Bridge Circuit Operation

TEMPERATURE DETECTION CIRCUITRY Temperature Detectors Bridge Circuit Operation The bridge operates by placing R_x in the circuit, as shown in Figure 8, and then adjusting R₃ so that all current flows through the arms of the bridge circuit. When this condition exists, there is no current flow through the ammeter, and the bridge is said to be balanced. When the bridge is balanced, the currents through each of the arms are exactly proportional. They are equal if R₁ = R₂. Most of the time the bridge is constructed so that R₁ = R₂. When this is the case, and the bridge is balanced, then the resistance of R_x is the same as R₃, or R_x = R₃. When balance exists, R₃ will be equal to the unknown resistance, even if the voltage source is unstable or is not accurately known. A typical Wheatstone bridge has several dials used to vary the resistance. Once the bridge is balanced, the dials can be read to find the value of R₃. Bridge circuits can be used to measure resistance to tenths or even hundredths of a percent accuracy. When used to measure temperature, some Wheatstone bridges with precision resistors are accurate to about + 0.1°F. Two types of bridge circuits (unbalanced and balanced) are utilized in resistance thermometer temperature detection circuits. The unbalanced bridge circuit (Figure 9) uses a millivoltmeter that is calibrated in units of temperature that correspond to the RTD resistance. Figure 9 Unbalanced Bridge Circuit IC-01 Page 12 Rev. 0