Temperatur Detection Circuitry : Bridge Circuit Construction
Bridge Circuit Construction
Figure 8 shows a basic bridge circuit which consists of three known resistances, R1, R2, and R3 (variable), an unknown variable resistor RX (RTD), a source of voltage, and a sensitive ammeter.
Resistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances for current flow through the ammeter. R3 is a variable resistor known as the standard arm that is adjusted to match the unknown resistor. The sensing ammeter visually displays the current that is flowing through the bridge circuit. Analysis of the circuit shows that when R3 is adjusted so that 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.
Since the values of R1, R2, and R3 are known values, the only unkown is Rx. The value of Rx can be calulated for the bridge during an ammeter zero current condition. Knowing this resistance value provides a baseline point for calibration of the instrument attached to the bridge circuit. The unknown resistance, Rx, is given by Equation 1-2. 
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