FIXED CAPACITORS 



capacitor they can be made to extremely close tolerance, i 1 per cent. Their 

 most important characteristic, however, is that their temperature coefficients 

 are accurately known. This means that in the design of RC filters and 

 oscillators the temperature coefficients of the resistors may be allowed for 

 by choice of suitable ceramic capacitors to go with them. The temperature 

 coefficient range obtainable is +0-01 per cent per degree C to —0-075 per cent 

 per degree C, and is stated on the component. The information may be 

 printed on the component in some such manner as this 



which means that the capacitance lies between 98 and 102 pF and the tem- 

 perature coefficient is negative, 150 parts per million per degree centigrade. 

 Alternatively, the component may be colour coded using the band-and-4 dot 

 system. The colours have this significance: 



Band 



\ 



2nd dot 9^^ '^o' 



, \ / 



H GOOD 



lU 



^ 



/ 3rd dot 

 1st dot 



High k ceramic capacitors {Plate 21.4, right) 



These are characterized by small physical size for their capacitance and 

 voltage rating, which makes them useful for miniature equipment. They 

 should be restricted to non-critical applications such as smoothing and 

 decoupling, since their capacitance values are subject to serious fluctuations. 

 Manufacturing tolerances are commonly of the order of —20 +100 per cent, 

 and they have a very large negative temperature coefficient, about ten times 

 bigger than that for any other kind of capacitor except the aluminium 

 electrolytic. In addition they are inclined to be 'lossy' (tan d = 0-02) and this, 

 coupled with their small size, means that they are liable to heat up unless the 

 alternating voltage across them is kept small. They are also rather leaky, of 

 the order of 100 MQ. /nF. 



301 



