Industrial, 4-20 mA current loop, measuring basics IV.

Continued from the previous post

Although the title says “basics”, here are some advanced circuits about the 4-20 mA current loop transmitters.

In the first example a USB connection is shown. Nowadays almost every transmitter incorporates a microcontroller. The transmitters can be programmed through their display, or through HART protocol, or via DIP switches.

Transmitters with HART communication are expensive, using displays also increases the cost, and using DIP switches provides a not so flexible way of configuration.

Some manufacturers use USB as a programming / configuration interface, but most of them are not loop powered, so they need a separate power supply, which increases the cabling costs.

The following solution shows a way of how to include the USB communication in the transmitter design, while the transmitter is powered from the loop.

USB provides +5V besides the communication lines, so it can power the “power hungry” FT232 IC, The R4-R5 voltage divider tells the MCU that USB connection is present, and its UART interface can be switched ON.

On the other hand USB can serve more power to the transmitter, so when USB is present the MCU can switch its own clock to a higher lever.


4-20 mA transmitter with USB communication.

Sometimes it is needed to give a potential free contact signalling some threshold. A signal relay would be useful, but these relays simply consume too much power to be fed from the 4-20 mA loop.

A possible solution would be to use a low power reed relay. The relay can get its power form the loop (connected serially with the transmitter).

For this to work, an opto isolation is needed because of the serial connection. If the realy is ON, some fraction of the loop current is flowing through it, if it is OFF, all of the loop current flows through the zener diode.

This way the 4-20 mA loop porvides the measured signal, while the relay output signals if the threeshold limit is exceeded or not.


4-20 mA transmitter with relay output

Most of the applications require galvanic isolation between the loop side and the sensor side. In the former posts, a transformer based schema was shown. here is this solution is greater detail.

Although the transformer serves power to the sensor side, the 4-20 mA current regulation should be done precisely, so a so called isolated current sense solution is needed. IL300 (a LED – photodiode paired IC) seems to be a good choice.

Placing its LED in series with the loop’s output give information about the actual output current, while placing the photodiode’s circuitry to the sensor side gives continuous current regulation driven by the MCU.


4-20 mA current regulation with IL300

The previous solution’s major problem is that the IL300 LED to photodiode current transfer ration varies widely with device to to device, with temperature and with time. So it is not today’s precise solution.

The picture below shows the correct 4-20 mA current loop regulation.

4_20ma_2wire_USB_transmitter_jfet_isolated_precisePrecise, 4-20 mA current loop regulation

Looking the schematics in more detail:

The input circuit is a switch mode buck regulator as shown in the previous posts. Is simply generates a fixed +5V to the rest of the circuit.

4_20ma_2wire_usb_transmitter_jfet_isolated_precise_1Input switch mode power supply

Then a formerly shown isolated +5V / +5V DC/DC converter is used to generate an isolated +5V to the sensor side.


Isolated power supply to the snesor side

The sensor side incorporates a USB communication as shown earlier.


USB communitaction at the sensor side

And here it is! The isolated, current measuring, servo circuit.

Most of the loop’s output current flows through IL300’s LED (U7/LED), this generates an equal photocurrent in both of its photodiodes. The total output current is measured with Rs, The voltage across Rs is sensed with R3.

U8 operational amplifier adjusts the LED’s current through the MOSFET (the MOSFET bypasses some portion of the LED’s current),  so the U7/PD1  always reflects an exact fraction of the output current.

4_20ma_2wire_usb_transmitter_jfet_isolated_precise_4Output current measurement servo circuit with IL300

The last block is the current regulation circuitry. The wonderful thing in the servo circuit is that it simply doesn’t matter how the LED to photodiode current transfer ratio fluctuates, since it only relies on the transfer ratio of IL300’s photodiode1 and photodiode2, which is very closely matched. The MCU gives the current output’s setpoint and the operational amplifier continuously adjusts the NPN transistor’s base in order to the U7/PD2 photodiode feed back current reflects the exact loop output current.

4_20ma_2wire_usb_transmitter_jfet_isolated_precise_5Output current regulation servo circuit with IL300

This solution is highly efficient since it doesn’t need much power form the sensor side, while maintains high precision with galvanic isolation.

The drawback of this solution is that it needs a well designed current regulation (it is basically two integrating controller connected in cascade).

The main principle is that the loop side regulation circuit shall be slower than the IL300 bandwidth and the sensor side’s regulation circuit shall be slower than the loop side regulation.

For example: IL300 bandwidth is 200 kHz, loop side regulation can be 10 kHz, and the sensor side regulation is 200 Hz.

200 Hz bandwidth seems slow, but these types of transmitter usually measure slowly varying signals (temperature, force, pressure…)


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