FIXED RESISTORS 



cyclic temperature changes during the hfe of the equipment it is in, its resist- 

 ance is subject to a small but progressive alteration, usually towards a higher 

 value. For this reason it is difficult to attach a temperature coefficient to 

 carbon resistors. The only way to minimize the effect is to choose resistors 

 with an ample margin of safety in power rating. 



Composition resistor noise has been briefly referred to in Chapter 17. In 

 addition to Johnson noise, these components develop a noise voltage pro- 

 portional to any direct current that may be passed through them. This 

 'current' noise is distributed in frequency in a similar manner to flicker noise 

 in valves, that is, the noise power in a given small band of frequencies is 

 inversely proportional to the mid-band frequency. It follows that current 

 noise is most serious in direct-coupled amplifiers. Its magnitude differs 

 widely from specimen to specimen of a particular type, and nominal value, 

 of resistor. Hence there is no simple expression which may be quoted for 

 giving the reader some idea of the importance of current noise. As a guide, 

 however, an empirical expression given by G.W. A. Dummer {Fixed Resistors, 

 Pitman) for the total noise allowed in composition resistors for use by the 

 Armed Services is 



'■^noise = -^ + ^^SlO | qqq 



microvolts per volts applied across the resistor, where R is the resistance in 

 ohms and the noise is measured over the band 200-10,000 c/s. Current noise 

 is generated to a much greater degree by resistors of low, rather than high, 

 power rating. Thus we have another reason for having an ample margin here. 

 The voltage rating for composition resistors varies considerably between 

 types. As a rough guide: 



Uninsulated— 1 watt, 1,000 V Insulated— 1 watt, 500 V 



— ^ watt, 500 V —I watt, 250 V 



'' Cracked carbon' , ^high stability^, or pyrolytic resistors 



In these resistors the resistive element is a piece of ceramic rod on to which 

 a surface film of carbon has been 'cracked' by exposing it to a hydrocarbon 

 vapour at about 1,000°C. Connections are then made to the ends of the rod 

 by pressing on metal caps to which the lead wires are secured, after which 

 the resistor may be adjusted to the proper value by machining away the 

 conducting film along a spiral path until the required value is reached. The 

 whole assembly is then protected by some kind of paint or varnish. In this 

 way components are produced whose resistance is stable, whose temperature 

 coefficient of resistance is less than iO-l per cent per degree centigrade, 

 whose resistance value is generally within 1 per cent of the nominal and whose 

 current noise is about 1-10 per cent of that for a composition resistor. The 

 resistance value in ohms and the tolerance are usually printed on the com- 

 ponent. The voltage rating is about 200 for the half-watt size and 400 for 

 the one-watt size. A group of pyrolytic resistors is shown in Plate 20.3. 



Wirewound resistors 

 These are of two kinds, general purpose and precision. 

 20 293 



