Sharing—principles of two-wire, three-wire, and four-wire systems

Thread Content

The two-wire, three-wire, and four-wire systems we are discussing refer to the differences in operating principles and structure among various transmitters that output analog DC current signals, and not merely to the wiring methods of these transmitters. Otherwise, measuring temperature with a thermocouple paired with a millivoltmeter could be considered the origin of the two-wire system! The term \"multi-wire system\" came into use only after the invention of two-wire transmitters. This is the result of the widespread use of electronic amplifiers in instruments; the essence of amplification is a process of energy conversion, which in turn requires a power supply. Therefore, the first to appear were four-wire transmitters ； That is, two wires are responsible for supplying power, while the other two wires are responsible for delivering the converted and amplified signals (such as voltage, current, etc.). With the advent of the DDZ-Ⅱ type electric combined instrument, four-wire transmitters that operate on 220V.AC power supply and output signals in the range of 0–10mA.DC have been widely used; they can still be found in some factories today. In the 1970s, China began producing DDZ-Ⅲ type electric unit combination instruments, adopting the International Electrotechnical Commission (IEC) standards for analog signals in process control systems. That is, the instrument transmission signal uses 4-20mA.DC, while the communication signal uses 1-5V.DC; it is a signal system that employs current for transmission and voltage for reception. By using a 4-20mA.DC signal, the field instrument can operate in a two-wire system. However, due to certain constraints, at that time the two-wire system was only used in pressure and differential pressure transmitters, while temperature transmitters and similar devices still used the four-wire system. Currently, the range of two-wire transmitters available in China has also **expanded**, with an increasing number of application areas. Meanwhile, transmitters imported from abroad are also mostly two-wire type. To implement a two-wire transmitter, the following conditions must be met simultaneously: 1. V ≤ Emin – Imax·RLmax. The output voltage V of the transmitter is equal to the specified minimum supply voltage minus the voltage drop caused by the current across 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 transmitter’s output current. 3. P＜Imin(Emin-IminRLmax): The minimum power consumption P of the transmitter must not exceed the value given in this formula; it is usually less than 90 mW. In the formula: Emin = the minimum supply voltage; for most instruments, Emin = 24(1–5%) = 22.8 V, with 5% representing the allowable negative variation for a 24 V supply ； Imax="20mA" ； Imin="4mA" ； RLmax="250"Ω + resistance of the transmission wire. If the transmitter meets the above three conditions in its design, two-wire transmission can be achieved. The so-called two-wire system means that the power supply and the load are connected in series, sharing a common point; the signal communication as well as the power supply between the field transmitter and the instruments in the control room are accomplished using just two wires, which serve both as power lines and signal lines. In two-wire transmitters, the current at the signal start point is 4mA.DC, which provides a static operating current for the transmitter. Meanwhile, the electrical zero point of the instrument is also 4mA.DC, and it does not coincide with the mechanical zero point. This \"dynamic zero point\" facilitates the detection of faults such as power outages and broken connections. Moreover, the two-wire system also facilitates the use of safety barriers, contributing to safety and explosion prevention. As shown in Figure 1, a two-wire transmitter is powered by 24V DC, with an output signal of 4-20mA DC. The load resistance is 250Ω. The negative terminal of the 24V power supply has the lowest potential; this terminal serves as the signal common line. For smart transmitters, an FSK modulated signal based on the HART protocol can also be added to the 4-20mA DC signal. Figure 1: Schematic diagram of wiring for two-wire transmitters. Due to the widespread use of the 4-20mA.DC (1-5V.DC) signal standard, it is necessary to standardize this signal format in control systems to facilitate connections. As a result, certain instruments that are not electrically powered, such as those used for on-line analysis, measurement of mechanical or electrical quantities, are required to be able to output signals in the 4-20mA.DC format. However, due to complex conversion circuits and high power consumption, it is difficult for all such instruments to meet these requirements; hence they cannot operate in a two-wire configuration. In such cases, an external power supply must be used to create four-wire transmitters that can output signals at 4-20mA.DC. As shown in Figure 2, four-wire transmitters are typically powered by 220V AC, although some are also powered by 24V DC. The output signal can be 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 design of the output circuit. Figure 2: Schematic diagram of the wiring for a four-wire transmitter. Some instrument manufacturers, in order to reduce the size and weight of the transmitter, improve its resistance to interference, and simplify the wiring, change the power supply from 220V AC to low-voltage DC. If the power is supplied from a 24V DC power supply unit, the low-voltage supply facilitates the use of a common negative wire, which leads to the development of three-wire transmitter models. A three-wire transmitter is shown in Figure 3. By three-wire system, it means that one wire is used for the positive terminal of the power supply, another wire for the positive terminal of the signal output, and one common wire is used for both the negative terminal of the power supply and the negative terminal of the signal. Its power supply is typically 24V.DC; the output signal can be 4-20mA.DC with a load resistance of 250Ω, or it can be 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 design of the output circuit. Figure 3: Schematic diagram of wiring for a three-wire transmitter. In all three diagrams shown above, the input to the measuring instrument is a current signal; however, if a resistor RL is connected in parallel, then a voltage signal will be received.