POWER PACKS 



expensive — it is poor economy not to buy the best — any saving here is 

 welcome. Smoothing and input capacitors are commonly between 0-1 and 

 0-5 ^F and the smoothing resistor about 100k. 



E HT + 



° — rAAArr 14 



EHT + 



(a) 



VWw 



— rvvvv-- 



1- 



(b) 



Mains 



IX. 



E H T - 



Mains 



(c) 



EHT - 

 Figure 37.21 



(d) 



There are two important objections to the direct generation of EHT. One 

 is that a capacitor of, say, 0-25 fiF rated at 5,000 V is large and expensive. 

 The other is that, when charged, it is extremely dangerous. Radio-frequency 

 EHT methods overcome both these difficulties. 



::>— ^ 



EHT- 



(a) 



(b) 



RFEHT 



Figure 37.22 



In this method power is drawn from a HT supply to feed a radio-frequency 

 LC oscillator. The oscillator inductance forms the primary of a step-up 

 transformer, whose secondary voltage is rectffied and smoothed to form the 

 EHT. Because the frequency is high the necessary high voltage smoothing 

 and input capacitors can be quite small, of a capacity insufficient to hold a 

 lethal charge. The step-up transformer is a simple air-cored affair, and the 

 overall cost, despite the extra valve and associated small components, is 

 probably no greater than that incurred in generating EHT from 50 cycles. 



An RF supply giving —2,000 V at 1 mA has been described, including 

 details for making the RF transformer, by Dickinson^^ and has the circuit of 

 Figure 37.23. Notice that the EHT rectifier circuit is of the type d in Figure 

 37.21. This throws more strain than is absolutely necessary on the trans- 

 former insulation, but allows Dickinson to heat his rectifier valve from the 

 common heater supply. 



598 



