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Why do transmitters and sensors prefer 4-20ma signals?

  pubdate :2018-08-24 11:26  source:安盈   hot:

4-20mA. DC(1-5V.DC)Singel system is International Electrotechnical Commission analog signal transmission standard used in  process controll system. China also used the international standard signal system, the metter transmission signal used 4-20mA.DC, receiving signal adopts1-5.VDC. ,that is adopts the signal system of current transmission and voltage reception.

The signal current of general instrument is 4-20mA, refers to the minimum current of 4mA and  the maximum current of 20mA. When transmission signal, when transmission signal, because has resistance wire, if in the with the voltage transmission will produce a certain pressure drop, the signal at the receiving end will produce certain error, so the use of current signal as a general standard transmission transmitter.  

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

4~20mA.DC(1~5V.DC)signal system is International Electrotechnical Commission (IEC):analog signal standard for process control systems . In our country from DDZ-III electric instrument began to adopt the international standards signal system,the instrument transmission siganl adopts 4-20mA.DC, the contact signal adopts 1-5V.DC, that is the signalsystem of current transmission and voltage reception.

The Working Principle Of 4~20mA Current Loop:

In the industrial field, the use of an instrument amplifier to complete signal conditioning and long line transmission will cause the following problem:First,as the transimisson signal is the voltage signal, the transimission line will disturbed by noise;Second,the disturbed resistance of transimission line will produces voltage drop;Third,how to provide the working voltage of the instrument amplifier in the field is also a problem.


In order to solve the above problem and avoid the influence of related noises, we use current to transmit the signal, because current is not sensitive to noise. The current loop of 4~20mA represents zero signal with 4mA, full scale of signal with 20mA, and signals below 4mA and above 20mA are used to alarm various faults.

2、What's the advantage of 4~20mA.DC signal system?

Field instrument can realize two wire system, the so-called two wire system in series with the power supply, load, there is a common point, and the transmitter and the control roominstrumentation signal connection and power supply before using only two wires. Because of the origin current of signal is 4mA.DC, provides a static working current transducer, instrument electric zero at the same time for the 4mA.DC, not with mechanical zero overlap, this kinf of "zero" live power is helpful to identify and bolt failure. And the two wire system is also onvenient to use the safety grid, conducive to the safety explosion proof.

The instrument of controller room is adopt voltage parallel signal transmission, there is a common end between the instruments belong to the same control system, which is convenientfor test instruments, adjust instruments, computers, alarm devices and wiring.

The reason for using 4~20mA.DC communication signal between the field instument and the control room instrument is that:Because of the distance between the field and the control room, the resistance of the connecting wire is large, if the voltage signal is transmitted far, it is better than the partial voltage of the wire resistance of the receiving instrument, it will be a large error,the constant current source signal is used as remote transmission, as long as there is no branch in the transmission circuit, the current in the circuit will not change with the length of the wire,thus ensuring the accuracy of transmission.

The reason for using 1~5V.DC communication signal between instrument in the controller room  is that:In order to facilitate multiple instruments to receive the same signal together, and conducive to wiring and forming a varity of complex control system. If the current source is used as the contact signal, when mutiple instruments receive the same signal together, their input resistors must be connected in series, this makes the maximum load resistance exceed the load capacity of the negative, and the negative end potential of each receiving instrument is different,interference will be introduced and a singel centralized power supply cannot be achieved.

Adopt the voltage source signal connection,the current signal used in connection with the filed instrument must be converted to a boltage signal,the easiest way it that:Series in the current transfer circuit a 250Ω standard resistor, the 4~20mA.DC converted into 1~5V.DC, usually by the distributor to complete the task.

3、 Why does the transmitter choose 4~20mA.DC as the transmission signal?

1) First consider from safety of filed application

The safety emphasis is consider on explosion-proof safety spark type instrument, and on the premise of controlling the instrument energy, the static and dynamic power consumptionof the instrument is reduced to the minimum. A transmitter that Outputs 4~20mA.DC standard signals, whose power voltage is usually 24V.DC, the main reason for using DC voltage is that the distribution capacitance and inductance of the conductors connected by the transmitter and the control room instruments can be considered without the need of capacitors and inductors with large capacity ,for example, the conductor of 2mm2 has a distribution capacitance pf 0.05c/km;the inductance for a single line is about 0.4mh/km;It's much lower than detonating hydrogen, which is obviously very benefical for explosion protection.

