POWER PACKS 



To produce electronically forward- and backward-stabilized power 

 supplies, having output impedances of a fraction of an ohm, which are 

 also widely variable in voltage, requires a fairly elaborate circuit. The 

 difficulty with the conventional electronically stabilized supply is that the 

 output voltage cannot be reduced to zero because the voltage amplifier 

 valve — on whose action the device depends — is fed from it. The difficulty 

 may be overcome by feeding the cathode of the amplifier from an auxiliary 

 stabilized negative supply, so that volts are still available for the valve even 

 when its anode potential is zero with respect to earth. A stabilizer, due to 

 Scroggie', working on these principles is shown in Figure 37.13. 



HEATER POWER SUPPLIES 



For run-of-the-mill applications such as heater power for pulse circuits and 

 high level amplifier stages, a.c. excitation from a winding on the mains 

 transformer is perfectly satisfactory (Figure 37.14). It is usual to earth one 



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fe/ fe^ fe' 



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JL 



Mains 



Mains 



Figure 37.14 



Figure 37.15 



side of the winding in order to define the absolute potential of the heater 

 circuit. The only point to watch is that, in so doing, the heater-cathode 

 potential difference rating for none of the valves becomes excessive. For 

 most valves this rating is of the order of 100 V and as, in most circuits, the 

 cathodes operate at a potential not far from earth there is no problem here. 

 Nevertheless there are exceptions, of which probably the most important is 

 the thermionic diode, which often appears in circuit applications where 

 the cathode is very considerably positive or negative to earth. When this 

 happens it may be necessary to supply the diode heater from a separate 

 transformer winding. The heater-cathode rating for valves can be found 

 in the makers' literature. 



When low-level amplifier valve heaters are fed from raw a.c. there is a 

 risk that 50-cycle 'hum' may be introduced into signal circuits, either by 

 capacitive or inductive coupling. The effect of capacitive coupling may be 

 reduced by 'balancing' the heater circuit about earth so that the disturbing 

 effects of the two heater leads cancel. One procedure is to provide the 

 heater winding with a centre-tap and earth this {Figure 37.15) but the 

 method does not allow for the effects of physical asymmetries in the wiring. 



594 



