4-20mA current loop circuit that designers must know

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4-20mA current loop circuit that designers must know [Copy link]

There are many process control systems in industrial sites, from simple flow control to complex power grids, from environmental control systems to steel mill process control. These control systems are composed of many modules such as central processing units, input modules, analog outputs, digital outputs, power supplies, etc. Data communication is required between different modules. Among the many modern communication methods, there is also a relatively old communication method that is relatively special, which is the 4-20mA current loop. 4-20mA current loopyunrun.com.cn/tech/2055.html 4-20mA current loop is an analog communication method. Compared with various modern digital communication methods, it does not require complex encoding and decoding work, and the communication method is simple; due to the inherent anti-interference ability of the current loop, it has high reliability in industrial sites with complex noise environments; at the same time, the 4-20mA current loop communication has a relatively reliable fault diagnosis function. Therefore, designers believe that this communication and control method will continue to be used for many years. The zero-level signal of the 4-20mA current loop function is represented by a current of 4mA, and the full-scale is represented by a current of 20mA, hence the name "4-20mA current loop". Current output below 4mA is used for fault diagnosis, and current above 20mA is considered to be an over-range output. The specific meanings of different current outputs are shown in the following table.

Current Output mA	Status
0.0	Unit Fault
0.8	Unit Warm-up
1.2	Zero Drift Fault
1.6	Calibration Fault
2.0	Unit Production (Jump)
2.2 Unit Zero Adjustment 4.0-20.0 Normal Measurement Mode 4.0 Zero Gas Level 5.6 10% of Full Scale 8.0	25% of full scale
12.0	50% of full scale
16.0	75% of full scale
20.0	Full scale
>20.0	Over range

The figure below is a typical industrial control system structure diagram. The CPU receives the sensor signal using the 4-20mA current loop communication through the input module. After certain calculations, the control output module controls the actuator. Here, the communication between the gas sensor and the CPU is realized by the 4-20mA current loop. The implementation of this part is shown in the figure below. The entire communication loop consists of several parts, including gas sensors, transmitters, receivers, twisted pairs, and power supplies. The receiver can be any device that can detect the voltage across the current-sense resistor, and the output of the device can be either analog or digital. The current-sense resistor in the figure generally has a resistance between 100-500Ω. It is generally considered that 250Ω is the "standard" resistance value here, and 125Ω is also a relatively common resistance value. The resistance value of the current-sense resistor depends on the needs of the receiver and the circuit design, and is not unique. The transmitter is used to convert the sensor output signal into a 4-20mA current signal. Its circuit implementation forms are diverse, that is, it can be a current loop built by discrete devices or a dedicated integrated current loop chip, as long as the output current of the current loop can be proportional to the sensor output signal. When the sensor output signal is zero or the sensor output signal is at the lower limit of the output range, the transmitter output current is 4mA; when the sensor output signal is at the upper limit of the output range, the transmitter output current is 20mA. It should be noted that although the 4-20mA current loop is widely used in various occasions due to its simple structure and high reliability, it is generally considered that 4-20mA is not suitable for occasions requiring high-speed communication. The high resistance characteristics of the current source, the parasitic parameters of the transmission cable and other factors limit the frequency response of the entire loop. 4-20mA current loop circuit implementation As mentioned earlier, the 4-20mA current loop can be implemented in various forms. The following introduces several commonly used circuit implementations. The figure below shows the simplest implementation of a voltage-controlled current source. It is composed of an operational amplifier U1A, a current-expanding MOSFET Q1, and a current-sensing resistor R6, forming a typical current feedback circuit structure. The resistor R6 and the forward input voltage of the operational amplifier determine the size of the circuit output current, that is, I0=Vi+/R6. Other components in the circuit play an auxiliary role and are used to improve circuit performance. For example, the voltage regulator diode further steps down the 12V input voltage, and resistors R2 and R3 are used to limit the voltage across VR1. D1 provides a negative voltage to the operational amplifier to ensure that its minimum output can drop to 0V. D3 and C2 are used to protect the MOSFET, and C2 can also prevent the MOSFET from oscillating. Careful friends may have discovered that there is a problem with the above circuit. The current output of the transmitter is at the high end, that is, the transmitter and the receiver do not share a common ground. This may be a problem in some occasions. The circuit in the figure below changes the current source output to a low-end output by adding a current conversion circuit. In addition to the above two voltage-controlled current source circuits composed of discrete devices, major IC manufacturers have also launched their own 4-20mA integrated solutions. Such as ADI's AD420/421 series, AD5410/5420 series, TI's XTR110 series, etc. The following figure shows the internal structure of the AD5410/5420 series. Its implementation principle is similar to the circuit composed of discrete devices, but because the single chip integrates DAC and voltage-controlled current source circuit, it provides a convenient single-chip solution for the 4-20mA current loop, greatly simplifies the design complexity, and improves the reliability of the circuit. The figure below shows the internal structure block diagram of TI's XTR111. This circuit is very suitable for directly connecting to the sensor output to form a standard 4-20mA transmitter.

han0097

Is there any good solution for 4-20ma reception?

jiesuheshang

How to use multiple XTR117? I'm confused recently

northwester

Very professional article.

bobde163

I looked at these integrated chips and found them to be very expensive. Circuits built with discrete devices are cheaper.