Two-wire, three-wire, four-wire system refers to a variety of output is an analog signal transmitter DC current difference between its working principle and structural, not only refers to the wiring in the form of the transmitter. Otherwise thermocouple temperature measurement with millivolt meter is called a two-wire originator!

     Several title-wire, two-wire transmitter is some after birth. This is the result of electronic instrumentation amplifier widely used, is the essence of an energy amplification conversion process, which is inseparable from power. Therefore, it appears first is a four-wire transmitter; that is responsible for two lines of power supply, the other two lines are responsible for converting the amplified output signal (such as voltage, current, etc.). DDZ-Ⅱ electric unit combination meter appears, powering 220V.AC, the output signal is 0--10mA.DC four-wire transmitter has been widely used at present in some factories may also see its shadow.

     Seventies China began production DDZ-Ⅲ electric unit combination meter, and the International Electrotechnical Commission (IEC) for: process control systems using analog signal standards. Namely, instrument transmission signal using 4-20mA.DC, contact signals using 1-5V.DC, which uses the current transmission voltage signal received by the system. Use 4-20mA.DC signal, field instruments can achieve two-wire system. But limited conditions, when only two-wire pressure, the differential pressure transmitter with a temperature transmitter, and so still use four-wire system. Now the domestic two-wire transmitter product range has greatly expanded, more and more applications. At the same time coming from abroad is also a two-wire transmitter majority.

    Because to achieve the two-wire transmitter must meet the following conditions:

    1. V≤Emin-ImaxRLmax

    Minimum supply voltage of the transmitter output terminal voltage V is equal to minus a predetermined voltage drop in the load current and resistance transmission line resistance.

    2. I≤Imin

    Normal operating current transmitter I must be less than or equal to the transmitter output current.

    3. P <Imin (Emin-IminRLmax)

    Transmitter minimum power consumption P can not exceed the above formula, usually <90mW.

Where: Emin = minimum supply voltage for most instrumentation that Emin = 24 (1-5%) = 22.8V, 5% for the 24V power supply allows negative variation;

     Imax = 20mA;

     Imin = 4mA;

     RLmax = 250Ω + transmission line resistance.

    If the transmitter is designed to meet the above-mentioned three conditions, it can achieve two-wire transmission. That so-called two-wire power supply, in series with the load, there is a common point, and the signal contact with the control room and the field transmitter power meter between the only two wires, two wires both power line and signal line. Since the two-wire transmitter signal starting current 4mA.DC, the transmitter provides a quiescent current, while zero for the electric meter 4mA.DC, does not coincide with the mechanical zero, this "living zero" in favor of recognition off The isoelectric and disconnection fault. But also easy to use two-wire barrier, which will help secure explosion.

     Two-wire transmitter, the power supply is 24V.DC, the output signal is 4-20mA.DC, load resistance of 250Ω, the lowest negative 24V power supply line potential, which is the signal common line, but also in the smart transmitter loading HART protocol on 4-20mA.DC signal FSK keying signal.

      Due to the popularization and application 4-20mA.DC (1-5V.DC) signal system, the control system in order to facilitate the connection, requires a unified signal system, for which the requirements of non-electric unit combination of instruments, such as online analysis mechanical volume, power and other instruments, can be employed as 4-20mA.DC output signal system, but because of its conversion circuit complexity, power consumption and other reasons, it is difficult to meet all of the above three conditions, but can not do two-wire, We can only use an external power supply output 4-20mA.DC way to do a four-wire transmitter up.

     Four-wire transmitter, the power supply mostly 220V.AC, but also the power supply for the 24V.DC. Output signals 4-20mA.DC, load resistance of 250Ω, or 0-10mA.DC, the load resistance is 0-1.5KΩ; some have mA and mV signals, but the load resistance or input resistance, due to the output circuit in the form of different The values vary.

     Some Factory In order to reduce the volume and weight of the transmitter, and can improve performance, simplify wiring, and the transmitter is powered by 220V.AC to low voltage DC power supply, such as power supply box from 24V.DC access, since the low voltage power supply to the common negative line to create the conditions so that there will be a three-wire transmitter products.

     The so-called three-wire is the positive power supply terminal with a line, a positive signal output terminal with a cable, power supply negative terminal and a negative terminal share a signal line. Its power supply mostly 24V.DC, output signals 4-20mA.DC, load resistance of 250Ω or 0-10mA.DC, the load resistance is 0-1.5KΩ; some have mA and mV signals, but the load resistance or input resistance, due to different forms of output circuit and the values are different.

     Above, the input current signal is received the instrument, such as a resistor RL parallel connection, then the voltage signal is received.

     As can be seen from the above described, due to the different working principle and structure of the various transmitters, which appeared in different products, it will determine the two-wire transmitter, three-wire or four-wire connection form. For users, the selection should be based on the actual situation of the unit, such as a unified signal system, explosion-proof requirements, the requirements of the receiving device, investment and other issues to consider options.

     It is noted that the signal 4-20mA.DC three-wire and four-wire transmitter output, due to its principle and structure of the output circuit of two-wire system is not the same, so in the application and its ability to output negative terminal negative phase 24V power line? can common ground? it is to be noted, isolation measures can be taken when necessary, such as the use with electrical appliances, and other barriers to electricity and other common instruments, common ground and avoid additional interference .

    Finally, talk about the two-wire to a four-wire, four-wire two-wire system reform issues.

    From the foregoing various wire transmitter can exist, and that there is always a reason to exist, otherwise there would be so much wire, the wire by the user to change it is difficult, moreover also practical significance little.

     If you want the transmission signal is 0-10mA.DC four-wire transmitter to two-wire system, the first problem is that its initial current is zero at zero current state, the transmitter electronics amplifier is unable to establish a working point, so it would be difficult to work. If the DC power supply, ensure that the instrument and the original constant current characteristic, dynamic feedback resistor when the transmitter is in the load resistance is 0-1.5KΩ when its series about 2KΩ, when the output is 10mA, the two parts of the voltage drop will be greater than 24V, that is to say with 24V.DC power load of 0-1.5KΩ time, to ensure a constant current characteristic is impossible, not to mention with a two-wire transmission.

     Factory 70 years had done to 0-10mA.DC four-wire transmitter to work two-wire transmitter, which would be: the original transmitter circuit is improved and the supply voltage increased to 48V.DC, but the starting current transmitter still can not be zero, for use to offset the initial negative current output 4mA load resistor current. But these products are not able to get the promotion and application.

     If you want two-wire to four-wire system, there is no need, moreover it is a technological step backward.

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