The signal current of general instrumentation is 4-20mA, meaning a minimum current of 4mA and a maximum current of 20mA. When transmitting the signal, because there is resistance in the wire, if the voltage transmission will produce a certain voltage drop in the wire, then the signal at the receiving end will produce a certain error, so generally use the current signal as the standard transmission of the transmitter.
I. What is the 4~20mA.DC (1~5V.DC) signal system?
DC (1~5V.DC) signal system is the International Electrotechnical Commission (IEC): the analog signal standard for process control systems. This international standard signal system has been adopted in China since the DDZ-Ⅲ electric instrument, which uses 4~20mA DC for the instrument transmission signal and 1~5V DC for the contact signal, i.e. a signal system with current transmission and voltage reception.
4~20mA current loop working principle.
In industrial sites, the use of an instrumentation amplifier to complete the conditioning of the signal and to carry out long line transmission, will produce the following problems: first, because the transmitted signal is a voltage signal, the transmission line will be subject to noise interference; second, the distribution resistance of the transmission line will produce a voltage drop; third, how to provide the working voltage of the instrumentation amplifier in the field is also a problem.
In order to solve the above problems and avoid the influence of the relevant noise, we use current to transmit the signal, because the current is not sensitive to noise. 4 ~ 20mA current loop is to use 4mA to indicate the zero signal, with 20mA to indicate the full scale of the signal, and lower than 4mA higher than 20mA signal is used for various fault alarms.
Second, 4 ~ 20mA.DC (1 ~ 5V.DC) signal system advantages?
Field instrumentation can be achieved two-wire system, the so-called two-wire system that power, load in series, there is a common point, and the field transmitter and control room instrumentation before the signal contact and power supply with only two wires. DC, providing a static working current for the transmitter, while the instrument’s electrical zero point is 4mA.DC, not coinciding with the mechanical zero point, this “live zero point” is conducive to the identification of power failure and disconnection and other faults. The two-wire system also facilitates the use of safety grids for safety and explosion protection.
The control room instrumentation uses voltage parallel signal transmission, with a common terminal between the instruments belonging to the same control system, which facilitates the use of detection instruments, regulation instruments, computers and alarm devices, and facilitates wiring.
DC is the reason: because of the distance between the site and the control room, the resistance of the connecting wire is larger, if the voltage signal is used for remote transmission, it is better than the voltage division of the wire resistance and the input resistance of the receiving instrument, which will produce a larger error, while the constant current source signal is used as a remote transmission, as long as the transmission circuit does not branch, the current in the circuit will not with the wire length and change, thus ensuring the accuracy of the transmission.
The reason for using 1 to 5 V.DC for the contact signal between the control room instruments is that it is convenient for several instruments to receive the same signal together and facilitates wiring and the formation of various complex control systems. If the current source is used as the contact signal, when more than one instrument receives the same signal, their input resistance must be connected in series, which will make the maximum load resistance exceeds the load capacity of the transformer, and the negative potential of each receiving instrument varies, which will introduce interference, and cannot be done with a single centralised power supply.
The use of voltage source signal contact, contact with the site instrumentation with the current signal must be converted to a voltage signal, the simplest way is: in series with a 250Ω standard resistor in the current transmission circuit, the 4 ~ 20mA.DC converted to 1 ~ 5V.DC, usually by the distributor to complete this task.
Third, why transmitters choose 4 ~ 20mA.DC for the transmission signal?
1、First of all, the safety consideration from the field application
Safety focus is to consider the explosion-proof safety spark type instrument, and to control the instrument energy as a prerequisite to maintain the normal operation of the instrument static and dynamic power consumption is reduced to a minimum. DC standard signal output transmitter, the supply voltage is usually 24V.DC, the main reason for using DC voltage is that it can be used without large-capacity capacitors and inductors, you only need to consider the transmitter and control room instrumentation connected to the capacitance and inductance of the wire distribution, such as 2mm2 wire capacitance of the distribution of 0.05μ/km or so; for a single line of inductance of 0.4mH/km; much lower than the detonation of hydrogen km or so; much lower than the value of the detonation of hydrogen, which is obviously very favourable to explosion-proof.
