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Current Loop Circuits
 

Last Updated: December 31, 2017 06:47 AM

Circuits Designed by Dave Johnson, P.E.

4 to 20ma Current Loop Senses TemperatureI 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 . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

4-20ma Current Loop TesterThis circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps. . . Circuit by David Johnson P.E.-May, 2012

Monitors Pump MotorThis circuit solves this problem in a novel way.  It clips on the outside of the pump motor’s power cable and provides a transistor switch closure whenever the circuit detects current flowing to the motor.  The circuit draws so little current that a small lithium cell will power the circuit for yearsCircuit Solution by David Johnson P.E.-November, 2004

Precision Low Capacitance MeterThis circuit was originally designed to measure the volume of fluid inside a medical syringe.  As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Pump Motor MonitorThis 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. . . Circuit by David A. Johnson P.E.-November, 2004

Temperature Sensor with 4 to 20ma Current LoopI 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. . . Circuit by David Johnson P.E.-July, 2000

Tester 4-20ma Current LoopThis circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2012




Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

3.3V/10mA Regulator from a 4mA to 20mA Loop -  This circuit is a 3.3 V/10mA regulator that extracts its power from a 4mA to 20mA loop.  This circuit demonstrates how an LTC1474/LTC1475-based regulator is easily optimized for such low current applications.  The 2W sense resistor limits the peak inductor current to 40mA to minimize current ripple and provide good efficiency __ Linear Technology/Analog Devices App Note, Mar 16th 2011

3V/ 5V DACs Support Intelligent Current Loop -  App Note/ 722 / Jul-03___ Maxim Integrated

4 to 20ma Current Loop Senses Temperature -  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 . . . Hobby Circuit designed by Dave Johnson P.E.-July, 2000

4-20ma Current Loop Tester -  This circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps. . . Circuit by David Johnson P.E.-May, 2012

4-20-mA loop needs no external power source -  09/13/01 EDN-Design Ideas The simple circuit in Figure 1 uses a low-current-drain MAX4073H amplifier to sense the current flowing through a 4- to 20-mA loop.  The circuit senses the current through a 1Ω resistor with a fixed gain of 100 and uses no battery__ Circuit Design by Shyam Tiwari, Sensors Private Ltd, Gwalior, India

4-20mA Loop Transmitter -  Transmission of industry standard 4-20mA current loop signals to values and other actuators is a common requirement.  Resistive line losses and actuator impedances require current transmitters to be able to force a compliance voltage of a least 20V.  Because of this, 5V powered systems usually cannot meet current loop transmitter __ Linear Technology/Analog Devices App Note, Mar 16th 2011

7.4mA DC Supply from 4mA to 20mA Current Loop -  The LTC3255 is a switched-capacitor step-down DC/DC converter that produces a regulated output   (2.4V to 12.5V adjustable) from a 4V to 48V input.  In applications where the input voltage exceeds twice the output voltage, 2:1 capacitive charge pumping extends output current capability beyond input supply current limits.  At no __ Linear Technology/Analog Devices App Note, Sep 19th 2013

A Closed-Loop, Wideband, 100A Active LoA - D -  Linear Technology AN133 __ Designed by Jim WilliamsOct 12th, 2011

Add fault protection to a 4- to 20-mA loop supply -  22-Jan-04 EDN-Design Ideas A 4- to 20-mA current loop consists of a power source and a current-measuring device at the control end and a field transmitter that senses process-variable information, such as temperature or pressure, and converts it to a current  (Figure 1).  Most such industrial current loops are powered by 24V dc, but that voltage can range from 12 to 36V__ Circuit Design by Mark Pearson, Maxim Integrated Products, Sunnyvale, CA

AN133A Closed-Loop, Wideband, 100A Active LoA - D -  Linear Technology AN133 __ Designed by Jim WilliamsOct 12th, 2011

Bipolar 20mA current-loop driver for robotic valves & other applications -  01/06/14  EDN-Design Ideas Drive bipolar current loops using a single supplyThis Design Idea drives the ±20mA current loops that are often used to control two-way proportional pneumatic or hydraulic servo valves in high power robotics applications.  Most current drivers are capable of driving unipolar outputs only.  Bipolar current drivers often require negative supply voltages.  __ Circuit Design by Clive Bolton

Brute Force Marries Controlled Speed -  Linear Technology AN133 __ Designed by Jim WilliamsOct 12th, 2011

Circuit provides 4-to 20-mA loop for microcontrollers -  05/27/04  EDN-Design Ideas The 4- to 20-mA current loop is ubiquitous in the world of controls in manufacturing plants.  Discrete logic, microprocessors, and microcontrollers easily cover the digital portions of control schemes, such as limit switches, pushbuttons, and signal lights.  Interfacing a 4- to 20-mA output to a rudimentary microcontroller can be problematic__ Circuit Design by Robert Most, Dow Corning Corp, Auburn, MI

Circuit Translates TTY Current Loop to RS-232 -  08/01/98 EDN-Design Ideas (NOTE : Multiple circuits in this file, scroll to find this one.)The circuit in Figure 1a provIdeas signal translation between a passive current-loop (TTY) interface and a duplex RS-232C port.  The current flowing in the receiver loop causes the transistor to pull down RxD; when the transistor turns off, R1 pulls up RxD.  In like manner, the current in the transmitter loop switches on for a negative TxD voltage and off for a positive voltage.  __ Circuit Design by Jerzy Chrzaszcz, Warsaw University, Poland

