VARIABLE RESISTORS OR POTENTIOMETERS 



instead of straight, Plate 20.7; alternatively, the track may be made by 

 spraying the resistive material on to an insulating supporting base, e.g. a 

 fibreboard or a plastic {Plate 20.8). Power ratings for carbon track variable 

 resistors range from about \\ watts for the standard size models to 0-1 watt 

 for the tiny components, less than f in. in diameter, intended for deaf aids. 

 The 'law' of these devices may be linear, log, semi-log, linear tapered, 

 inverse log, inverse semi-log, or inverse linear tapered. The significance of 

 these terms is explained in Figure 20.4. The main application of log law 



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c 



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i_ 



E 

 £ 



x 



(0 



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en 

 c 



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80 

 60 

 AO 

 20 



A 

 B 

 C. 



20 AO 60 80 100 



Percentage of maximum rotation 

 (clockwise) 

 ..Linear D... .Linear tapered 

 Log E Inverse log 



..Semi-log F .Inverse semi-log 

 G . Inverse linear tapered 



Figure 20.4 



potentiometers is the volume controls of radio sets. The object here is to 

 arrange that the subjective loudness obtained is proportional to the rotation 

 of the control. In scientific work these components go some of the way 

 towards providing a control roughly linear in dB's. Log law potentiometers 

 may be made in the moulded track type by progressively varying the carbon 

 content of the mix along the track length, and in the sprayed type by spraying 

 the track a number of times, each time through a different specially shaped 

 mask so that a non-uniform conducting layer is built up. 



Wirewound variable resistors 



With wirewound variable resistors of the 'radio' type under discussion, the 

 law is nearly always linear though other laws may be had to special order. 

 The range of values obtainable is typically 1 ohm-100 k, by way of the 

 1-2-5-5-10 progression, as for carbon components. The tolerances on 

 nominal value are about 10 per cent. Manufacturing procedure is to wind 

 nickel chrome wire on to a flat strip of insulating material, afterwards bending 

 the strip round to the horseshoe shape necessary to fit into the insulating 

 body of the component {Plate 20.9). The oxide film on the wire surface is 

 relied upon for inter-turn insulation. Size for size, wirewound resistors have 

 a much higher power rating than carbon — the popular size can dissipate 5 



295 



