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CURRENT LOOP CIRCUITS
Circuits designed by David Johnson, P.E.
Last Updated on:
Wednesday, April 28, 2010 03:53 PM
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- 4-20ma Current
Loop Tester: Using just a few components, this circuit can simulate
signals from various instruments, which source a constant current.
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Pump Motor Monitor
This circuit can turn on a light or
sound a beeper, whenever an AC line powered pump motor turns on. It is
designed to detect the motor current using a small inductor placed on the
outside of the motor’s power cable. No direct wire contact is needed.
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TEMPERATURE SENSOR WITH 4 TO 20mA CURRENT LOOP
I designed a circuit similar to this one years ago to accurately measure the air
temperature inside a building 1000s of feet from a control room. The
circuit uses a very robust current loop method. It uses a highly accurate
semiconductor temperature sensor and an equally accurate voltage reference. The
circuit includes a diode bridge, so it is polarity independent. By using
the component values indicated, the circuit should not require calibration.
It has a range from –40F to +120F and an accuracy of +/- one degree F.
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