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Pressure transmitter two-wire, three-wire, four-wire wiring

Pressure transmitter two-wire, three-wire, four-wire wiring

2023-08-07

First, let's look at their definitions
Two-wire system: Two wires transmit power and transmit signals, that is, the load output by the sensor and the power supply are connected in series, and the power supply is introduced from the outside, and connected in series with the load to drive the load.

Three-wire system: The three-wire sensor is that the positive terminal of the power supply and the positive terminal of the signal output are separated, but they share a COM terminal.

Four-wire system: two wires for power and two wires for signal. Power and signal work separately.

The title of a few-wire system came into being only after the birth of the two-wire system transmitter. This is the result of the wide application of electronic amplifiers in instruments . The essence of amplification is an energy conversion process, which cannot be separated from power supply. Therefore, the four-wire transmitter appeared first; that is, two wires are responsible for the power supply, and the other two wires are responsible for outputting the converted and amplified signal (such as voltage , current , etc.). But at present, many transmitters use two-wire system. Next, let's take a look at the differences between different wire transmitters?

Differences between different wire transmitters

One, two-wire system

To realize a two-wire transmitter, the following conditions must be met at the same time:

1. V≤Emin-ImaxRLmax

The output voltage V of the transmitter is equal to the specified low power supply voltage minus the voltage drop of the current on the load resistance and transmission lead resistance.

2. I≤Imin

The normal working current I of the transmitter must be less than or equal to the output current of the transmitter.

3. P<Imin(Emin-IminRLmax)

The minimum power consumption P of the transmitter cannot exceed the above formula, usually <90mW.

In the formula: Emin=low power supply voltage, for most instruments Emin=24(1-5%)=22.8V, 5% is the allowable negative variation of 24V power supply;

Imax=20mA;

Imin=4mA;

RLmax=250Ω+transmission wire resistance.

If the pressure transmitter meets the above three conditions in design , two-wire transmission can be realized. The so-called two-wire system means that the power supply and the load are connected in series, and there is a common point, while the signal connection and power supply between the field transmitter and the control room wires , which are both power lines and signal lines. The two-wire transmitter provides a static working current for the transmitter because the starting point current of the signal is 4mA DC. 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 conducive to identifying power failure and Faults such as disconnection. Moreover, the two-wire system is also convenient to use a safety barrier, which is conducive to safety and explosion protection.


Figure 1 is the wiring diagram of the two-wire converter . The DC power supply U (24V) supplies power to the on-site converter through two wires, and these two wires are also transmission lines for the output signal (4-20mA). The output current signal needs to be converted into a voltage UL=IRL= (1～5) U through the standard resistance RL=250Ω and sent to the controller. The characteristic of this two-wire converter is that it is only used for detecting signals and has no control function.

Two, three-wire system

The wiring diagram of the three-wire converter is shown in Figure 2. In addition to the two current transmission lines, the current output system also has a power line to provide power to the transformer . The three-wire system means that one wire is used for the positive terminal of the power supply, one wire is used for the positive terminal of the signal output, and one wire is used for the negative terminal of the power supply and the negative terminal of the signal. Most of its power supply is DC 24V, the output signal is DC4-20mA, the load resistance is 250Ω or DC0-10mA, the load resistance is 0-1.5kΩ; some also have mA and mV signals, but the load resistance or input resistance depends on the output circuit Different forms have different values.

Due to the popularization and application of DC4-20mA and DC 1-5V signal systems, in order to facilitate connection in the application of control systems, the unification of signal systems is required. For this reason, some non-electric unit combination instruments are required, such as on-line analysis, mechanical quantities, Electricity and other instruments can adopt the output of DC4-20mA signal system, but due to the complexity of the conversion circuit and large power consumption, it is difficult to meet all the above three conditions and cannot achieve the two-wire system, so the external power supply can only be used. The method is to make a four-wire transmitter with an output of DC4-20mA. Most of the four-wire transmitters are powered by AC 220V, and some are powered by DC24V. The output signal has DC 4-20mA, the load resistance is 250Ω, or DC 0-10mA, the load resistance is 0-1.5kΩ; there are also mA and mV signals, but the load resistance or input resistance has different values due to different output circuit forms. different, as shown in Figure 4.3.



The input to the receiving instrument is a current signal, if the resistor RL is connected in parallel, the received signal is a 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, and four-wire wiring forms of the transmitter.

Three and four wire system

Figure 3 Wiring diagram of four-wire transmitter

The four-wire transmitter is shown in Figure 3, and its power supply is mostly 220V AC, and some power supply is 24V DC. The output signal has 4-20mA DC, the load resistance is 250Ω, or 0-10mA DC, the load resistance is 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance varies with the output circuit form different.

In the above three figures, the input to the receiving instrument is a current signal. If the resistor RL is connected in parallel, the received signal is a voltage signal.

It can be seen from the above description that due to the different working principles and structures of various transmitters, different products have emerged, which also determines the two-wire, three-wire, and four-wire wiring forms of the transmitter. For users, the selection should be based on the actual situation of the unit, such as the unification of signal system, explosion-proof requirements, requirements of receiving equipment, investment and other issues to comprehensively consider the choice.

It should be pointed out that the 4-20mA DC signal output by the three-wire and four-wire transmitters is different from the two-wire system in principle and structure of the output circuit, so whether the output negative terminal can be compared with 24V in the application The negative line of the power supply is connected? Can it be shared? It should be noted that if necessary, isolation measures can be taken, such as using power distributors , safety barriers, etc., in order to share electricity and ground with other instruments and avoid additional interference.


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