GB1269931A - Improvements in or relating to apparatus for detecting temperature - Google Patents

Abstract

1,269,931. Measuring resistance. BRENTFORD ELECTRIC Ltd. and M.G.J. FRY. April 10, 1969 [March 11, 1968], No. 11787/68. Heading G1U. To measure the resistance R of an unknown resistor 11 through which a direct current I is passing, a signal proportional to the current I is applied to a first resistor R I , a signal proportional to the voltage V across the unknown is applied to a second resistor R V and either the first or second resistor R I , R V is adjusted to maintain the ratio of the voltages across them constant. The unknown resistor 11 may be a coiled electric cable which is to be heated and dried before the application of a plastics insulation, the amount by which the first or second resistor R I , R V has to be adjusted providing an indication of the resistance, and hence temperature, of the cable. The signal proportional to the direct current I is derived by a direct current transformer arrangement 12 having a primary circuit constituted by the current-carrying conductor 8 and a secondary circuit comprising a pair of oppositely-wound windings connected in series with an A.C. source 16 and a bridge rectifier 14 supplying the resistor R I . The signal proportional to the voltage V is derived similarly (13). The ratio of the voltages across the resistors R I , R V is maintained at unity by connecting the two resistors in series and automatically adjusting R I to keep the difference voltage V r zero. A relay 26 operates every time the difference voltage V r reaches a threshold level to cause a motor or solenoid device S to adjust resistor R I by one step, and the number of adjustment steps made is counted to provide an indication of temperature change. The count may be indicated by electronic indicator tubes, or by stepping switches which may also be arranged to operate auxiliary circuits when particular counts (temperatures) are reached. Resistor R I includes a part R 4 pre-set in accordance with ambient temperature, and a resistor R 1 in series with transformer 13 is pre-set in accordance with the initial cable resistance.