- Current sources are better than voltage sources for signal transmission
Because the distance between the site and the control room is far, when the resistance of the connecting wire is large, if the voltage source signal is used for remote transmission, a large error will be generated due to the voltage division 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 the transmission circuit does not branch, the current in the circuit will not change with the length of the wire, thus ensuring the accuracy of the transmission.
3、The reason for choosing 20mA as the maximum signal current
The choice of maximum current 20mA is based on safety, practicality, power consumption and cost considerations. DC for flammable hydrogen is also safe, for 24V.DC hydrogen detonation current is 200mA, far above 20mA, in addition to comprehensive consideration of the connection distance between the instruments at the production site, the load carried and other factors; there is also the issue of power consumption and cost, the requirements for electronic components, the power supply power requirements and other factors.
- Reasons for choosing 4mA for the starting signal current
The output is 4 ~ 20mA transmitter to the majority of two-wire system, two-wire system that is the power supply, load in series, there is a common point, and the site transmitter and control room instrumentation between the signal contact and power supply with only two wires. This is based on two points: one is the transmitter circuit without static working current will not work, the signal starting current 4mA.DC, not coincident with the mechanical zero point, this “live zero point” is conducive to the identification of power failure and disconnection and other faults.
Fourth, the origin of the 4 ~ 20mA sensor?
The reason for using the current signal is not easy to interference, and the current source of infinite internal resistance, wire resistance in series in the circuit does not affect the accuracy, in the ordinary twisted pair can be transmitted hundreds of meters.
The reason for using a current signal is that it is not susceptible to interference, because the amplitude of the noise voltage at an industrial site may reach several V, but the noise power is very weak, so the noise current is usually less than the nA level and therefore brings very little error to the 4-20mA transmission; the internal resistance of the current source tends to be infinite, and the wire resistance in series in the loop does not affect the accuracy, so it can be transmitted over ordinary twisted-pair cables. Due to the large internal resistance of the current source and the constant current output, at the receiver end we only need to place a 250 ohm resistor to ground to obtain 0-5V. The advantage of a low input impedance receiver is that the nA level of input current noise produces only very weak voltage noise.
The upper limit of 20mA is taken because of the explosion protection requirement: the spark energy caused by the interruption of a 20mA current is not sufficient to ignite the gas. The lower limit is not taken as 0mA because it enables the detection of a broken line: in normal operation it will not fall below 4mA, and when the transmission line is broken due to a fault, the loop current drops to 0. 2mA is often taken as the alarm value for a broken line. Current type transmitters convert physical quantities into 4~20mA current outputs, which necessarily require an external power supply for them. Most typically, the transmitter requires two power supply lines, plus two current output lines, a total of four lines to be connected, called a four-wire transmitter. Of course, the current output can be connected to the power supply with a common wire common VCC or GND, saving a wire, called a 3-wire transmitter. In fact, you may have noticed that the 4-20mA current itself can supply the transmitter. The transmitter is equivalent to a special load in the circuit, the special thing is that the power consumption current of the transmitter varies between 4 to 20mA depending on the sensor output. The display instrument only needs to be connected in series in the circuit. This type of transmitter requires only 2 external wires and is therefore called a 2-wire transmitter. The lower limit of the industrial current loop standard is 4mA, so the transmitter has at least 4mA supply as long as it is within the range.
Therefore, the 4-20mA signal output is generally not susceptible to interference and is safe and reliable, so the two-wire 4-20mA power output signal is commonly used in industry. However, in order to better handle the sensor signal, there are now more other forms of output signals: 3.33MV/V; 2MV/V; 0-5V; 0-10V etc.
A simple circuit diagram of a 4 to 20mA to voltage signal is also attached.
This diagram uses a 250 ohm resistor to convert a 4 to 20mA current signal to a 1 to 5V voltage signal, and then uses an RC filter plus a diode (forgive me for being bad at analogue circuits, I don’t know what that means) to the AD conversion pins of the microcontroller.
