What are the reasons for using 4-20mA current to transmit analog quantity?

The reason for using the current signal is that it is not easy to have interfered, because the amplitude of the noise voltage in the industrial field may reach several volts, but the power of the noise is very weak, so the noise current is usually less than the nA level, so the error caused by the 4-20mA transmission is very large. small;

The internal resistance of the current source tends to infinity, and the resistance of the wire in series in the loop does not affect the accuracy, so hundreds of meters can be transmitted on the ordinary twisted pair;

Due to the large internal resistance and constant current output of the current source, we only need to place a 250-ohm resistor to the ground at the receiving end to obtain a voltage of 0-5V. The advantage of a low input impedance receiver is nA-level input current noise. Only very weak voltage noise is generated.

The upper limit is 20mA because of the explosion-proof requirement: the spark energy caused by the on-off of the 20mA current is not enough to ignite the gas. The reason why the lower limit is not set to 0mA is to be able to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is broken due to a fault, the loop current drops to 0. 2mA is often used as the disconnection alarm value. The current transmitter converts the physical quantity into a 4-20mA current output, which must be powered by an external power source.

The most typical is that the transmitter needs two power cords, plus two current output lines, a total of 4 wires must be connected, which is called a four-wire transmitter. Of course, the current output can share a line with the power supply (common VCC or GND), which can save one line, so now the four-wire transmitter is basically called a three-wire transmitter. In fact, you may have noticed that the 4-20mA current itself can supply power to the transmitter. The transmitter is equivalent to a special load in the circuit. This type of transmitter only needs two external wires, so it is called two-wire Transmitter.

Generally need to design a VI converter, input 0-3.3v, output 4mA-20mA, can use op-amp LM358, power supply +12v.

Let's take a systematic look at why analog devices prefer to use 4~20mA to transmit signals~

The 4-20mA. DC (1-5V.DC) signal system is an analog signal transmission standard adopted by the International Electrotechnical Commission (IEC) process control system. Our country also adopts this international standard signal system. The instrument uses 4-20mA.DC for signal transmission and 1-5V.DC for receiving a signal, that is, the signal system adopts current transmission and voltage reception.

The signal current of general instruments is 4-20mA, which means that the minimum current is 4mA and the maximum current is 20mA. When transmitting signals, because there are resistances on the wires, if voltage transmission is used, a certain voltage drop will be generated in the wires, and the signal at the receiving end will have a certain error, so current signals are generally used as the standard transmission of the transmitter.

1. What is 4~20mA.DC (1~5V.DC) signal system?

The 4~20mA.DC (1~5V.DC) signal system is the International Electrotechnical Commission (IEC): the analog signal standard for process control systems. Our country has adopted this international standard signal system from the DDZ-electric instrument. The instrument transmission signal adopts 4-20mA.DC and the contact signal adopts 1~5V.DC, that is, the signal system adopts current transmission and voltage reception.

Working principle of 4~20mA current loop:

2. What are the advantages of 4~20mA.DC (1~5V.DC) signal system?

The field instrument can realize a two-wire system. The so-called two-wire system means that the power supply and the load are connected in series and have a common point. However, the signal communication and power supply between the field transmitter and the control room instrument only use two wires. Because the starting current of the signal is 4mA.DC provides static working currently for the transmitter. At the same time, the electrical zero points of the instrument are 4mA.DC, which does not coincide with the mechanical zero points. This "live zero point" is helpful for identifying faults such as power failure and disconnection. . Moreover, the two-wire system is also convenient to use the safety barrier, which is beneficial to the safety and explosion-proof.

The instrument in the control room adopts voltage parallel signal transmission, and there is a common terminal between the instruments belonging to the same control system, which is convenient for the use of detection instruments, adjustment instruments, computers, and alarm devices, and is convenient for wiring.

The reason why the communication signal between the field instrument and the control room instrument is 4~20mA.DC is: because the distance between the field and the control room is longer, the resistance of the connecting wire is larger, if the voltage signal is used for remote transmission, it is better than the wire The voltage divider between the resistance and the input resistance of the receiving instrument will produce a large error, and a constant current source signal is used as a remote transmission. As long as the transmission loop does not branch, the current in the loop will not change with the length of the wire, thus ensuring The accuracy of the transmission.

The reason for using 1~5V.DC as the communication signal between the instruments in the control room is: in order to facilitate multiple instruments to receive the same signal together and to facilitate the wiring and the formation of various complex control systems. If a current source is used as a contact signal when multiple instruments receive the same signal together, their input resistances must be connected in series, which will cause the maximum load resistance to exceed the load capacity of the transmitting instrument, and the signal negative terminal potential of each receiving instrument Each is different, it will introduce interference, and it is impossible to achieve a single centralized power supply.

Using voltage source signal connection, the current signal used for communication with the field instrument must be converted into a voltage signal. The simplest way is to connect a 250Ω standard resistor in series with the current transmission loop to convert 4-20mA.DC to 1~5V. .DC, usually by the distributor to complete this task.