Convert 1 to 5V signal to 4-to 20-mA output -  04/19/12  EDN-Design Ideas Despite the long-predicted demise of the 4- to 20-mA current loop, this analog interface is still the most common method of connecting current-loop sources to a sensing circuit.  This interface requires the conversion of a voltage signal—typically, 1 to 5V—to a 4- to 20-mA output.  Stringent accuracy requirements dictate the use of either expensive precision resistors or a trimming potentiometer to calibrate out the initial error of less precise devices to meet the design goals.  __ Circuit Design by Thomas Mosteller, Linear Technology Corp

Current Loop Interface -  This is a very simple circuit to interface a current loop sensor to an input which is designed for a voltage, such as that from a standard potentiometer.  This page is technical so that interested persons can build the circuit themselves, but 4QD can also supply the interface on a 'postage stamp' sized (22mm x 27mm) board as shown in the first diagram.  It is a relatively trivial circuit!  __ Designed by Richard Torrens

Current loop transmits AC measurement -  01/28/13EDN-Design Ideas Originally published in the Aug 6, 1992, EDN-Design Ideas -- Process-control applications use current loops to send information as an analog signal over long distances with high noise immunity.  Using the three-chip circuit in Figure 1, you can measure alternating current or voltage and transmit the results on a 4- to 20-mA current loop.  The circuit accepts a 0- to 10-mV ac RMS input and provIdeas a 4- to 20-mA output.   __ Circuit Design by Mark Fazio, David Scott, and Bob Clarke, Analog Devices, Wilmington, MA

Industrial Sensor 4mA to 20mA Current Loop Power Tap -  The LTC3105 is a high efficiency step-up DC/DC converter that can operate from input voltages as low as 225mV.  A 250mV start-up capability and integrated maximum power point controller (MPPC) enable operation directly from low voltage, high impedance alternative power sources such as photovoltaic cells, TEGs (thermoelectric __ Linear Technology/Analog Devices App Note, Mar 16th 2011

Isolated 4-20mA Current Loop -  This circuit is a computer-controlled 4 – 20mA current loop.  It is designed to operate on a single supply over a range of 3.3 V to 30V.  The circuit’s zero output reference signal, 4mA, is set by R1 and calibrated using R2, and its full-scale output current is set by R3 and calibrated using R4.  The zero and full-scale output __ Linear Technology/Analog Devices App Note, Mar 16th 2011

Less Than 200 μA, Low Power, 4 mA-to-20 mA, Process Control Current Loop  -  Located at the control unit, the 12-bit AD5621 DAC produces an output voltage, VDAC, between 0 V and 5 V as a function of the input code.  The code is set via an SPI interface.  The ideal relationship between the input code and output voltage is given by where: V REF is the output of ADR125 and the power supply to the AD5621.  __ Analog Devices from the Lab

Loop powers current transmitter -  10/28/99 EDN-Design Ideas In the circuit in Figure 1 , the loop supply is 20 to 30V dc, and the loop current is 10 to 20 mA. IC 1 operates as a constant-current (I P) source. IC 2 , R 1 , and R 2 operate as a shunt regulator, which provIdeas 12V V CC . IC 2 is a TLO32 op amp.  The sensor circuit could be a resistance-temperature-detector circuit or any other sensor- or signal-conditioning circuit.   __ Circuit Design by N Kannan

Monitors Pump Motor -  This circuit solves this problem in a novel way.  It clips on the outside of the pump motor’s power cable and provides a transistor switch closure whenever the circuit detects current flowing to the motor.  The circuit draws so little current that a small lithium cell will power the circuit for yearsCircuit Solution by David Johnson P.E.-November, 2004

Precision Low Capacitance Meter -  This circuit was originally designed to measure the volume of fluid inside a medical syringe.  As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2004

Provides 4- to 20-mA loop for microcontrollers -  05/27/04  EDN-Design Ideas The 4- to 20-mA current loop is ubiquitous in the world of controls in manufacturing plants.  Discrete logic, microprocessors, and microcontrollers easily cover the digital portions of control schemes, such as limit switches, pushbuttons, and signal lights.  Interfacing a 4- to 20-mA output to a rudimentary microcontroller can be problematic__ Circuit Design by Robert Most, Dow Corning Corp, Auburn, MI

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. . . Circuit by David A. Johnson P.E.-November, 2004

Selectable Range Current Loop  -  App Note/ 1085 / Aug-08___ Maxim Integrated

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. . . Circuit by David Johnson P.E.-July, 2000

Tester 4-20ma Current Loop -  This circuit injects an adjustable current through a wire loop.  Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2012

Translate TTY Current Loop to Rs-232 -  08/01/98 EDN-Design Ideas (NOTE : Multiple circuits in this file, scroll to find this one.)The circuit in Figure 1a provIdeas signal translation between a passive current-loop (TTY) interface and a duplex RS-232C port.  The current flowing in the receiver loop causes the transistor to pull down RxD; when the transistor turns off, R1 pulls up RxD.  In like manner, the current in the transmitter loop switches on for a negative TxD voltage and off for a positive voltage.  __ Circuit Design by Jerzy Chrzaszcz, Warsaw University, Poland


Current Loop Circuits


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