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The so-called two-wire system, three wire system, and four wire system refer to various outputssimulationdirect currentcurrentsignaloftransmitter, itsworking principleAnd the difference in structure, not just referring to the wiring form of the transmitter.

The term 'several wire system' only emerged after the birth of two-wire transmitters. This iselectronicamplifierThe widely used result in instruments is that amplification is essentially an energy conversion process, which cannot be separated from itpower supplyTherefore, a four wire transmitter appeared; Two lines are responsible forpower supplyThe other two wires are responsible for outputting converted and amplified signals (such as voltage, current, etc.). The emergence of DDZ-II electric unit combination instrument, with a power supply of 220VacThe output signal is 0-10mAdcThe four wire transmitter has been widely used and can still be seen in some factories.

In the 1970s, China began producing DDZ-III electric unit combination instruments and adopted international standardselectricianCommittee (IEC): Processcontrol systemUsing analog signal standards. The instrument transmission signal adopts 4-20ma.dc, and the contact signal adopts 1-5v.dc, which is a signal system that uses current transmission and voltage reception. By using 4-20mA.dc signals, on-site instruments can achieve a two-wire system. However, due to limitations, the two-wire system was only used on pressure and differential pressure transmitters at that time, while temperature transmitters still used the four wire system. Currently, there are two wire transmitters in ChinaproductThe scope has also greatly expanded, and the application areas are becoming more and more diverse. At the same time, transmitters imported from abroad are mostly two-wire.

Because to achieve a two-wire transmitter, the following conditions must be met simultaneously:

1．v≤emin-imaxrlmax

The output voltage v of the transmitter is equal to the specified power supply voltage minus the current at the loadresistanceThe voltage drop across the resistance of the transmission wire.

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 <min(min-iminrlmax)

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

In the formula: emin=power supply voltage. For most instruments, emin=24 (1-5%)=22.8v, and 5% is the negative variation allowed by the 24V power supply;

imax=20ma；

imin=4ma；

RLmax=250 Ω+transmission wire resistance.

If the transmitter meets the above three conditions in design, two-wire transmission can be achieved. The so-called two-wire system refers to the power supply and load being connected in series with a common point, while the signal communication and power supply between the on-site transmitter and the control room instrument only use two wires, which are both power and signal lines. The two-wire transmitter provides a static working current for the transmitter due to the signal starting current of 4ma.dc, while the instrumentelectricalZero point is 4ma.dc, not related tomachineZero point coincidence, this "active zero point" is beneficial for identifying faults such as power outages and disconnections. Moreover, the two-wire system also facilitates the use of safety barriers, which is beneficial for safety and explosion prevention.

The two-wire transmitter is shown in Figure 1, with a power supply of 24V. DC and an output signal of 4-20mA. DC. The load resistance is 250 Ω, and the negative line potential of the 24V power supply is the signal common lineintelligenceThe transmitter can also load the FSK key control signal of the Hart protocol on the 4-20mA.dc signal.

Due to the popularity and application of the 4-20ma.dc (1-5v. dc) signal system, in order to facilitate connection in control system applications, it is required that the signal system be unified. Therefore, some non electric unit combination instruments, such as online analysis, mechanical quantity, electric quantity, etc., can use the 4-20ma.dc signal system output. However, due to the complex conversion circuit and high power consumption, it is difficult to meet all three conditions mentioned above, and it is impossible to achieve a two-wire system. Therefore, an external power supply can only be used to make a four wire transmitter with an output of 4-20ma.dc.

The four wire transmitter, as shown in Figure 2, is mostly powered by 220V AC, but there are also those powered by 24V DC. The output signal is 4-20mA.dc with a load resistance of 250 Ω, or 0-10mA.dc with a load resistance of 0-1.5k Ω; Some also have ma and mv signals, but the values of load resistance or input resistance may vary depending on the output circuit form.

In order to reduce the size and weight of the transmitter, improve anti-interference performance, and simplify wiring, some instrument manufacturers have changed the power supply of the transmitter from 220v.ac to low-voltage DC power supply. For example, when the power supply is taken from a 24V DC power box, the low-voltage power supply creates conditions for negative line sharing, resulting in three wire transmitter products.

The three wire transmitter is shown in Figure 3. The so-called three wire system means that the positive end of the power supply uses one wire, the positive end of the signal output uses one wire, and the negative end of the power supply and the negative end of the signal share one wire. Its power supply is mostly 24V/DC, with an output signal of 4-20mA/DC, a load resistance of 250 Ω or 0-10mA/DC, and a load resistance of 0-1.5K Ω; Some also have ma and mv signals, but the values of load resistance or input resistance may vary depending on the output circuit form.

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

From the above description, it can be seen that due to the different working principles and structures of various transmitters, different products have emerged, which determines the wiring forms of transmitters in two-wire, three wire, and four wire systems. For users, the selection should be based on the actual situation of their unit, such as the standardization of signal systems, explosion-proof requirements, requirements for receiving equipment, investment, and other issues to comprehensively consider.

It should be pointed out that the 4-20mA.dc signals output by the three wire and four wire transmitters are affected by their outputPrinciples of Electric CircuitsThe structure is different from the two-wire system, so can its output negative terminal be connected to the negative line of a 24V power supply in application? Can we share the land? This should be noted, and isolation measures can be taken if necessary, such as using distributors, safety barriers, etc., in order to share power and ground with other instruments and avoid the generation of additional interference.

Discuss the issue of changing from a two-line system to a four line system and from a four line system to a two-line system.

From the above, it can be seen that various wire based transmitters can exist, and there is always a reason for their existence. Otherwise, there would not be so many wire based transmitters, and it is difficult for users to modify them. Moreover, the practical significance is not significant.

If a four wire transmitter with a transmission signal of 0-10mA.dc is to be changed to a two-wire system, the first problem encountered is that its starting current is zero. In the zero current state, the electronic amplifier of the transmitter cannot establish a working point, making it difficult to operate normally. If a DC power supply is used and the original constant current characteristics of the instrument are guaranteed, when the transmitter has a load resistance of 0-1.5k Ω, the feedback coil resistance connected in series with it is about 2k Ω. When the output is 10ma, the voltage drop of these two parts will be greater than 24V. That is to say, it is impossible to ensure constant current characteristics when using 24V/DC power supply and the load is 0-1.5k Ω, so it cannot be said to use two-wire transmission.

In the 1970s, an instrument factory worked on changing a four wire transmitter from 0-10mA. DC to a two-wire transmitter. The specific method was to improve the original transmitter circuit and increase the power supply voltage to 48V. DC, but the starting current of the transmitter could not be zero. Therefore, a negative current was used to offset the starting output current of 4mA on the load resistor. But such products have also not been promoted and applied.