Temperature DetectorsTEMPERATURE DETECTION CIRCUITRYFigure 8 Bridge CircuitResistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances forcurrent flow through the ammeter. R3 is a variable resistor known as the standard arm that isadjusted to match the unknown resistor. The sensing ammeter visually displays the current thatis flowing through the bridge circuit. Analysis of the circuit shows that when R3 is adjusted sothat the ammeter reads zero current, the resistance of both arms of the bridge circuit is the same.Equation 1-1 shows the relationship of the resistance between the two arms of the bridge.(1-1)R1R3R2RxSince the values of R1, R2, and R3 are known values, the only unkown is Rx. The value of Rxcan be calulated for the bridge during an ammeter zero current condition. Knowing thisresistance value provides a baseline point for calibration of the instrument attached to the bridgecircuit. The unknown resistance, Rx, is given by Equation 1-2.(1-2)RxR2R3R1Rev. 0 Page 11 IC-01
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