2) A current source is superior to a voltage source for transmitting signals

Because of the distance is far between the field and the control room, when the resistance of the connecting wire is large, if the voltage source is transmitted far away, there will be a large error due to the partial voltage between the wire resistance and the input resistance of the receiving instrument, if the current source signal is used for remote transmission, as long as there is no branch in the transmission loop, the current in the loop will not change with the length of the wire, thus ensuring the transmission accuracy.

3) The reason of signal maximum current is 20mA  

The choice of maximum current of maximum current 20mA is based on safety, utility, power consumption and cost. The safety spark meter can only use low voltage、low current, 4~20mA current and 24V.DC it is safe for inflammable hydrogen, the detonation current of 24V.DC hydrogen is 200mA, well over 20mA, in addition, it is necessary to take into account the connection distance between theinstruments on the production site, the load and other factors;there are also power and cost issues, requirement for electronic components and power supply requirements.

4) The reason of signal starting current is chosen 4mA

In 4~20mA output transducer in the majority with two wire system, two wire system in series with the power supply, load, there is a common point, and the scene between the transimitter and the control room instrumentation signal connection and power supply using only two wires. Why not the signal staring is 0mA? This is based on two points: First, thetransmitter circuit will not work without static working current, the signal starting current is 4mA, do not coincide with mechanical zero, this "live zero" helps to identify failures such as power outages and power outages.

4、4~20mA sensor original?

The reason why the current signal is that is not easy to be interfered, and the internal resistance of the current source is infinite, the wire resistance series has no influence on the accuracy of the circuit, and it can transmit hundreds of meters on the common twisted wire.

Adopt the cause of the current signal is not susceptible to interference, because the scene of the industrial noise voltage amplitude could reach several V, but the noise power is very weak. so the level of noise current is usually less than nA, so bring 4~20mA transmission ereor is very small;The internal resistance of the current source tends to be infinite, and thewire resistance series does not affect the precision in the loop, so it can transmit hundreds of the meters on the common twisted wire;Due to the large current source internal resistance and constant current output, we need to place on the receiving end a resistance to the ground of 250 ohms can get 0 to 5V voltage, low input impedance of the receiver are the input current noise level nA very weak voltage noise only.

The upper limit of 20mA is for explosion protection:The spark energy caused by the 20mA power failure is not enough to ignite the gas. The reason the 0mA limit is not taken is to detect the broken line:when the transmission line breaks down due to failure, the loop current drops to 0. Usually take 2mA as the line breaking alarm value. Current type transmitter converts the physical quantity into 4~20mA current output, which must be supplied by an external power source. Typically, the transmitter requires two power codes, plus two current output lines, to connect a total of four wires, called a four-wire transmitter. Of coursr, the current output can share a line with the power supply to share VCC or GND, saving a line called a three-wire transmitter. As you may have noticed, the 4~20mA current itself can power the transmitter. The transmitter is equivalent to a special load in the circuit, the special point is that the power consumption current of the transmitter varies between 4 and 20mA according to the output of the sensor. The display instrument only needs to be strung in the circuit. The transmitter is called a two-wire transmitter because it only needs two externals wires. The lower limit of the industrial current loop standard is 4mA, so the transmitter will have at least 4mA power supply as long as it is within the measuring range.

Therefore、the signal output of 4-20mA is generally not easy to be interfered and safe and reliable, so the power output signal of 4~20mA of second-line system are commonly used in industry. However, in order to better process the signal of the sensor, there are more other kinds of output signals at present: 3.33MV/V; 2MV/V; 0-5V; 0-10V, etc.

A simple circuit diagram of a 4~20mA converter signal is also attached:


This picture using a 250 ohm resistance to 4~20mA current signal is converted into 1~5V voltage signal, and then use a RC filter to add a diode to MCU AD conversion pin.

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