VARIABLE CAPACITORS 



apparatus using non-thermionic rectifiers, or thermionic rectifiers of the 

 directly heated kind, the HT is available before the valves have warmed up 

 to consume it, and during this transient period the HT voltage 'surges' to 

 an excessive value. 



The mechanical form of electrolytic capacitors is either tubular, with a 

 wire emerging at each end (see Plate 21.5) or enclosed in a cylindrical can, 

 secured to the apparatus by a single large nut or by a clip, and having all 

 the connections at one end. Where several capacitors are enclosed in one 

 case the latter construction is usual, and the capacitors often have a common 

 negative pole which is internally connected to the can itself. 



Future developments 



At the time of writing there are two further types of capacitor of great 

 promise which are not widely used, probably because at present they are too 

 expensive. 



Polystyrene film capacitors — These have low loss (tan d = 0-0005) and 

 phenomenally low leakage, about 30 times less than a mica capacitor and 

 100 times less than impregnated paper and foil. They are made from 100 pF 

 to 4 fxF and should be ideal for coupling purposes in low-level stages. 



Tantalum electrolytic capacitors — These are even smaller for their size and 

 working voltage than aluminium electrolytic components. One 50 ^i¥ 70 V 

 unit is about the size of a stack of half-a-dozen halfpennies. In addition they 

 have better tan 6, about 0-05, and extremely low leakage, about a hundred 

 times better than conventional electrolytics. 



VARIABLE CAPACITORS 



Variable capacitors are of two types: (1) the truly variable type intended 

 for frequent readjustment, employing air as dielectric, and famihar to anyone 

 who has looked inside a wireless set as the 'tuning condenser'; (2) 'preset', 

 screwdriver-adjusted types, otherwise 'trimmers', meant to be initially set 

 up to a correct value and left there. 



VW^ 



Frame 



WAW 



Frame 



(a) (t.) 



Figure 21.5 

 True variable capacitors 



These are useful for the accurate control of variable-frequency filters and 

 oscillators. The most common size is variable up to 500 pF and may be had 



303 



