GB946858A - Improvements in or relating to indicating devices - Google Patents

Abstract

946,858. Photo-electric control arrangements; bridge circuits. INDICATING DEVICES. May 20,1960 [ Feb. 24, 1959], No. 6373/59. Headings G1A and G1U. A voltage or current measuring device comprises a reversible motor 21, Fig. 1, an indicating member 14 which is rotatable under control of motor 21 over a range of indicating movement which is more than one complete revolution, a variable resistance 24 driven by motor 21 and forming part of a self-balancing bridge network 31, Fig. 2 whose balance output circuit includes the operating winding 12 of a moving coil 10 in series with the voltage or current to be measured, e.g. from a thermocouple 30. The unbalance current from bridge 31 causes winding 12 to rotate relative to main body 11 over a limited range, less than one revolution. A blade 16 mounted on coil spindle 13 is interposed, towards the ends of this range,between a light source 17a or 17b and an associated photo-cell 18a or 18b mounted on body 11. Photo-cells 18a, 18b control respective relays 33, 34 so that when illumination of a photo-cell is interrupted, current is supplied to motor 21 to rotate body 11 in the same directtion as the coil rotation whereby the coil may continue to rotate. A slider 23 of helical potentiometer 24 adjusts bridge 31 towards balance, and when balance is approached, the deflection of coil 12 decreases with respect to body 11, but does not necessarily reach zero,both photo-cells are again illuminated and motor 21 is stopped, pointer 14 then indicating the thermocouple output. In another embodiment, Fig. 3, coil 12 rotates a mirror 38 so that photo-conductive elements 40,41 are unevenly illuminated when bridge 31 is unbalanced. The elements 40, 41 are connected in a further, A.C. supplied, bridge circuit whose unbalance voltage is applied to one phase winding of motor 21 through a phase-shifting capacitor 49 and an amplifier 48. In this case motor 21 drives pointer 14 directly, slider 23 adjusting bridge 31 to balance as before. Application No. 6372/59 and Specification 901, 213 are referred to.