Patent GB-644082-A (GB)
Title: Improvements relating to variable impedance devices
IPC: H01C10/46
Filed: 1947-11-28
Abstract: 644,082. Variable contact resistances. FERRANTI, Ltd., and WALKER, D. F. Nov. 28, 1947, No. 31495. [Class 37] A rotary potential divider comprises a chain of seriesconnected resistors 1-9, Fig. 1, so evaluated as to give a variation of output voltage V<SP>1</SP> in accordance with some given law, for example, a cosine law, Fig. 3, with at least two, as in Fig. 1, but not all the resistors 1-9 at any instant individually in parallel with a potentiometer (10 or 12 in Fig. 1) of high resistance value relative to that of any of the resistors 1-9, and a slider or tap 11 which traverses the identical potentiometers 10, 12, but is only in contact with one at any instant (at the instant in Fig. 1 the output voltage V<SP>1</SP> represents the cosine of an angle between 20 and 30 degrees) and means (such as in Fig. 5 rotating pairs of brushes P<SP>1</SP>, S<SP>1</SP>, R<SP>1</SP>, Q<SP>1</SP>) actuated by the control which moves the slider 11 (in Fig. 5 the shaft 40 whose position represents the angle #) to transfer potentiometer 10 from connection to resistor 1 to bridge successively resistors 3, 5, 7, 9 and potentiometer 12 from resistor 2 to bridge successively resistors 4, 6, 8 so that tap 11 moves from one end of the chain of resistors to the other end, or over the entire circumferential length of an endless chain 0-360 degrees, Fig. 8 (not shown), traversing potentiometers 10, 12 successively and finally potentiometer 10 so that output voltage V<SP>1</SP> varies smoothly from a voltage representing cos 0 degrees to a voltage representing cos 90 degrees. The brushes P<SP>1</SP>, S<SP>1</SP>, R<SP>1</SP>, Q<SP>1</SP> which sweep over contact segments A<SP>1</SP>-J<SP>1</SP> on fixed discs 30, 32, Fig. 5, rotate with potentiometers 10, 12 in opposite direction to slider 11 and the relative speeds are such as to satisfy, Fig. 1. The Specification shows the operation of Fig. 5 in detail, Fig. 6 (not shown). In Fig. 5, the gearing system provides for the introduction of a fixed angle a whereby the potential of slider 11 varies in accordance with cos (#+α). As shown, Fig. 3, the potential change with slider 11 moving between the potentiometer ends follows a linear law and the extent of divergence of the straight line from the cosine curve varies throughout the latter. To overcome this defect the cosine potentiometer is A.C. operated as shown, Fig. 4, and a transformer 20 has its primary between earth and the higher potential end (P for potentiometer 10, as shown) of that potentiometer engaged at any given moment by the slider 11. The resistance value of any of the resistors 1-9 is assumed zero in comparison with the resistance of potentiometers 10 or 12 (identical). The secondary 22 in series with a very high resistance 23 is connected between the slider 11 and end P. A compensating P.D. is therefore created except when the slider 11 is at end P or Q. The practical equipment, Fig. 5, may be modified to include this compensating circuit as illustrated in Fig. 7 (not shown), wherein slider 11 is ganged to a wiper co-operating with a commutator associated with potentiometers 10, 12 to effect connection to the higher potential end of whichever potentiometer 10 or 12 is engaged by slider 11. Fig. 4 may be modified by transferring the primary connection from P to Q (the lower potential end of potentiometer 10). To cover the range 0-180 degrees the chain in Fig. 1 is lengthened by a further nine resistors with J earthed and the input voltage V across the entire chain. To cover 0-360 degrees (a closed chain), Fig. 8 (not shown), resistors are only needed between 0-180 degrees. Fig. 8 may be a continuously rotating sine or cosine potentiometer.