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The bridge circuit is used whenever extremely accurate resistance measurements are required (such as RTD measurements). EO 1.9 Given a simplified schematic diagram of a basic bridge circuit, STATE the purpose of the following components: a. Rl and R2 b. RX c. Adjustable resistor d. Sensitive ammeter EO 1.10 DESCRIBE the bridge circuit conditions that create a balanced bridge. EO1.11 Given a block temperature diagram of a basic instrument detection and control system, STATE the purpose of the following blocks: a. RTD b. Bridge circuit c. DC-AC converter d. Amplifier e. Balancing motor/mechanical linkage EO 1.12 DESCRIBE the temperature instrument indication(s) for the following circuit faults: a. Short circuit b. Open circuit EO 1.13 EXPLAIN the three methods of bridge circuit compensation for changes in ambient temperature. 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.
Figure 8 Bridge Circuit Resistors Rl 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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