814 BELL SYSTEM TECHINAL JOURNAL 



To obtain the maximum dependable performance from the various power 

 transformers and coils employed in the radar receiver power supplies under 

 the severe military held conditions, a considerable development program 

 was carried on throughout the past war. At the beginning of this program 

 the only available method of insuring adequate transformer winding insula- 

 tion under extreme liumidity conditions involved the sealing of the structure 

 in a metal container. P^or aircraft service, where weight is of prime con- 

 cern, this added weight could not be tolerated so that here an open type of 

 structure was extensively employed, with the protection to the winding 

 being furnished in the form of several coats of varnish followed by an enamel 

 overcoat. With the increased emphasis on high-altitude oj)eration of mili- 

 tary aircraft and the rapid temperature and pressure changes involved, a 

 development [)rogram was instituted to improve the open-type transformer 

 sealing process. The result of this program has produced the Flexseal 

 process whereby the service life of this type of power transformer has been 

 increased as much as ten times that obtained with the varnish process for- 

 merly employed. This process involves a multiple-dip varnish coating 

 method where the varnish is thickened by the addition of talc, a very 

 tine low-gravity wettable inert filler. This process results in the formation 

 of a relatively thick plastic shell which completely surrounds the trans- 

 former structure. One of the features of this process is found in its sim- 

 plicity, whereby no special equipment was required to carry out the pro- 

 cedure, an important factor during a w^artime production program. Figure 

 75 illustrates a number of typical open-type power transformers which 

 were employed on various military radar projects, all of which incorporate 

 the Flexseal treatment for improved service life. 



2.83 High Voltage Power Supplies 



The high voltage, as required for radar receiver cathode-ray tube indicator 

 purposes, varies from 2000 volts to 5000 volts, and for the TR tube keep- 

 alive potentials of the order of 1000 volts must be provided. In these cases, 

 however, the d-c current requirements are quite small and generally no 

 regulation means are required for stabilization of the voltage, the stability 

 of the primary source of power usually being sufficient. 



The design problems encountered in this type of power supply are con- 

 cerned {)rimarily with the requirements of reliability of operation under 

 severe military ()])erating conditions, and furlhcr require that only circuit 

 elements having well-defined factors of safety be employed in such appli- 

 cations. 



Figure 76 illustrates a number of typical high-voltage transformer designs 

 which have been employed in mihtary radar systems during the past war 

 period. Both air insulated and oil immersed types of structures are shown. 



