The term "wire system" refers to the number of wires used to supply power to the instrument and to transmit standard signals. The term "several-wire system" was introduced after the birth of the two-wire transmitter. This is the result of the widespread use of electronic amplifiers in instrumentation. The nature of amplification is an energy conversion process, which cannot be separated from the power supply. Therefore, the first four-wire transmitters appeared, i.e., two wires are responsible for the power supply, and the other two wires are responsible for the output of the converted and amplified signals (e.g., voltage, current, etc.).
Instruments powered by 220VAC must be four-wire.
Four-wire signal instrumentation refers to: power supply circuit and signal circuit are independent of each other, separated by an isolation circuit, for the isolation of the 4 ~ 20mA standard signal. So the signal must be isolated and usually not grounded. The power supply can be 24VDC or 220VAC.
When the power provided by the two-wire system is not enough to supply power to the instrument, the three-wire method is used, and the signal is a 4～20mA standard signal.
The three wires of the three-wire system are: one for the power supply (usually 24VDC+), one for the signal, and one common wire for the power supply and the signal for the voltage reference point and the current loop.
Non-4 to 20mA standard signals are not referred to as two-wire.
The two-wire system is two wires powered by the simultaneous transmission of 4 to 20mA standard signals. Non-this standard signal is not called two-wire system. The minimum power supply capacity of the two-wire system is: 4mA × 24V - power consumption of the receiving instrument - line loss.
So in the actual field operation, four-wire system, three-wire system, two-wire system is the main power supply?
Most of the four-wire power supply for 220V.AC, there are also power supply for 24V.DC.
Four-wire transmitter as shown in the figure below, most of its power supply for 220V.AC, there are also powered by 24V.DC. 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 different output circuit forms and different values.
Most of the three-wire power supply is 24V.DC.
Three-wire transmitter as shown in the figure below, the so-called three-wire system is the positive end of the power supply with a wire, the positive end of the signal output with a wire, the negative end of the power supply and the negative end of the signal share a wire. DC, the majority of its power supply for 24V.DC, the output signal has 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 forms of output circuits and different values.
Two-wire power supply is 24V.DC.
Two-wire transmitter shown in the figure below, its power supply is 24V.DC, the output signal is 4-20mA.DC, the load resistance is 250Ω, the negative line of the 24V power supply has the lowest potential, which is the signal common line, for the intelligent transmitter can also be loaded on the 4-20mA.DC signal on the HART protocol FSK keying signal.
Due to the different working principles and structures of various transmitters, which led to the emergence of different products, it also determines the two-wire, three-wire, four-wire wiring form of the transmitter. How to choose four-wire, three-wire, two-wire instrument?
Power greater than 10W, higher precision requirements, the general choice of four-wire instrumentation.
The application of four-wire instrumentation, the earliest began with the emergence of the DDZ-II type electric unit combination instrumentation, power supply for 220V.AC, followed by the output signal for 0 ~ 10mA.DC four-wire transmitter has been widely used. Due to the complexity of the conversion circuit, power consumption and other reasons, many instruments are still prioritized four-wire system, the most common like the Koch force mass flowmeter and electromagnetic flowmeter, now or four-wire system, because the mass flowmeter needs to be vibration power consumption, electromagnetic flowmeter needs to be excitation power consumption, the 2 table power to 10W or more, so it has to be four-wire system.
Reduce non-safety voltage supply, the general choice of three-wire instrumentation.
The application of three-wire system, in order to meet the instrumentation needs of the power supply power design, the transmitter's power supply from 220V.AC to low-voltage DC power supply, such as power supply from the 24V.DC power supply box to take, reduce the non-safety voltage supply. This way there is a three-wire transmitter products.
Less power and affordable, generally choose the two-wire system.
The application of two-wire instrumentation, the use of 4-20mA.DC signals, field instruments can be realized two-wire system, and now the domestic two-wire transmitter products are widely used.
In summary, 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 noted that the three-wire and four-wire transmitter output 4-20mA.DC signal, due to its output circuit principle and structure and two-wire system is not the same, so in the application of the negative end of its output and the negative line of the 24V power supply can be connected to a common ground this is to pay attention to, and if necessary, can be taken to take isolation measures such as the use of distributors, safety barriers, etc. in order to share power with other instruments, common ground and to avoid the generation of additional interference. This is to pay attention to whether the negative end of the 24V power supply can be connected to the common ground.