Industrial, 4-20 mA current loop, measuring circuits basics II.

Continued from the previous post.

The same priciple is true for the followings, temperature to current transmitter. In this case the input voltage is propotional to the measured temperature, not the rotation.

4_20ma_2wire_transmitter_tempTemperature input, full analog, 2 wire, 4-20 mA loop powered transmitter

U3 is a low cost, NTC based, integrated temperature sensor.

The next schematic is a differential pressure to current transmitter.

4_20ma_2wire_transmitter_pressureDifferential pressure input, full analog, 2 wire, 4-20 mA loop powered transmitter

This circuit needs a little explanation.

MPX5x00 sensors are integrated, differential pressure sensors with wide pressure ranges. They need 5 V supply voltage, but with maximum 10 mA supply current. This 10 mA is well above the minimum 4 mA loop current, so we have to replace the linear regulator (U2) with a switching regulator. This can serve more output current without using more then 4 mA in its input line. The switching regulator can be TPS54062.

In industrial applications it is often desirable to know a motor’s rotor position or the velocity of the rotor.

4_20ma_2wire_transmitter_velocity_rotationVelocity / rotation input, microcontroller based 2 wire, 4-20 mA loop powered transmitter

Measuring rotation or rotation speed contactlessly needs incremental encoders. The sensor can be Hall element with magnetic ring, or reflective type optocoupler. The picture above shows an optocoupler based solution. Although the previous examples included only analogue components, it incorporates a microcontroller, since it is the most flexible and space saver way to detect angular changes. The optocouplers can be TCRT5000 type reflective optical sensors. Their emitting diodes must be driven with constant current. This is done by U1A.

The microcontroller unit (MCU) measures the pulses coming from the optocouplers’ phototransistors, calculates the position and then supplies a voltage to U1B through R10.

This schematic is also suitable for a tachometer application, but for an angular position transmitter, two limit switches are needed (to detect the start and end position). This way the whole position transmitter is self-learning, since the intial and end position is continuously updated when reaching the limit switches.

When the input side of the transmitter is connected to the protective earth (PE) conductor (eg. thermocouple based measurements), galvanic isolation must be used between the sensor and the current loop side. This can be easily done with a small high frequency transformer and optocouplers. The schematic shown below describes this, where U2A is an integrated transformer driver, U3 is the optocoupler, U5 is a digital to analogue converter, the rest of the circuit is the same as the formers.

isolated_2_wire_transmitter2 wire, isolated input-output, 4-20 mA transmitter

U2 can be SN6501.

Loop powered process metering, visualization

Sometimes it is necessary to make visualisation of the measured value at the measuring place, not only at the data acquition and visualization center. In this situation we need a loop powered process meter, which measures the current flowing in the loop and displays it.

2-wire-transmitter-with-process-indicator2 wire, 4-20 mA transmitter with loop pwered process indicator

Since the 4-20 mA current corresponds to a physical value range (eg. 0-6 bar pressure), the measured value should be scaled. This scaling can be easily done with a microcontroller, so this function is also easier to solve with programmable digital circuits, not only with analogue components.

loop_powered_process_indicatorLoop powered, microcontroller based, 4-20 mA process indicator / process meter

The loop current is measured by the circuit formed by U1A. The output voltage of U1A is digitized by a microcontroller which then serves data to the display. Setting the scaling from 4-20 mA to physical value is done by the two adjustment switches.

The supply voltage is „generated” by Z1 zener, but this voltage can slightly vary with different currents flowing through Z1. If one wants more accurate supply voltage, a shunt regulator can be used. This solution is shown below.

 shunt_regulatorCreating more accurate voltage using precision shunt regulator

Current loop galvanic isolation

When the distance between the transmitter and the measuring unit is too long, it is often adviseable use a galvanic isolator device. This takes place according to the picture below.

2-wire-transmitter-with-galvanic-isolation4-20 mA current loop with galvanic isolation

The galvanic isolator simply copies the current measured at the input to its output terminals.

In the simplest solution, both the input and the output is passive, so an additional power supply is necessary.

loop_powered_signal_isolatorLoop powered, analog, optocoupler based 4-20 mA signal isolator

The sensing element is a linear optocoupler, HCNR200 or IL300 type.

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8 thoughts on “Industrial, 4-20 mA current loop, measuring circuits basics II.”

    1. The posts are mainly guidelines, I don’t really wanted to do a complete design process, just make a clear thought of the topics.
      There can be problems while implementing, but every development has problems. I don’t know what will be particularly, but if you are a good engineer, the problems will be neglible.

      Liked by 1 person

  1. Can you please clarify how switching regulator can reduce the MPx5x00 sensor current from 10mA to 4mA? I thought LDOs are just not efficient, but should not cause higher current requirement for sensor… Thanks.


    1. Let’s see an example:
      the MPx5x00 sensor operates at 5V with 10 mA current consumption.

      The switching regulator is 90% efficient, and has 16V at its input, with a maximum of 4 mA of input current.

      The switching regulator converts this 16V@4mA to 5V@11.52mA (including efficiency loss) This available current is adequate for the pressure sensor ahd the rest of the electronics.

      Hope this helps, don’t hesitate if you has further questions.


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