RESISTORS 



the other, gives the first digit of the resistance, the second band the second 

 digit, the third band the number of zeros and the fourth band the tolerance. 

 If the fourth band is absent it means the tolerance is ^20 per cent. 



Uninsulated resistors use the 'body-tip-spot' system (Figure 20.2), where 

 the body colour gives the first digit, the tip colour the second and the spot 



2nd digit Tolerance 



t 



§ 



1st digit 



/ 



21 



Number of 

 zeros 



Figure 20.1 



Silver, gold 

 or absent 



the number of zeros. There may be a tip colour — silver or gold — at the other 

 end which indicates the tolerance; if there is none, the tolerance is ±20 per 

 cent. 



In the case of resistors between 1 and 9 ohms inclusive the values — so far 

 as applying the code is concerned — are 01 to 09. Thus a 6 ohm uninsulated 

 resistor ±20 per cent has a first digit zero (black body), second digit 6 (blue 

 tip) no zeros (black spot) no tolerance mark (other tip unmarked). The 

 component is therefore entirely black except for one blue tip. 



Newcomers to electronics are ocasionally puzzled by the peculiar values 

 of resistances which seem to be common, 47 k, 68 k, etc. These numbers 

 follow from the use of the 'preferred value' system, in which the nominal 

 resistor values which should be manufactured are not arbitrary, but are 

 dictated by the tolerance required. This is explained in Figure 20.3 which 



1-5 



2-2 



33 



A 7 



68 



10 



i20% 



1 



15 



22 



27 



3 3 



39 



1 1-1 1-2 13 1-5 16 18 2-0 2-2 2 A 27 30 3'3 3-6 39 A-3 A 7 51 5-6 6 2 6 8 7-5 8-2 9-1 10 +5°/o 



A7 



56 



6 8 



8-2 



10 



110% 



8 9 10 



-H 1 1 



Figure 20.3 Table of preferred values 



shows how the 'preferred values' in each tolerance range are cell centre values 

 for a range of adjacent cells of approximately equal width. The actual 

 resistance of a component whose nominal value is the cell-centre value will 

 lie somewhere within that cell. In this way any resistor manufactured could 

 be sold under one or other of the nominal values — none need be wasted. In 

 fact composition resistors can be manufactured to within ±20 per cent of 

 the required nominal value, and the ± 10 and ±5 per cent specimens are then 

 obtained by measurement and selection. Figure 20.3 enables one to choose 

 the nearest preferred value to a calculated required one. 



Carbon composition resistors suffer from two disadvantages, their insta- 

 bility and their noise. The instability takes the form of a non-reversibility 

 in the resistance-temperature characteristic; as the component undergoes 



292 



