"Wire system" refers to the number of wires used for power supply and standard signal transmission in an instrument. The term "multi-wire system" only came into use after the advent of two-wire transmitters. This was a result of the widespread application of electronic amplifiers in instruments; amplification is essentially an energy conversion process, which requires a power supply. Therefore, the first transmitters to appear were four-wire transmitters, where two wires supplied the power and the other two wires output the converted and amplified signal (such as voltage, current, etc.).
Instruments powered by 220VAC must be four-wire.
A four-wire signal instrument refers to an instrument where the power supply circuit and the signal circuit are independent and separated by isolation circuits, used for isolating a standard 4-20mA signal. Therefore, the signal must be isolated and is typically not grounded. The power supply can be 24VDC or 220VAC.
When the power provided by the two-wire system is insufficient to power the instrument, a three-wire system is used, with a standard 4-20mA signal.
The three wires in a three-wire system are: one power line (usually 24VDC+), one signal line, and one common line for both power and signal, used for voltage reference and current loop.
Signals that are not 4-20mA standard are not called two-wire systems.
Two-wire transmission uses two wires to simultaneously supply power and transmit a standard 4-20mA signal. Signals not conforming to this standard are not considered two-wire. The minimum power supply capability for a two-wire system is: 4mA × 24V - power consumption of the receiving instrument - line loss.
So, in actual field operations, what are the main power supply methods for four-wire, three-wire, and two-wire systems?
Four-wire power supplies are mostly 220V AC, but some are 24V DC.
The four-wire transmitter is shown in the figure below. Most of them are powered by 220V AC, but 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 varies depending on the output circuit configuration.
Three-wire power supplies are mostly 24V DC.
A three-wire transmitter is shown in the figure below. The term "three-wire" means that the positive power supply terminal uses one wire, the positive signal output terminal uses another wire, and the negative power supply terminal and the negative signal terminal share a common wire. Most are powered by 24V DC, with output signals ranging from 4-20mA DC and a load resistance of 250Ω, or 0-10mA DC with a load resistance of 0-1.5KΩ. Some also offer mA and mV signals, but the load resistance or input resistance varies depending on the output circuit configuration.
The two-wire power supply is 24V DC.
The two-wire transmitter is shown in the figure below. Its power supply is 24V DC, the output signal is 4-20mA DC, and the load resistance is 250Ω. The negative line of the 24V power supply has the lowest potential, which is the signal common line. For intelligent transmitters, the HART protocol FSK keying signal can also be loaded onto the 4-20mA DC signal.
Because various transmitters have different working principles and structures, different products have emerged, which in turn determines the two-wire, three-wire, and four-wire wiring configurations of the transmitters. How to select a four-wire, three-wire, or two-wire instrument?
When the power is greater than 10W and the accuracy requirement is high, a four-wire instrument is generally selected.
The application of four-wire instruments began with the advent of the DDZ-II type electric unit combination instrument, which was powered by 220V AC. Subsequent four-wire transmitters with output signals of 0~10mA DC were widely used. Due to the complexity of the conversion circuit and high power consumption, many instruments still prioritize four-wire systems. The most common examples are Coriolis mass flow meters and electromagnetic flow meters, which are still four-wire systems because mass flow meters require vibration power consumption, and electromagnetic flow meters require excitation power consumption; both meters have power consumption exceeding 10W, thus necessitating a four-wire system.
To reduce unsafe voltage power supply, three-wire instruments are generally selected.
In the application of three-wire systems, to meet the power supply requirements of the instrument, the transmitter's power supply is changed from 220V AC to low-voltage DC, such as drawing power from a 24V DC power supply box, reducing the supply of unsafe voltages. This is how three-wire transmitter products came about.
It has lower power consumption and is more economical, so a two-wire system is generally chosen.
The application of two-wire instruments, using 4-20mA DC signals, allows field instruments to achieve two-wire operation. Currently, two-wire transmitters are widely used in China.
In conclusion, users should consider their specific circumstances, such as signal standardization, explosion-proof requirements, receiving equipment requirements, and investment costs, when making a selection.
It should be noted that the 4-20mA DC signal output by three-wire and four-wire transmitters has a different output circuit principle and structure than that of two-wire transmitters. Therefore, it is important to pay attention to whether the negative terminal of the output can be connected to the negative line of the 24V power supply and whether it can share a common ground. If necessary, isolation measures can be taken, such as using a distributor or safety barrier, to ensure that it shares power and ground with other instruments and to avoid the generation of additional interference.
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