3. Why the transmitter chooses 4~20mA.DC as the transmission signal?

1. First of all, from the safety considerations of field applications

The focus of safety is to consider explosion-proof and safe spark-type instruments and to control the energy of the instrument as a prerequisite to reduce the static and dynamic power consumption to maintain the normal operation of the instrument to a minimum. For transmitters that output 4-20mA.DC standard signals, their power supply voltage is usually 24V.DC. The main reason for using DC voltage is that large-capacity capacitors and inductors are not needed, and only the transmitter and control room instruments are needed. The distributed capacitance and inductance of the connecting wire, for example, the distributed capacitance of a 2mm2 wire is about 0.05μ/km; the inductance for a single wire is about 0.4mH/km; it is much lower than the value of detonating hydrogen, which is obviously very beneficial for explosion protection.

2. The current source for signal transmission is better than the voltage source

Because the distance between the field and the control room is relatively long, when the resistance of the connecting wire is large, if the voltage source signal is used to transmit remotely, due to the partial pressure of the wire resistance and the input resistance of the receiving instrument, a large error will occur. The source signal is used as a remote transmission. As long as the transmission loop does not branch, the current in the loop will not change with the length of the wire, thus ensuring the accuracy of transmission.

3. The reason why the maximum signal current is 20mA

The choice of the maximum current of 20mA is based on safety, practicality, power consumption, and cost considerations. Safety spark meters can only use low voltage and low current. 4-20mA current and 24V.DC is also safe for flammable hydrogen. The detonation current for 24V.DC hydrogen is 200mA, which is far above 20mA. In addition, comprehensive consideration should be given to production. Factors such as the connection distance between field instruments and the load they carry; there are also power consumption and cost issues, requirements for electronic components, and power supply requirements.

4. The reason why the signal starting current is 4mA

Transmitters with an output of 4-20mA are mostly two-wire systems. The two-wire system means that the power supply and the load are connected in series, and there is a common point. However, the signal communication and power supply between the field transmitter and the control room instrument only use two Wire. Why is the starting signal not 0mA? This is based on two points: First, the transmitter circuit will not work without a static working current. The signal starting current is 4mA.DC, which does not coincide with the mechanical zero points. This "live zero point" is conducive to identifying faults such as power failure and disconnection.

4. The origin of the 4-20mA sensor?

The reason for adopting the current signal is that it is not easy to have interfered, and the internal resistance of the current source is infinite. The resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted-pair wire.

The reason for using the current signal is that it is not easy to have interfered, because the amplitude of the noise voltage in the industrial field may reach several volts, but the power of the noise is very weak, so the noise current is usually less than the nA level, so the error caused by the 4-20mA transmission is very large. small;

The internal resistance of the current source tends to infinity, and the resistance of the wire in series in the loop does not affect the accuracy, so hundreds of meters can be transmitted on the ordinary twisted pair;

Due to the large internal resistance and constant current output of the current source, we only need to place a 250-ohm resistor to the ground at the receiving end to obtain a voltage of 0-5V. The advantage of a low input impedance receiver is nA-level input current noise. Only very weak voltage noise is generated.

The upper limit is 20mA because of the explosion-proof requirement: the spark energy caused by the on-off of the 20mA current is not enough to ignite the gas. The reason why the lower limit is not set to 0mA is to be able to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is broken due to a fault, the loop current drops to 0. 2mA is often used as the disconnection alarm value.

The current transmitter converts the physical quantity into a 4-20mA current output, which must be powered by an external power source. The most typical is that the transmitter needs two power cords, plus two current output lines, a total of 4 wires must be connected, which is called a four-wire transmitter. Of course, the current output can share VCC or GND with the same line as the power supply, which can save one line, which is called a three-wire transmitter.

In fact, you may have noticed that the 4-20mA current itself can power the transmitter. The transmitter is equivalent to a special load in the circuit. The special feature is that the current consumption of the transmitter varies between 4 and 20 mA according to the sensor output. The display instrument only needs to be strung in the circuit. This kind of transmitter only needs 2 external wires, so it is called a two-wire transmitter. The standard lower limit of the industrial current loop is 4mA, so as long as it is within the range, the transmitter has at least 4mA power supply.

Therefore, the 4-20mA signal output is generally not susceptible to interference and is safe and reliable. Therefore, the two-wire 4-20mA power output signal is commonly used in the industry. But in order to better process the sensor signal, there are more other forms of output signal: 3.33MV/V; 2MV/V; 0-5V; 0-10V, etc.

Attached is a simple circuit diagram of a 4-20mA to voltage signal: 

This picture uses a 250-ohm resistor to convert a 4-20mA current signal into a 1 to 5V voltage signal, and then uses an RC filter plus a diode (forgive me for the bad analog circuit, I dont know what it means). The AD conversion pin of the microcontroller.