CAPACITORS 



in gangs up to 4. Thus by parallel connecting in pairs one can make 2 X 

 0-001 [jF max. for controlling Wien bridge type oscillators and filters, but a 

 word of warning is necessary — the frame of these capacitors is common to 

 one set of plates in each gang {Figure 21.5). Thus when they are used in 

 Wien bridges the frame is live and the component must be mounted on 

 insulating pillars and operated via an insulating shaft. This sharing of one 

 terminal means ganged variable capacitors cannot be used for tuning parallel 

 T"s. It is necessary to devise some other method of mechanically coupling 

 individual variable capacitors, such as the use of nylon gears. 



Trimmers 



These commonly have maximum capacitances of about 50 pF; a group 

 are shown in Plate 21.6: (a) is virtually a tiny air dielectric variable capacitor: 

 (b) is another air-dielectric type, range 3-30 pF, obtained by screwing in and 

 out of one another two sets of concentric annular plates; (c) is a low k 

 ceramic type, range 5 to 40 pF, with a stated temperature coefficient (either 

 —750 p.p.m./°C or +100 p.p.m./°C); (d) is a 'mica compression' type. It is 

 in effect a foil and mica capacitor with arrangements for varying the pressure 

 which holds the assembly together and thus adjusting the capacitance value. 

 The pressure is controlled by turning the screw. 



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