210 BELL SYSTEM TECHNICAL fOURNAL 



care. Power driven time delay thermistors tre best fitted for applications 

 where close limits on the time interval arc not required. In some com- 

 munications uses it is satisfactory to permit a six to one ratio between maxi- 

 mum and minimum times as a result of the simultaneous variation from 

 nominal values of all the following factors which affect the delay : operating 

 voltage ± 5 per cent; ambient temperature 20 degrees centigrade to 40 

 degrees centigrade; operating current of the relay ± 25 per cent; relay 

 resistance zt 5 per cent; and thermistor variations such as occur from 

 unit to unit of the same type. 



After a timing operation a power driven time delay thermistor should bs 

 allowed time to cool before a second operation. If this is not done, the 

 second timing interval will be shorter than the first. The cooling period 

 depends on particular circuit conditions and details of thermistor design, 

 but generally is several times the working time delay. In telephone relay 

 circuits requiring a timing operation soon after previous use, the thermistor 

 usually is connected so that it is short circuited by the relay contacts at the 

 close of the working time delay interval. This pe: nits the thermistor to 

 cool during the period when the relay is locked up. If this period is suffi- 

 ciently long, the thermistor is available for use as soon as the relay drops 

 out. Time delay thermistors have been operated more than half a million 

 times on life test with no significant change in their timing action. 



To avoid the limitations of wide timing interval limits and extended cool- 

 ing period between operations usually associated with the power driven time 

 delay thermistor, a cooling time delay method of operation has been used. 

 In this arrangement, two relays or the equivalent are employed and the 

 thermistor is heated to a low resistance-value by passing a relatively large 

 current through it for an interval short compared with the desired time 

 interval. The current then is reduced automatically to a lower value and 

 the thermistor cools until its resistance increases enough to reduce the cur- 

 rent further and trip the working relay. This part of the operating cycle 

 accounts for the greater part of the desired time interval. With this ar- 

 rangement, the thermistor is available for re-use immediately after a com- 

 pleted timing interval, or, as a matter of fact, after any part of it. By proper 

 choice of operating currents and circuit values, wide variations of voltage 

 and ambient temperature may occur with relatively little effect upon the 

 time interval. The principal variable left is the cooling time of the thermis- 

 tor itself. This is fixed in a given thermistor unit, but may vary from unit 

 to unit, depending upon dissipation constant and thermal capacity, as 

 pointed out above. 



In addition to their use as definite time delay devices, thermistors have 

 been used in several related applications. Surges can be prevented from 



