First, let's take a look at their definitions.
Two-wire system: two wires transmit both power and signal, that is, the load and power output of the sensor are connected in series. The power is introduced from the outside and connected in series with the load to drive the load. Three-wire system: A three-wire sensor is a sensor in which the positive end of the power supply and the positive end of the signal output are separated, but they share a common COM terminal. Four-wire system: two wires for power supply and two wires for signal. The power supply and signal work separately. The term "several-wire system" only came into being after the birth of the two-wire transmitter. This is the result of the widespread use of electronic amplifiers in instruments . The essence of amplification is an energy conversion process, which is inseparable from power supply. Therefore, the four-wire transmitter appeared first; that is, two wires are responsible for the supply of power, and the other two wires are responsible for outputting the converted and amplified signals (such as voltage , current , etc.). But at present, many transmitters use a two-wire system. Next, let's take a closer look at the differences between transmitters with different wire systems. Differences between transmitters with different wire systems 1. 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 the transmission wire 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 minimum consumption of the transmitterThe power P 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 negative change allowed by the 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 achieved. The so-called two-wire system means that the power supply and load are connected in series, with a common point, and the signal communication and power supply between the field transmitter and the control room wires , which are both power lines and signal lines. Since the signal starting current of the two-wire transmitter is 4mA DC, it provides a static working current for the transmitter. 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 faults such as power failure and line break. In addition, the two-wire system is also convenient for using safety barriers, which is conducive to safety and explosion-proof. Figure 1 is the wiring diagram
of the two-wire converter . The DC power supply U (24V) supplies power to the converter on site through two wires, and these two wires are also the transmission lines of the output signal (4~20mA). The output current signal needs to be converted into a voltage UL=IRL= (1~5) U through a standard resistor RL=250Ω and sent to the controller. The characteristic of this two-wire converter is that it is only used to detect signals and has no control function. Two-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 the transformer.
Provide power. The three-wire system uses one wire for the positive end of the power supply, one wire for the positive end of the signal output, and one wire for the negative end of the power supply and the negative end of the signal. Most of the power supply is DC 24V, and the output signal is DC4-20mA, with a load resistance of 250Ω or DC0-10mA, with a load resistance of 0-1.5kΩ; some also have mA and mV signals, but the load resistance or input resistance has different values due to different output circuit forms.
Due to the popularity and application of DC4-20mA and DC 1-5V signal systems, in order to facilitate connection in control system applications, the signal system is required to be unified. For this reason, some non-electric unit combination instruments, such as online analysis, mechanical quantity, and electrical quantity instruments, can use the DC4-20mA signal system for output. However, due to the complexity of its conversion circuit and high power consumption, it is difficult to meet all the above three conditions and cannot achieve a two-wire system. Only an external power supply method can be used to make a four-wire transmitter with an output of DC4-20mA. The power supply of the four-wire transmitter is mostly AC 220V, and there are also DC24V power supplies. The output signal is DC 4-20mA, with a load resistance of 250Ω, or DC 0-10mA, with a load resistance of 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, 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.
3. Four-wire system
Figure 3 Schematic diagram of wiring of four-wire transmitter The
four-wire transmitter is shown in Figure 3. Most of them are powered by 220V AC, and some are powered by 24V DC. The output signal is 4-20mA DC, with a load resistance of 250Ω, or 0-10mA DC, with a load resistance of 0-1.5KΩ; some also have mA and mV signals, but the load resistance or input resistance has different values due to different output circuit forms.
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 appeared, which also determines the two-wire, three-wire and four-wire wiring forms of the transmitter. For users, when selecting, they should consider and choose according to the actual situation of the unit, such as the unification of the signal system, explosion-proof requirements, requirements of the receiving equipment, investment and other issues.
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 terms of its output circuit principle and structure. Therefore, in the application, can the negative end of its output be connected to the negative line of the 24V power supply? Can it be grounded? This should be noted. If necessary, isolation measures can be taken, such as using distributors , safety barriers, etc., so as to share power and ground with other instruments and avoid the generation of additional interference.