Temperature DetectorsTEMPERATURE DETECTION CIRCUITRYThe battery is connected to two opposite points of the bridge circuit. The millivoltmeter isconnected to the two remaining points. The rheostat regulates bridge current. The regulatedcurrent is divided between the branch with the fixed resistor and range resistor R1, and the branchwith the RTD and range resistor R2. As the electrical resistance of the RTD changes, the voltageat points X and Y changes. The millivoltmeter detects the change in voltage caused by unequaldivision of current in the two branches. The meter can be calibrated in units of temperaturebecause the only changing resistance value is that of the RTD.The balanced bridge circuit (Figure 10) uses a galvanometer to compare the RTD resistance withthat of a fixed resistor. The galvanometer uses a pointer that deflects on either side of zero whenthe resistance of the arms is not equal. The resistance of the slide wire is adjusted until thegalvanometer indicates zero. The value of the slide resistance is then used to determine thetemperature of the system being monitored.Figure 10 Balanced Bridge CircuitA slidewire resistor is used to balance the arms of the bridge. The circuit will be in balancewhenever the value of the slidewire resistance is such that no current flows through thegalvanometer. For each temperature change, there is a new value; therefore, the slider must bemoved to a new position to balance the circuit.Rev. 0 Page 13 IC-01
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