of relays with valves 

 and transistors may produce circuits which although unorthodox both to 

 telephone and electronic engineers are nevertheless highly efficient and 

 elegant. 



Spark-quench circuits 



When an inductive circuit is interrupted the energy stored in the induc- 

 tance, unless otherwise dissipated, will appear as a spark at the interrupting 

 contacts. The contacts will eventually become pitted by the repeated spark- 

 ing and unreliable operation will result ; furthermore, radio interference will 

 be produced, transmitting impulses to nearby high-gain amplifiers. Indeed, 

 a relay operated from a sensitive amplifier has been known to go into a state 

 of continuous oscillation due to feedback of spark interference into the 

 amplifier input. 



It is rarely necessary to use spark-suppression circuits on contacts control- 

 ling other relays, but if uniselectors, motors or other heavy and highly 

 inductive loads are switched, some form of spark quenching is essential. 

 Figure 34.18 shows several possible circuits which provide a path for the 

 stored inductive energy. The most generally useful circuit is that of Figure 

 34.18c in which a non-linear resistor (e.g. Metrosil, Atmite) is used. Under 

 normal conditions little current flows through the non-linear resistance, but 

 the high voltages developed at the interruption of the inductive circuit cause 

 its resistance to fall, and large momentary currents pass. The voltage across 

 the contacts is therefore kept below the value at which sparking will occur. 



All spark-suppression circuits, by providing low resistance paths for 

 circulating currents, delay the collapse of magnetic flux in the inductive load ; 



524 



