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Current Loop Circuits
Designed by David A. Johnson, P.E.

Last Updated on: Friday, July 18, 2008 11:34 AM
Master Category List - Dave's Circuits
 
  • 4-20ma Current Loop Tester:  Using just a few components, this circuit can simulate signals from various instruments, which source a constant current. 
  • 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.

  • 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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