CAPACITORS 



electrophysiological spectrum: the low-frequency ends are, however, of 

 considerable importance. The principal reason for the rise in loss at low 

 frequencies with a particular dielectric is the increasing importance of the 



Loss current 



Ideal 

 capacitive 

 current 



Resultant 



total 

 current 



V 



Figure 21.2 



leakage current in comparison with the capacitative current. It follows that 

 the leakage current of capacitors for low-frequency use gives a fair indication 

 of the condition of the capacitor: this is a useful fact for the worker not 

 possessing a capacitance bridge. Capacitor leakage resistance may be 

 measured in a number of ways, of which the simplest is probably to charge 

 the component and observe the rate at which it discharges through its own 

 leakage with an electrostatic voltmeter (or high resistance moving-coil volt- 

 meter in the case of low voltage electrolytics). The voltage will fall to 1/e 

 of its original value in RC seconds, where C is in /iF and R in megohms. 

 As a guide to the values of leakage which are to be expected, approximate 

 figures are included where applicable in this chapter. The leakage tends to 

 be proportional to the capacitance, so the figures are in megohm-/^F. To 

 get the expected leakage resistance for a particular capacitor, divide the 

 figure given by the capacitor value in ^F. 



Impregnated paper capacitors 



In these capacitors the dielectric is paper impregnated with mineral oil, 

 paraffin wax or petroleum jelly. The electrodes may be either of aluminium 

 foil or may be made by spraying or vacuum-depositing a metal film directly 

 on to the dielectric paper itself. The method of construction is to roll up a 

 sandwich of foils and paper, or two layers of metallized paper into a cylin- 

 drical bundle, as indicated in the end views {Figure 21.3). The bundle is then 

 enclosed in a tubular container or flattened and fitted into a rectangular one 

 {Plates 21.1 and 21.2). It is clear that one or other electrode must be outer- 

 most with this construction and will form an electrostatic screen for the other. 

 The connection to the outer electrode is marked by a black band on the 

 container {Figure 21.4). The point is of importance in decoupling apphcations, 

 where it is usual to earth the outside foil. 



Maximum and working voltages for paper and foil capacitors range from 

 hundreds to thousands, the most common ratings being 350 and 500. Metal- 

 lized foil types range from 75 to 600. Tolerances are about ±20 per cent. 

 Values proceed by the 1-2-5-10 sequence, sometimes including 2-5 and 3. 



298 



