CHOKES AND TRANSFORMERS 



(2) A high voltage secondary winding for the generation of HT. This may 

 be either for use with half-wave or bridge rectification, in which case there 

 is a single winding with two ends brought out, or for full-wave rectification, 

 in which case the winding has twice as many turns and both ends and the 

 centre-tap are brought out. Full-wave HT secondaries are more common. 

 A half-wave or bridge winding which delivers 350 V R.M.S. at a max per- 

 missible load of 120 mA would be described as 350 V, 120 mA. A full-wave 

 counterpart would be designated 350-0-350 V, 120 mA. HT secondaries 

 range from about 150-0-150 to 500-0-500 for voltage and from about 60 mA 

 to 350 mA. The most popular and generally available sizes are probably 

 350-0-350 V 60 mA and 350-0-350 V 120 mA. It should be remembered 

 that the rectified HT voltage of a 350-0-350 V secondary, using the usual 

 capacitor input scheme, and on light load, is much higher than 350. First 

 the R.M.S. output on light load of a winding designed to give 350 on full 

 load is likely to be about 380. Secondly the rectifier system will deliver the 

 peak voltage, (2)^/^ X 380 = 540 V. The point should be borne in mind 

 when choosing the voltage ratings of capacitors. 



(3) A low voltage secondary winding delivering 5 V at about 2 amps. 

 This is intended for the heater of a thermionic rectifier where one is used 

 (see Chapter 24). 



(4) One or more 6-3 V windings delivering between 1 and 3 amps maximum 

 for valve heaters generally and for such accessories as dial lamps. These 

 windings may be provided with centre-taps; these should be earthed, the 

 purpose being to balance out the electrostatic interference caused by the 

 two leads comprising the heater wiring. 



In addition there may be an odd wire — frequently uninsulated — and this 

 goes to an electrostatic screen between the primary and secondary windings. 

 It should be earthed. The idea is to keep mains-borne interference at radio 

 frequency (e.g. diathermy apparatus) out of the equipment. 



SPECIAL MAINS TRANSFORMERS 

 The autotransformer 

 This is shown in Figure 22.2, from which it is clear that the component 



5 amp 

 Samp 



Figure 22.2 



220V 



110V 



1 1 



i 



T 5 amp 



10 amp 

 -o 



I 110V 



o 

 o 



1. 



Figure 22.3 



possesses only one winding. Autotransformers are commonly used for running 

 American llOV equipment from British 220-odd V mains, or vice versa. 

 Not only is the autotransformer advantageous in that — having only one 

 winding — it is easier to make, but also the component can be smaller. This 



306 



