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Introduction Of Output Signal - 4-20ma

Author:Vacorda    Source:www.vacorda.com    Date: 2020-08-31 17:17:04

Introduction:

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 signal, that is, a signal system using current transmission and voltage reception.

 output signal 4-20ma

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 the current signal is generally used as the standard transmission of the transmitter.

 

1. Working Principle:

In the industrial field, using an instrumentation amplifier to complete signal conditioning and long-line transmission will cause the following problems: first, because the transmitted signal is a voltage signal, the transmission line will be interfered by noise; second, the distributed resistance of the transmission line will produce Voltage drop; Third, how to provide the working voltage of the instrumentation amplifier in the field is also a problem.

  figure 1

In order to solve the above problems and avoid the influence of related noise, we use current to transmit signals, because current is not sensitive to noise. The 4-20mA current loop uses 4mA to represent the zero signal, and 20mA to represent the full scale of the signal, and signals lower than 4mA and higher than 20mA are used for various fault alarms.

 

2. What are the advantages of the 420mA.DC (15V.DC) signal system?

 output signal 4-20ma

The field instrument can realize the 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, it provides static working current for the transmitter. At the same time, the electrical zero point of the instrument is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero point" is helpful for identifying faults such as power failure and disconnection. . Moreover, the two-wire system also facilitates the use of safety barriers, which is conducive to safe explosion protection.

 

The control room instruments adopt voltage parallel signal transmission, and there are common terminals between the instruments belonging to the same control system, which is convenient for detecting instruments, regulating instruments, computers, and alarm devices, and convenient for wiring.

  figure 2

 

The communication signal between the field instrument and the control room instrument adopts 420mA.DC. The reason is: because the distance between the field and the control room is relatively long, the resistance of the connecting wire is large. If the voltage signal is used to transmit remotely, it is better than the voltage division of the wire resistance and the input resistance of the receiving instrument, which will cause a larger error. Using constant current source signal as 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, thereby ensuring the accuracy of transmission.

 

The communication signal between the instruments in the control room adopts 15V. The reason 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, will introduce interference, and cannot achieve a single centralized power supply.

Using voltage source signal connection, the current signal used to communicate 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 420mA.DC as the transmission signal?

 output signal 4-20ma

1. The first is to consider the safety of field application

 

The focus of safety is to consider the explosion-proof safety spark-type instrument, and to control the energy of the instrument as the premise, 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 of.

 

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 for remote transmission, due to the voltage division 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 the considerations of safety, practicality, power consumption and cost. Safety spark meters can only use low voltage and low current. 4-20mA current and 24V.DC are also safe for flammable hydrogen. The detonation current for 24V.DC hydrogen is 200mA, which is far above 20mA. In addition, comprehensive consideration must 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 have 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 static working current. The signal starting current is 4mA.DC, which does not coincide with the mechanical zero point. This "live zero point" is conducive to identifying faults such as power failure and disconnection.

【Figure 3

 

 

Appendix: Simple circuit diagram of 420mA to voltage signal

 output signal 4-20ma

【Figure 4】

This figure 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 and a diode to connect to the AD conversion pin of the micro controller.

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