FIXED CAPACITORS 



Metallized foil capacitors are characterized by smaller size than foil and 

 paper types for the same capacitance and working voltage, but also by 

 greater leakage and loss. Reasonable figures are : 



Foil and paper tan 6 > 001 Leakage resistance > 5000 Mfi . //F 



Metallized paper tan d > 002 Leakage resistance > 100 Mf2 . ^F 



The low leakage resistance of metaUized paper capacitors may render them 

 unsuitable for intervalve coupling purposes, and their recommended function 



Metallizing 

 Paper. 



Metallizing—, 

 Paper j^* 



Figure 21.3 



is decoupling and smoothing. The author has tried them for intervalve 

 coupling and found that, in addition, they seem to generate noise, which 

 makes them unsuitable in low-level stages. A useful attribute of metallized 

 paper capacitors is a measure of self-protection against breakdown from over- 



Inside foil 

 connection 



Outside 

 toil mark 



Outside foil 

 connection 



Figure 21.4 



voltage; the fault current produces enough heat to evaporate the metalliza- 

 tion in the damaged region, thus effectively removing that part of the com- 

 ponent from circuit. 



G. W. A. Dummer {Fixed Capacitors, London; Pitman) has drawn 

 attention to an additional limitation with capacitors employing paper 

 dielectric, which is that, however high the direct voltage rating of the com- 

 ponent, any alternating voltage applied across it must never exceed about 

 350 peak. The reason is bound up with eventual burning of the paper as a 

 result of repeated ionization of residual air bubbles within the component 

 at each cycle of the applied voltage. 



The temperature coefficient of paper capacitors is about +0-02 per cent 

 per degree C. 



Mica capacitors 



Here the dielectric is mica and once again the electrodes may be either 

 actual metal foils or may be metal films — commonly silver — applied direct to 



299 



