EHT SUPPLIES 



supply derived by rectification of the a.c. mains may be smoothed by floating 

 an ordinary accumulator across it, but if this is done care has to be taken to 

 arrange matters so that the accumulator is neither gradually discharged — 

 because the rectified voltage is too low — nor the water all electrolysed off by 

 overcharging because the rectified voltage is too high : in addition the usual 

 maintenance procedures are necessary. In these special smoothing and 

 stabihzing cells the chemical cycle is closed, no gas is evolved and the device 

 is hermetically sealed. Any current up to a stated maximum can be passed 

 through the cell and overcharging is impossible; therefore the external 

 circuit can be arranged to pass a steady 'charging' current through, and the 

 cell will maintain a steady potential difference across itself of between 1-4 

 and 1-45 V. The temperature coefficient is about —0-17 per cent/°C and the 

 maximum current ratings lie between 20 mA for a model weighing 0-1 oz., 

 and 1 amp for one weighing 2| oz. The effective equivalent capacitance of 

 the 400 mA version is stated to be 60,000 fiF. Further details may be had 

 from Mercia Enterprises, Ltd., 30 Silver Street, Coventry. 



EHT SUPPLIES 



For ad hoc apparatus such as, for example, might be used in vision experiments 

 involving photomultipUer cells, it may be worth considering deriving EHT 

 from a chain of series-connected dry batteries of the layer type. Such a 

 supply has the merits of stability and freedom from noise — provided the 

 batteries are reasonably new — and may easily be earthed at either pole or at 

 any intermediate point as required. For more permanent installations, 

 however, mains-driven supplies are more satisfactory and fall into two types, 

 those in which EHT is generated directly by transformation, rectification and 

 smoothing of the mains, and those which involve the generation of radio 

 frequency oscillations. 



Direct methods 



Because the current demands on EHT supplies are low — seldom more than 

 a milliamp — two useful simplifications follow: half-wave, rather than full- 

 wave, rectification is adequate, and resistance, rather than choke, smoothing 

 is sufficient. A half-wave positive supply may have the configuration of 

 Figure 37.21a or b. Similarly a negative supply could be as in Figure 37.2lc 

 or d. However, it is not hard to see that, whereas in a and c the maximum 

 voltage the transformer insulation has to stand is equal to the peak secondary 

 voltage, in b and d the maximum voltage stress is nearly twice as much. For 

 this reason the former configurations are general, and EHT transformers 

 usually have one end of the secondary winding internally earthed. It follows 

 that when a thermionic rectifier valve is used {Figure 37.22) the transformer 

 has in addition to have a special rectifier heater winding, whose insulation 

 also must be able to stand the peak secondary voltage to earth, and in the 

 case of the positive supply must in addition withstand twice the peak voltage 

 between itself and the live end of the main secondary. 



From this it is clear that non-thermionic rectifiers are advantageous, quite 

 apart from their longer life, in that — particularly for positive supplies — they 

 make possible the use of a cheaper transformer. As EHT transformers are 



40 597 



