Loop powers current transmitter

-October 28, 1999

In the circuit in Figure 1, the loop supply is 20 to 30V dc, and the loop current is 10 to 20 mA. IC1 operates as a constant-current (IP) source. IC2, R1, and R2 operate as a shunt regulator, which provides 12V VCC. IC2 is a TLO32 op amp. The sensor circuit could be a resistance-temperature-detector circuit or any other sensor- or signal-conditioning circuit. The IC2/Q1 combination operates as a current sink, with IC=VIN/RE. The total loop current is IP+IC. Because of its common-mode input-range limitation, IC2 requires a minimum 2V input for proper operation. Hence, the VIN signal range must be 2 to 10V. With VIN=2V, you adjust potentiometer P1 to set ILOOP at 10 mA. At this operating point, IC=4 mA, and the balance of the current (IP=6 mA) goes to the shunt regulator, the op amp, and the sensor-circuit load. The maximum load current is typically 5 mA. As VIN increases from 2 to 10V, ILOOP increases proportionally, solely from the increase in IP, because IC is constant:

For every 1V increase in VIN, the change in VR is 0.4V. The circuit has good accuracy and temperature stability and operates with loop supplies of 20 to 30V. The maximum available sensor-circuit current is approximately 5 mA. Currents greater than this value result in poor VCC regulation. (DI #2421).

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