22 



CHOKES AND TRANSFORMERS 



These are the practical versions of the theoretical self and mutual inductances*. 

 It has been intimated before, in Part I, that it is a good plan to keep inductors 

 of both kinds out of electronic equipment as far as possible, and that one 

 reason for this is that whereas one can go into a radio shop and buy a resistor 

 or capacitor with for practical purposes any value over a range of several 

 orders, the same is not true of inductors. One may get what one wants by 

 improvization ; a particular radio tuning coil or old intervalve transformer 

 may happen to have a required self inductance, but if this is so it is fortuitous. 

 In general, if one wants inductors one has to make them oneself. 



There are some important exceptions : the mains transformers and chokes 

 sold as replacements for radio sets are quite suitable for the power packs 

 of electronic equipment, and it is often feasible to press the multi-ratio types 

 of 'output transformer' — for coupling power valves to loudspeakers — into 

 services for which they were never intended. 



Core 



HT 

 Secondary - 



350^ 



CT 



Oo- 



•-SSOc^- 



5 volt secondary 

 for rectifier heater 



{ 



One or more6-3voU 

 windings possibly 

 centre -taoped for . 

 other valve heaters 



CT 



Earthed 

 screen 



1/ 



-o 2/.0V 1 



-o 220V 

 -o 200 V 



^ 



- Primary 



Figure 22.1 



MAINS TRANSFORMERS 



The most important class of these {Figure 22.1) have about 4 distinct windings : 

 (1) A primary winding, for connection to the mains and provided with 

 taps at, say, 200, 220 and 240 V for adjustment to the appropriate local 

 voltage. 



* Except in the world of LC filters, where the inductances retain their academic name — 

 inductors. 



305 



