Patent GB-677057-A (GB)
Title: Means for deriving constant unidirectional potential
IPC: G05F1/52
Filed: 1951-02-07
Abstract: 677,057. Automatic temperature control systems; rectifying. BRISTOL'S INSTRUMENT CO., Ltd. (Bristol Co.). Feb. 7, 1951, No. 3033/51. Classes 38 (ii) and 38 (iv). In a system for producing a constant direct voltage, the temperature difference between two sets of thermocouples connected in series opposition is maintained constant by automatic control of A:C. electric heating of one set of thermocouples in accordance with the temperature difference. In Fig. 1 a constant electromotive force, e.g. for measuring or control purposes, is produced between terminals 35, 36 by maintaining constant the difference in temperature of two sets of thermocouples 14, 22 connected in series opposition. Thermocouples 14 are housed within a tube 13 of thermally conducting material which is mounted within another tube 16 of similar material, the two tubes having lugs 17, 18 connected together and to a base-plate 10 so that the temperature of thermocouples 14 is substantially that of the base-plate. Thermo-couples 22 are housed within a tube 21 of thermally-conducting material which is mounted within another tube 24 of similar material the two tubes having lugs 25, 26 connected together but thermally insulated from base-plate 10 so that the temperature of thermocouples 22 is determined by the energization of a non-inductive electric heater winding 28 round the outside of tube 24. Changes in the temperature difference of the two sets of thermcouples are detected by non- inductive temperature sensitive resistances 15, 23, which are respectively associated with the two sets of thermocouples and form with resistances 43, 44 an A.C. bridge circuit energized from a secondary winding 51 of mains transformer 49. A potentiometer 45 is so adjusted that the output of this bridge circuit is zero when the temperature difference is such that the E.M.F. between terminals 35, 36 has a desired value. Deviations from this temperature difference then result in an A.C. output from the bridge circuit corresponding in magnitude and phase-sense to the magnitude and sense of the deviation which, through a triode 65, varies in the grid-cathode circuit of a gas-filled triode 32, the magnitude and sense of a voltage component which is in phase with the anode voltage of triode 32. A fixed component of voltage in quadrature with the anode voltage of triode 32 is supplied to the gridcathode circuit of triode 32 from a secondary winding 52 of mains transformer 49. Deviation in the temperature difference thus produces a phase shift of the grid-cathode voltage of triode 32 and since triode 32 is in series with the supply from A.C. source 31 to heater 28 deviation is corrected by variation in the energization of heater 28. In Figs. 2 and 3 (not shown), an additional resistance responsive to the temperature of one set of thermocouples is associated with the bridge circuit to compensate for effect of absolute temperature changes if the relationship between temperature and thermocouple E.M.F. is non-linear. Use of a thermocouple of platinum and a nickel-chromium alloy to avoid the necessity for this compensation is referred to.