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.

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

Continued from the previous post

There’s a major problem with the circuits shown earlier. They all have a capacitor between their output pins (marked C3 in the following picture).

4_20ma_2wire_transmitter_capacitorThe capacitor shown in the red circle

This capacitor is needed by the voltage regulator (LDO) and provides other features like filtering and stability, so it cannot be omitted.

Let’s see the following situation:

2-wire-transmitter_simple2 wire transmitter elements in the measuring loop at startup

We have a 24V power supply with a maximum of 2A output current, a 2 wire transmitter and a 4-20 mA analogue PLC input (with measureing resistance of 50 ohms).

The power supply is switched off, so the transmitter is de-energised (not working because the lack of power). At the time we switch on the power supply, the transmitter’s internal (output, C3) capacitor is beginning to charge and actually it is functioning like a short circuit. For a little time, the power supply’s output 24V falls across the transmitter’s output resistor (10 ohms) and the PLC’s input 50 ohms, which means 400 mA current.

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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.

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


Let me begin with an example, which is well known by everyone. It is the public lighing. Its scheme is show below.

At sunset, when it is becoming dark, the public lighting turns on, and when the sun is rising, it turns off.

The sunshine is sensed by a light sensor, which serves an electrical signal propotional to the ambient light intensity. When this electrical signal falls below a certain point, the measuring unit decides to turn on the lights.


Public lighting scheme

In this article I will focus on the sensor’s inside, how it works, and what if we want to measure other types of physical properties, eg. pressure, temperature or rotation.

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Thermocouple Measurement Clarification

This is just a brief overview of how to do thermocouple measurements correctly and simply.
The principles of measuring with thermocouples are described by a lot of web pages. However, one can easily misinterpret the informations obtained form the internet.
The first confusion usually comes from the use of “ice bath” as cold junction compensation, the other is when someone tries to use the thermocouples’ mV – temperature tables.

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Switch debouncing in PLC software

When a switch or a pushbutton has to be connected to a digital circuit it is often (almost always) a nasty thing, because its unwanted behaviour.

It is a common practice to connect a button or a switch to a digital system (to a microcontroller, to a PLC, or to a PC). The simplest way to do it is shown in the picture below:


Connecting a pushbutton to a microcontroller

If we measure the voltage at the input of the microcontroller with an oscilloscope, it would show the following:


Contact bouncing phenomena

The problem is obvious. There isn’t a simple, clean low to high transition. If the microcontroller is fast enough, it senses this switch on event as if there were several pushbutton pressings.

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Simple Overvoltage Protection

In industrial environment sometimes it is essential to deal with unplanned harsh electrical conditions.
For example: if the power infeed is a standard 230 V type, then the designer usually cannot believe that the power input is around 230 V most of the time, and is always in the 10% limits.
There are always voltage spikes and other nasty things that put some unwanted overstress to the sensitive electronics, mainly to the switch mode power supplies.

The other case is when the requirements include a high degree of isolation. If for eg. 8 kV of isolation is needed between the mains input and the internal 24 V electronics then it is very hard to find an appropriate power supply off-the-self at an affordable price.

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