tailieunhanh - The Alarm, Sensor and Security Circuit P2

Most instrumentation system rely on a from of the basic bridge ciruit ti monitor all type of physical phenmena | Bridge sensors 29 Most instrumentation systems rely on a form of the basic bridge circuit to monitor all types of physical phenomena using light-sensitive resistors RTD sensors and pressure strain and flow sensors. Many precision instrumentation systems utilize a precision op amp or chopper-stabilized amplifier. The output of the op amp can be directed to a digital panel meter or an A D converter card inside a personal computer. A precision instrumentation bridge circuit is shown in Fig. 1-36. A resistive sensor R4 unbalances the bridge producing a 30 Sensors and Detection Circuits tiny output that is coupled to a National Semiconductor LH0038 three-stage precision instrumentation-amplifier chip with internal gain resistors. The amplifier has a gain factor up to 2000. A 10-V reference produced from the LH0070 is applied to the bridge to provide an accurate source voltage or reference. The system has Maxwell bridge 31 excellent common-mode rejection and can be used in precision measuring applications. A shielded cable should be used when connecting the sensor if it is not located near the electronics. Any resistive-type sensor could be substituted for a strain gauge such as a pressure sensor or light-detection circuits. The bridge amplifier is used in many precision measurement circuits including seismology water resource management ballistics etc. Instrumentation-grade bridge amplifier parts list Quantity Part Description 3 Rl R2 R3 10-kQ -W resistor 1 R4 10-kQ resistive sensor 2 C2 C5 25-V capacitor disk 1 Cl 25-V capacitor 2 C3 C4 10-pF 25-V electrolytic capacitor 1 U1 LH0070 10-V reference National Semiconductor 1 U2 LH0038 instrumentation amplifier Maxwell bridge The basic bridge circuit is not limited to resistive de circuits. A device called the Maxwell bridge can measure an unknown capacitance or inductance. A Maxwell bridge is shown in Fig. 1-37. IC1 and IC2 form an oscillator that can be made to oscillate between 1 and 10-kHz. A small 3 1 .