First of all, let's look at their definitions
Two-wire system: two wires and the transmission of power and signal transmission, that is, the sensor output load and power are connected in series, the power is introduced from the outside, and the load in series to drive the load.
Three-wire system: three-wire sensor is the positive end of the power supply and signal output is separated from the positive end, but they share a COM terminal.
Four-wire system: Two wires for power and two wires for signal. The power and signal are working separately.
The designation of several-wire system came into existence after the birth of two-wire transmitter. This is the result of the widespread use of electronic amplifiers in the instrumentation, the essence of amplification is an energy conversion process, which can not be separated from the power supply. Therefore, the first to appear is a four-wire transmitter; that is, two wires are responsible for the supply of power, and the other two wires are responsible for the output of the converted and amplified signals (such as voltage, current, etc.). Currently, however, many transmitters use a two-wire system. Below, let's take a look at what are the differences between the different wire system transmitters?
Differences between the different wire system transmitters
I. Two-wire system
To realize a two-wire transmitter, the following conditions must be met simultaneously:
1. V≤Emin-ImaxRLmax
The voltage V at the output of the transmitter is equal to the specified low supply voltage minus the voltage drop of the current across the load resistance and the transmission lead resistance.
2. I≤Imin
The normal operating current I of the transmitter must be less than or equal to the output current of the transmitter.
3. P＜Imin(Emin-IminRLmax)
The small power consumption P of the transmitter must not exceed the above equation, usually <90mW.
Where: Emin = low supply voltage, for most instruments Emin = 24 (1-5%) = 22.8V, 5% for the 24V power supply allows the amount of negative changes;
Imax=20mA;
Imin=4mA;
RLmax=250Ω + transmission lead resistance.
If the pressure transmitter is designed to meet the above three conditions, the two-wire transmission can be realized. The so-called two-wire system that the power supply, load in series together, there is a common point, and the field transmitter and control room instrumentation between the signal contact and power supply with only two wires, these two wires are both power lines and signal lines. Two-wire transmitter as a result of the signal starting current of 4mA DC, for the transmitter to provide a static operating current, while the instrumentation electrical zero point of 4mA DC, not coincide with the mechanical zero point, this "live zero" is conducive to the identification of power outages and disconnections and other faults. Moreover, the two-wire system also facilitates the use of safety barriers, which is conducive to safety and explosion-proof.
II. three-wire system
The wiring diagram of the three-wire converter is shown in Fig. 2. The current output system has a power supply line to provide power to the transformer in addition to the two current transmission lines. Three-wire system is the positive end of the power supply with a line, the positive end of the signal output with a line, the negative end of the power supply and the negative end of the signal share a line. Most of its power supply for DC 24V, the output signal DC4-20mA, load resistance of 250Ω or DC0-10mA, load resistance of 0-1.5kΩ; some have mA and mV signals, but the load resistance or input resistance, due to the different forms of output circuits and different values.
Due to the popularity and application of DC4-20mA, DC 1-5V signaling system, in order to facilitate the connection in the control system application, it is required that the signaling system is unified, and for this reason, it is required that some non-electrical unit combinations of instrumentation, such as on-line analysis, mechanical quantities, and electric quantity, etc., can be used with an output of DC4-20mA signaling system, but due to the complexity of the conversion circuitry and the large power consumption, it is difficult to meet all of the above three conditions and cannot achieve the two-wire system. However, due to the complexity of the conversion circuit, power consumption and other reasons, it is difficult to meet all of the above three conditions and can not do two-wire system, you can only use the external power supply method to do the output for the four-wire DC4-20mA transmitter. Most of the four-wire transmitters are powered by AC 220V, and some are powered by DC24V. Output signals are DC 4-20mA, load resistance of 250Ω, or DC 0-10mA, load resistance of 0-1.5kΩ; there are mA and mV signals, but the load resistance or input resistance, due to the different forms of output circuits and the value is different.
Input to the receiving instrument is a current signal, such as resistance RL parallel access, it is the receiving voltage signal. Due to the different working principles and structures of various transmitters, different products have emerged, which also determines the two-wire, three-wire, four-wire wiring form of the transmitter.
III, four-wire system
Four-wire transmitter as shown in Figure 3, most of its power supply for 220V AC, there are also power supply for 24V DC. Output signal 4-20mA DC, load resistance of 250Ω, or 0-10mA DC, load resistance of 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance, due to the different output circuit forms and different values.
In the above three diagrams, the input to the receiving instrument is a current signal, such as the parallel access to the resistor RL, it is the voltage signal received.
As can be seen from the above description, due to the working principle and structure of the various transmitters are different, resulting in the emergence of different products, which also determines the transmitter of the two-wire, three-wire, four-wire wiring form. For the user, the selection should be based on the actual situation of the unit, such as the unity of the signal system, explosion-proof requirements, the requirements of the receiving equipment, investment and other issues to consider the choice.
It should be pointed out that the three-wire and four-wire transmitter output 4-20mA DC signal, due to its output circuit principle and structure and the two-wire system is not the same, so in the application of the negative end of its output can be connected to the negative line of the 24V power supply? Can it be common ground? This is to pay attention to, if necessary, isolation measures can be taken, such as the use of distributors, safety barriers, etc., in order to and other instruments and common power, common ground and to avoid the generation of additional interference.