21 

 CAPACITORS 



FIXED CAPACITORS 



Fixed capacitors are available in values ranging from about 1 pF (1 pF = 

 10~i2 F) to 2,000 /x¥ (1 ^F = 10"^ F). In choosing a component one has to 

 bear in mind not only the capacitance required but also the maximum 

 potential difference that will occur between the plates of the capacitor and 

 the nature of the dielectric material. 



The dielectric material is determined to a considerable extent by the 

 capacitance required. Figure 21.1 shows approximately the capacitance 



-Tubular metallized paper capacitors 



High voltage 

 •—Low A ceramic capacitors -~ • „ '"^electrolvtics"* 



I I '" Tufcjiar paper and foil capacitors ' ' , '7 electrolyticsT ''' 



IpF lOpF lOOpF 500 IDOOpF IQOOOpF 



OOl^F 01[j.F IpiF IOjulF 20pF WOpF 1000|liF 



Figure 21.1 



range occupied by capacitors employing the various common dielectrics, 

 from which it is clear that for most values there are alternatives. To enable 

 the reader to make an appropriate choice we shall mention briefly the 

 characteristics of the various materials, but before doing so it is necessary to 

 say a little about capacitors in general. 



The current through a real capacitor when connected to a source of 

 sinusoidally alternating voltage leads that voltage by an angle somewhat 

 less than 90 degrees, which implies that in addition to the true capacitive current 

 there is a small component of current in phase with the apphed voltage which 

 represents energy delivered to the capacitor and not recovered, i.e. a 'loss'. 

 The angle between the actual current vector and the ideal current vector is a 

 measure of the loss occurring in the capacitor and is called 6 {Figure 21.2). 



A.c. bridges for the measurement of capacitors are commonly provided 

 with two dials, both of which have to be set to the correct position in order 

 to achieve balance; one is caUbrated in capacitance, the other in 'tan d\ 

 To assist readers who possess a capacitance bridge to check the condition of 

 suspected capacitors of various kinds (and to show which kinds of capacitor 

 are intrinsically 'lossy') very approximate values of tan d are given in this 

 chapter. 



Each dielectric material has a band of frequencies over which tan d is 

 reasonably constant, but at the ends of the band the loss rises steeply. 

 Electrobiology being a low-frequency technique, we are not concerned with 

 losses at the upper end of these bands, all of which are much above the 



297 



