Patent GB-1246819-A (GB)
Title: Improvements relating to transducers
Inventor: WILLIAMSON DENNIS
IPC: G01R17/10
Filed: 1968-12-21
Abstract: 1,246,819. Automatic current control. FERRANTI Ltd. Jan.22, 1970 [Dec. 21, 1968], No.60885/68. Heading G3R. [Also in Division G1] A system for providing an output current proportional to the resistance of an unknown resistor incorporates a four arm bridge network containing reference resistance R 5 , R 6  in one arm, the unknown RX in an adjacent arm and resistors R 7 , R 8  in the other two arms, the bridge being energized by current which is made constant at 12 where it flows through the unknown and reference resistors. A high gain amplifier arrangement 13, 14, energized by the same supply as the bridge, derives the output current from the bridge output. At least part of the output current is returned via a negative feedback loop to one or more of the bridge resistors, e.g. R 5 , other than the unknown resistor, to maintain the output of the bridge network substantially at zero despite variations of the value of the unknown resistor. The system is particularly of use with a variable resistance temperature sensor having four leads R 1 , R 2 , R 3 , R 4 , two at end end, to enable the variable resistance to be balanced irrespective of the effects of the small resistances of the leads. To minimize the effects of R 1 , R 2 , the bridge output points L1 is connected to each end of R 1  via high resistors R 9 , R 10 , R 10 , being in series with R 2 . The output corner B of the bridge is thus represented by the mid point of R 1 . The effects of variations in L1 are mitigated because the two halves there are in adjacent bridge arms. The effects of variations in R 2  are mitigated by being in series with the much larger resistance R 10 . At the other end of RX, R 4  is in the constant current path and thus has no effect, while R 3  is in series with much larger resistance R 8 . The current feedback for the bridge is to a medium value resistor R 5 , and the system settles to an equilibrium wherein the feedback current balances bridge output voltage variations due to changes in RX, the current (representing RX) being indicated on ammeter 15. Further feedback to the constant current supply 12 enables the current to be altered to compensate for non-linearity in the temperature variable resistor. Modifications are described for situations in which the temperature sensor has only three or two leads Figs. 3-4 (not shown). In such circumstances, a pair of leads at opposite ends of the sensor must be matched. Current feedback may be to the arm R<SP>7</SP> not including a sensor lead. The constant current circuit Fig. 5 incorporates resistors R 12 , R 13  and Zener diode 72 across the stabilized supply 11, the voltage across the Zener diode supplying the emitter of transistor T1 whose collector connects via R 14  to the supply and to the base of T2. This passes a current determined by its emitter resistor and base potential to the bridge, and its emitter potential controls the base of T1, such that any current increase through R 15  reduces the current through T1 to reduce the current through T2. The constant current level is controlled by varying the current applied to R 13  (from the bridge output current).