37 

 POWER PACKS 



A power pack is that part of an electronic equipment which, drawing energy 

 from the electric supply mains, is responsible for supplying appropriate 

 power to the remainder. A complete apparatus may draw all its power 

 from a master power pack — or 'power unit' — or the several units compri- 

 sing the apparatus may each be endowed with their own, individual small 

 power packs. The latter scheme is conducive to greater flexibility, but if 

 high grade power supplies are necessary it is usually uneconomic. 

 Three types of supply are usually involved : 



(1) HT supplies for valves — Invariably a positive supply, of a few hundred 

 volts above earth, and very frequently — particularly with direct-coupled 

 and pulse apparatus — a negative supply, a few hundred volts below earth. 



(2) LT supplies for valve heaters — 6-3 V a.c. or d.c. depending on the 

 application, is the usual heater rating for general electronic work. 



(3) EHT supplies of a few thousand volts, most particularly for cathode 

 ray tubes, but also for Geiger-Miiller tubes and Multiplier Photo Cells. 



These three types of supply will now be discussed in detail. 



HT SUPPLIES 



Unstabilized supply 



The most elementary type of HT supply is shown, in its commonest form, 

 in Figures 37.1 and 37.2, using thermionic and non-thermionic rectification 

 respectively. A full-wave capacitor-input rectifier circuit feeds a simple 

 smoothing filter comprising a single choke and capacitor; the performance 

 of such a filter has been discussed in Part I; typically the ripple on the 

 output is of the order of a volt. In the thermionic case the valve manu- 

 facturer sometimes specifies the use of series anode resistors as shown, to 

 limit the valve current during the switching-on surge. Notice the protective 

 fuses, also the primary fuse and main switch connected to the phase, not 

 the neutral, side of the mains. In laboratories still fitted with two-pin plug 

 and socket distribution systems, where the sense of connection can be 

 reversed, double-pole switching and fusing is advisable, 



A simple unstabilized supply of the above kind has the disadvantages 

 that its output depends on the mains voltage, and that its output impedance 

 is rather high, a few hundred ohms; that is, the regulation is poor. It would 

 be quite suitable for feeding, say, a small loudspeaker amplifier; the latter 

 draws, on the average, a constant load current (the output valve being in 

 class A) so that the high output impedance does not matter, and fluctuation 

 in HT voltage caused by fluctuating mains voltage might produce small 

 fluctuations in gain, but these would be quite unimportant. It would not be 



585 



